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• can bind successfully to the ldap server, but needs to know how to find user w/i AD

  - by Brad

  I create a login form to bind to the ldap server, if successful, it creates a session (which the user's username is stored within), then I go to another page that has session_start(); and it works fine. What I want to do now, is add code to test if that user is a member of a specific group. So in theory, this is what I want to do if(username session is valid) { search ldap for user -> get list of groups user is member of foreach(group they are member of) { switch(group) { case STAFF: print 'they are member of staff group'; $access = true; break; default: print 'not a member of STAFF group'; $access = false; break; } if(group == STAFF) { break; } } if($access == TRUE) { // you have access to the content on this page } else { // you do not have access to this page } } How do I do a ldap_search w/o binding? I don't want to keep asking for their password on each page, and I can't pass their password thru a session. Any help is appreciated.

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• How do I unit test the methods in a method object?

  - by Sancho

  I've performed the "Replace Method with Method Object" refactoring described by Beck. Now, I have a class with a "run()" method and a bunch of member functions that decompose the computation into smaller units. How do I test those member functions? My first idea is that my unit tests be basically copies of the "run()" method (with different initializations), but with assertions between each call to the member functions to check the state of the computation. (I'm using Python and the unittest module.)

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• Getting list of all existing vtables.

  - by Patrick

  In my application I have quite some void-pointers (this is because of historical reasons, application was originally written in pure C). In one of my modules I know that the void-pointers points to instances of classes that could inherit from a known base class, but I cannot be 100% sure of it. Therefore, doing a dynamic_cast on the void-pointer might give problems. Possibly, the void-pointer even points to a plain-struct (so no vptr in the struct). I would like to investigate the first 4 bytes of the memory the void-pointer is pointing to, to see if this is the address of the valid vtable. I know this is platform, maybe even compiler-version-specific, but it could help me in moving the application forward, and getting rid of all the void-pointers over a limited time period (let's say 3 years). Is there a way to get a list of all vtables in the application, or a way to check whether a pointer points to a valid vtable, and whether that instance pointing to the vtable inherits from a known base class?

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• C -- Basic Struct questions

  - by Ryan Yu

  So I'm trying to learn C right now, and I have some basic struct questions I'd like to clear up: Basically, everything centers around this snippet of code: #include <stdio.h> #include <stdlib.h> #define MAX_NAME_LEN 127 typedef struct { char name[MAX_NAME_LEN + 1]; unsigned long sid; } Student; /* return the name of student s */ const char* getName (const Student* s) { // the parameter 's' is a pointer to a Student struct return s->name; // returns the 'name' member of a Student struct } /* set the name of student s If name is too long, cut off characters after the maximum number of characters allowed. */ void setName(Student* s, const char* name) { // 's' is a pointer to a Student struct | 'name' is a pointer to the first element of a char array (repres. a string) s->name = name; } /* return the SID of student s */ unsigned long getStudentID(const Student* s) { // 's' is a pointer to a Student struct return s->sid; } /* set the SID of student s */ void setStudentID(Student* s, unsigned long sid) { // 's' is a pointer to a Student struct | 'sid' is a 'long' representing the desired SID s->sid = sid; } I've commented up the code in an attempt to solidify my understanding of pointers; I hope they're all accurate. So anyway, I have a feeling that setName and setStudentID aren't correct, but I'm not exactly sure why. Can someone explain? Thanks!

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• Accessing any structs members at run-time.

  - by jmgunn

  Is it possible to get access to an individual member of a struct or class without knowing the names of its member variables? I would like to do an "offsetof(struct, tyname)" without having the struct name or member variable name hard coded amoungst other things. thanks.

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• C++ Newbie Question

  - by JavaMan

  I am from a Java background and is learning C++. I encountered the following C++ code: String source = String::New("'Hello' + ', World'"); As what I understand so far, this should be a call to static member function 'New' of class 'String'. But, I've searched through the whole header file defining 'String', there is not any static member named 'New' in the String class or its super classes. Is there any special meaning attached to String class or the New member function in C++?

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• :any option for rails 3 routes

  - by user357523

  In rails 2 you can use the :any option to define a custom route that responds to any request method e.g. map.resources :items, :member => {:erase => :any} rails 3 doesn't seem to support the :any option resources :items do get :erase, :on => :member # works any :erase, :on => :member # doesn't work end does anyone know if this option has been removed or just renamed?

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• Decayed multidimensional array return from function

  - by paul simmons

  related to http://stackoverflow.com/questions/2520535/gcc-multi-dim-array-or-double-pointer-for-warning-free-compile , is there a way to return so-called "decayed array pointer" from a function? in summary (suppose 2 dim array) returning int (*a)[5] format rather than int** format? as far as I see, when returned int** pointer is sent to another function waiting (int*)[] parameter, it is not working correctly.

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• Useless variable name in C struct type definition

  - by user1210233

  I'm implementing a linked list in C. Here's a struct that I made, which represents the linked list: typedef struct llist { struct lnode* head; /* Head pointer either points to a node with data or NULL */ struct lnode* tail; /* Tail pointer either points to a node with data or NULL */ unsigned int size; /* Size of the linked list */ } list; Isn't the "llist" basically useless. When a client uses this library and makes a new linked list, he would have the following declaration: list myList; So typing llist just before the opening brace is practically useless, right? The following code basically does the same job: typedef struct { struct lnode* head; /* Head pointer either points to a node with data or NULL */ struct lnode* tail; /* Tail pointer either points to a node with data or NULL */ unsigned int size; /* Size of the linked list */ } list;

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• Could I ever want to access the address zero?

  - by Joel

  The constant 0 is used as the null pointer in C and C++. But as in http://stackoverflow.com/questions/2389251/pointer-to-a-specific-fixed-address there seems to be some possible use of assigning fixed addresses. Is there ever any conceivable need, in any system, for whatever low level task, for accessing the address 0? If there is, how is that solved with 0 being the null pointer and all? If not, what makes it certain that there is not such a need?

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• how to access child instances in a vector in c++

  - by tsubasa

  I have a parent class and child class (inherited from parent). In the child class, I have a member function named function_blah(); I used vector<parent*> A to store 5 parent instances, 3 child instances. So the total number of elements in the vector is 8. I can easily access to member functions of element A[0] to A[4], which are parent instances. But whenever I try to have access to member functions of element A[5] to A[7], the compiler complains that class parent has no member named 'function_blah' The way I access to elements is using index. e.x A[i] with i = 0..7. Is it correct? if not, how?

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• sending address of a variable declared on the stack?

  - by kobac

  I have a doubt concerning declaring variables, their scope, and if their address could be sent to other functions even if they are declared on the stack? class A{ AA a; void f1(){ B b; aa.f2(&b); } }; class AA{ B* mb; f2(B* b){ mb = b; //... } }; Afterwards, I use my AA::mb pointer in the code. So things I would like to know are following. When the program exits A::f1() function, b variable since declared as a local variable and placed on the stack, can't be used anymore afterwards. What happens with the validity of the AA::mb pointer? It contains the address of the local variable which could not be available anymore, so the pointer isn't valid anymore? If B class is a std::<vector>, and AA::mb is not a pointer anymore to that vector, but a vector collection itself for example. I would like to avoid copying all of it's contents in AA::f2() to a member AA::mb in line mb = b. Which solution would you recommend since I can't assign a pointer to it, because it'll be destroyed when the program exits AA::f2()

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• [Grails] HibernateException: No session currently bound to execution context

  - by user289206

  I'm trying to create a very basic REST-ish web service with Grails and Postgres. I have the read() & delete() methods working, but I can't get create() to work. Hibernate just gripes, "HibernateException: No session currently bound to execution context." Here's my create method: def create = { def member = new Member(params) member.save() render(status: 201) } Any advice would be great. Thanks.

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• walking list in KDB/kernel debugger

  - by user291849

  I need to walk a link list in the kernel debugger. How can I determine the head pointer and walk the list? I have a listing and can find the address and location in the code where I check to see if I have a head, so I know the specific code location and address. But not sure how to determine the pointer or how to determine the next element and pointer on the list.

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• linq2sql left join with "multiselect"

  - by just_azho

  Hi, folks I'm trying to achieve following by linq2sql, but not successful. I've Member and Reference tables. DB is design in such a manner that Member can have multiple (=0) References. What I want as a result of query is, list (rows) of members, where all references of the member are "collected" in one column. What I had achieved is following query, but for this one there exist a row for each Reference. var refs = (from m in db.Members join r in db.References on m.PID equals r.PID into g from o in g.DefaultIfEmpty() select new { member = m, name = (o == null ? "" : o.NameSurname) }); I feel I need to insert SelectMany somewher :) Could you please give hints on achieving the goal?

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• Linked List manipulation, issues retrieving data c++

  - by floatfil

  I'm trying to implement some functions to manipulate a linked list. The implementation is a template typename T and the class is 'List' which includes a 'head' pointer and also a struct: struct Node { // the node in a linked list T* data; // pointer to actual data, operations in T Node* next; // pointer to a Node }; Since it is a template, and 'T' can be any data, how do I go about checking the data of a list to see if it matches the data input into the function? The function is called 'retrieve' and takes two parameters, the data and a pointer: bool retrieve(T target, T*& ptr); // This is the prototype we need to use for the project "bool retrieve : similar to remove, but not removed from list. If there are duplicates in the list, the first one encountered is retrieved. Second parameter is unreliable if return value is false. E.g., " Employee target("duck", "donald"); success = company1.retrieve(target, oneEmployee); if (success) { cout << "Found in list: " << *oneEmployee << endl; } And the function is called like this: company4.retrieve(emp3, oneEmployee) So that when you cout *oneEmployee, you'll get the data of that pointer (in this case the data is of type Employee). (Also, this is assuming all data types have the apropriate overloaded operators) I hope this makes sense so far, but my issue is in comparing the data in the parameter and the data while going through the list. (The data types that we use all include overloads for equality operators, so oneData == twoData is valid) This is what I have so far: template <typename T> bool List<T>::retrieve(T target , T*& ptr) { List<T>::Node* dummyPtr = head; // point dummy pointer to what the list's head points to for(;;) { if (*dummyPtr->data == target) { // EDIT: it now compiles, but it breaks here and I get an Access Violation error. ptr = dummyPtr->data; // set the parameter pointer to the dummy pointer return true; // return true } else { dummyPtr = dummyPtr->next; // else, move to the next data node } } return false; } Here is the implementation for the Employee class: //-------------------------- constructor ----------------------------------- Employee::Employee(string last, string first, int id, int sal) { idNumber = (id >= 0 && id <= MAXID? id : -1); salary = (sal >= 0 ? sal : -1); lastName = last; firstName = first; } //-------------------------- destructor ------------------------------------ // Needed so that memory for strings is properly deallocated Employee::~Employee() { } //---------------------- copy constructor ----------------------------------- Employee::Employee(const Employee& E) { lastName = E.lastName; firstName = E.firstName; idNumber = E.idNumber; salary = E.salary; } //-------------------------- operator= --------------------------------------- Employee& Employee::operator=(const Employee& E) { if (&E != this) { idNumber = E.idNumber; salary = E.salary; lastName = E.lastName; firstName = E.firstName; } return *this; } //----------------------------- setData ------------------------------------ // set data from file bool Employee::setData(ifstream& inFile) { inFile >> lastName >> firstName >> idNumber >> salary; return idNumber >= 0 && idNumber <= MAXID && salary >= 0; } //------------------------------- < ---------------------------------------- // < defined by value of name bool Employee::operator<(const Employee& E) const { return lastName < E.lastName || (lastName == E.lastName && firstName < E.firstName); } //------------------------------- <= ---------------------------------------- // < defined by value of inamedNumber bool Employee::operator<=(const Employee& E) const { return *this < E || *this == E; } //------------------------------- > ---------------------------------------- // > defined by value of name bool Employee::operator>(const Employee& E) const { return lastName > E.lastName || (lastName == E.lastName && firstName > E.firstName); } //------------------------------- >= ---------------------------------------- // < defined by value of name bool Employee::operator>=(const Employee& E) const { return *this > E || *this == E; } //----------------- operator == (equality) ---------------- // if name of calling and passed object are equal, // return true, otherwise false // bool Employee::operator==(const Employee& E) const { return lastName == E.lastName && firstName == E.firstName; } //----------------- operator != (inequality) ---------------- // return opposite value of operator== bool Employee::operator!=(const Employee& E) const { return !(*this == E); } //------------------------------- << --------------------------------------- // display Employee object ostream& operator<<(ostream& output, const Employee& E) { output << setw(4) << E.idNumber << setw(7) << E.salary << " " << E.lastName << " " << E.firstName << endl; return output; } I will include a check for NULL pointer but I just want to get this working and will test it on a list that includes the data I am checking. Thanks to whoever can help and as usual, this is for a course so I don't expect or want the answer, but any tips as to what might be going wrong will help immensely!

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• gluNewQuadric() before opengl's initialization

  - by Schrödinger's cat

  Hello, I'm working on a c++ code that uses SDL/opengl. Is this possible to create a pointer to a quadric with 'gluNewQuadric()' before having initialized opengl with 'SDL_SetVideoMode'? The idea is to create a class with a (pointer to a) quadric class member that has to be instantiate before the 'SDL_SetVideoMode' call. This pointer is initialized in the class' constructor with a 'gluNewQuadric()' call.

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• troubles creating a List of doubles from a list of objects

  - by Michel

  Hi, i have a list with objects. The object has a property 'Sales' which is a string. Now i want to create a list of doubles with the values of all objects' 'Sales' properties. I tried this: var tmp = from n in e.Result select new{ Convert.ToDouble ( n.Sales) }; but this gives me this error: Error 106 Invalid anonymous type member declarator. Anonymous type members must be declared with a member assignment, simple name or member access.

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• possible implementations of casting in c++

  - by lego69

  I have this snippet of the code in my header: class A { private: int player; public: A(int initPlayer = 0); A(const A&); A& operator=(const A&); ~A(); void foo() const; friend int operator==(const A& i, const A& member) const; }; implementation of the operator== int operator==(const A& i, const A& member) const{ if(i.player == member.player){ return 1; } return 0; } and I need casting for this part of my code: A *pa1 = new A(a2); assert(i == *pa1); i - is some int, which my function receives I receive an error non-member function, How can I fix it? thanks in advance

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• Using `g_object_set_data` for passing user name.

  - by PP

  I am using g_object_set_data to set user name with event_box so in call back i can get it with in event_box pointer. g_object_set_data(G_OBJECT(event_box), "user_name", (gpointer)(user_name) ); but problem is that i am setting user_name which is not an pointer allocated string. It is an local string (not allocated on hip) which gets destroyed. So is it necessary to allocated and then use the pointer, i just want to associate one name with this event_box.

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• Java reference storage question

  - by aab

  In java, when you pass an object to a method as a parameter, it is actually passing a reference, or a pointer, to that object because objects in Java are references. Inside the function, it has a pointer to that object which is a location in memory. I am wondering where this pointer lives in memory? Is a new memory location created once inside the function to hold this reference?

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• how i should create a reference of a object of a class

  - by vishal_niist

  How i get address pointer of a object

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• Why does JAXB not create a member variable in its generated code when an XML schema base type and subtype have the same element declared in them?

  - by belltower

  I have a question regarding with regard to JAXB generated classes. As you can I see, I have a complex type, DG_PaymentIdentification1, declared in my schema. Its a restriction of PaymentIdentification1. DG_PaymentIdentification1 is also identical to PaymentIdentification1. I also have a type called DG_CreditTransferTransactionInformation10 which has a base type of CreditTransferTransactionInformation10 and is identical to it. I have included the relevant XML schema snippets below. <xs:complexType name="DG_PaymentIdentification1"> <xs:complexContent> <xs:restriction base="PaymentIdentification1"> <xs:sequence> <xs:element name="InstrId" type="DG_Max35Text_REF" minOccurs="0"/> <xs:element name="EndToEndId" type="DG_Max35Text_REF" id="DG-41"/> </xs:sequence> </xs:restriction> </xs:complexContent> </xs:complexType> <xs:complexType name="PaymentIdentification1"> <xs:sequence> <xs:element name="InstrId" type="Max35Text" minOccurs="0"/> <xs:element name="EndToEndId" type="Max35Text"/> </xs:sequence> </xs:complexType> <xs:complexType name="DG_CreditTransferTransactionInformation10"> <xs:complexContent> <xs:restriction base="CreditTransferTransactionInformation10"> <xs:sequence> <xs:element name="PmtId" type="DG_PaymentIdentification1"/> <xs:simpleType name="DG_Max35Text_REF"> <xs:restriction base="DG_NotEmpty35"> <xs:pattern value="[\-A-Za-z0-9\+/\?:\(\)\.,'&#x20;]*"/> </xs:restriction> </xs:simpleType> <xs:simpleType name="Max35Text"> <xs:restriction base="xs:string"> <xs:minLength value="1"/> <xs:maxLength value="35"/> </xs:restriction> </xs:simpleType> JAXB generates the following java class for DG_PaymentIdentification1: @XmlAccessorType(XmlAccessType.FIELD) @XmlType(name = "DG_CreditTransferTransactionInformDGion10") public class DGCreditTransferTransactionInformation10 extends CreditTransferTransactionInformation10 { } My question is why doesnt the DGCreditTransferTransactionInformation10 generated class have a variable of type DG_PaymentIdentification1 in the generated code? The base class CreditTransferTransactionInformation10 does have a type PaymentIdentification1 declared in it. Is there any way of ensuring that DGCreditTransferTransactionInformation10 will have a DG_PaymentIdentification1 in it?

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• From Binary to Data Structures

  - by Cédric Menzi

  Table of Contents Introduction PE file format and COFF header COFF file header BaseCoffReader Byte4ByteCoffReader UnsafeCoffReader ManagedCoffReader Conclusion History This article is also available on CodeProject Introduction Sometimes, you want to parse well-formed binary data and bring it into your objects to do some dirty stuff with it. In the Windows world most data structures are stored in special binary format. Either we call a WinApi function or we want to read from special files like images, spool files, executables or may be the previously announced Outlook Personal Folders File. Most specifications for these files can be found on the MSDN Libarary: Open Specification In my example, we are going to get the COFF (Common Object File Format) file header from a PE (Portable Executable). The exact specification can be found here: PECOFF PE file format and COFF header Before we start we need to know how this file is formatted. The following figure shows an overview of the Microsoft PE executable format. Source: Microsoft Our goal is to get the PE header. As we can see, the image starts with a MS-DOS 2.0 header with is not important for us. From the documentation we can read "...After the MS DOS stub, at the file offset specified at offset 0x3c, is a 4-byte...". With this information we know our reader has to jump to location 0x3c and read the offset to the signature. The signature is always 4 bytes that ensures that the image is a PE file. The signature is: PE\0\0. To prove this we first seek to the offset 0x3c, read if the file consist the signature. So we need to declare some constants, because we do not want magic numbers.   private const int PeSignatureOffsetLocation = 0x3c; private const int PeSignatureSize = 4; private const string PeSignatureContent = "PE";   Then a method for moving the reader to the correct location to read the offset of signature. With this method we always move the underlining Stream of the BinaryReader to the start location of the PE signature.   private void SeekToPeSignature(BinaryReader br) { // seek to the offset for the PE signagure br.BaseStream.Seek(PeSignatureOffsetLocation, SeekOrigin.Begin); // read the offset int offsetToPeSig = br.ReadInt32(); // seek to the start of the PE signature br.BaseStream.Seek(offsetToPeSig, SeekOrigin.Begin); }   Now, we can check if it is a valid PE image by reading of the next 4 byte contains the content PE.   private bool IsValidPeSignature(BinaryReader br) { // read 4 bytes to get the PE signature byte[] peSigBytes = br.ReadBytes(PeSignatureSize); // convert it to a string and trim \0 at the end of the content string peContent = Encoding.Default.GetString(peSigBytes).TrimEnd('\0'); // check if PE is in the content return peContent.Equals(PeSignatureContent); }   With this basic functionality we have a good base reader class to try the different methods of parsing the COFF file header. COFF file header The COFF header has the following structure: Offset Size Field 0 2 Machine 2 2 NumberOfSections 4 4 TimeDateStamp 8 4 PointerToSymbolTable 12 4 NumberOfSymbols 16 2 SizeOfOptionalHeader 18 2 Characteristics If we translate this table to code, we get something like this:   [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)] public struct CoffHeader { public MachineType Machine; public ushort NumberOfSections; public uint TimeDateStamp; public uint PointerToSymbolTable; public uint NumberOfSymbols; public ushort SizeOfOptionalHeader; public Characteristic Characteristics; } BaseCoffReader All readers do the same thing, so we go to the patterns library in our head and see that Strategy pattern or Template method pattern is sticked out in the bookshelf. I have decided to take the template method pattern in this case, because the Parse() should handle the IO for all implementations and the concrete parsing should done in its derived classes.   public CoffHeader Parse() { using (var br = new BinaryReader(File.Open(_fileName, FileMode.Open, FileAccess.Read, FileShare.Read))) { SeekToPeSignature(br); if (!IsValidPeSignature(br)) { throw new BadImageFormatException(); } return ParseInternal(br); } } protected abstract CoffHeader ParseInternal(BinaryReader br);   First we open the BinaryReader, seek to the PE signature then we check if it contains a valid PE signature and rest is done by the derived implementations. Byte4ByteCoffReader The first solution is using the BinaryReader. It is the general way to get the data. We only need to know which order, which data-type and its size. If we read byte for byte we could comment out the first line in the CoffHeader structure, because we have control about the order of the member assignment.   protected override CoffHeader ParseInternal(BinaryReader br) { CoffHeader coff = new CoffHeader(); coff.Machine = (MachineType)br.ReadInt16(); coff.NumberOfSections = (ushort)br.ReadInt16(); coff.TimeDateStamp = br.ReadUInt32(); coff.PointerToSymbolTable = br.ReadUInt32(); coff.NumberOfSymbols = br.ReadUInt32(); coff.SizeOfOptionalHeader = (ushort)br.ReadInt16(); coff.Characteristics = (Characteristic)br.ReadInt16(); return coff; }   If the structure is as short as the COFF header here and the specification will never changed, there is probably no reason to change the strategy. But if a data-type will be changed, a new member will be added or ordering of member will be changed the maintenance costs of this method are very high. UnsafeCoffReader Another way to bring the data into this structure is using a "magically" unsafe trick. As above, we know the layout and order of the data structure. Now, we need the StructLayout attribute, because we have to ensure that the .NET Runtime allocates the structure in the same order as it is specified in the source code. We also need to enable "Allow unsafe code (/unsafe)" in the project's build properties. Then we need to add the following constructor to the CoffHeader structure.   [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)] public struct CoffHeader { public CoffHeader(byte[] data) { unsafe { fixed (byte* packet = &data[0]) { this = *(CoffHeader*)packet; } } } }   The "magic" trick is in the statement: this = *(CoffHeader*)packet;. What happens here? We have a fixed size of data somewhere in the memory and because a struct in C# is a value-type, the assignment operator = copies the whole data of the structure and not only the reference. To fill the structure with data, we need to pass the data as bytes into the CoffHeader structure. This can be achieved by reading the exact size of the structure from the PE file.   protected override CoffHeader ParseInternal(BinaryReader br) { return new CoffHeader(br.ReadBytes(Marshal.SizeOf(typeof(CoffHeader)))); }   This solution is the fastest way to parse the data and bring it into the structure, but it is unsafe and it could introduce some security and stability risks. ManagedCoffReader In this solution we are using the same approach of the structure assignment as above. But we need to replace the unsafe part in the constructor with the following managed part:   [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)] public struct CoffHeader { public CoffHeader(byte[] data) { IntPtr coffPtr = IntPtr.Zero; try { int size = Marshal.SizeOf(typeof(CoffHeader)); coffPtr = Marshal.AllocHGlobal(size); Marshal.Copy(data, 0, coffPtr, size); this = (CoffHeader)Marshal.PtrToStructure(coffPtr, typeof(CoffHeader)); } finally { Marshal.FreeHGlobal(coffPtr); } } }     Conclusion We saw that we can parse well-formed binary data to our data structures using different approaches. The first is probably the clearest way, because we know each member and its size and ordering and we have control about the reading the data for each member. But if add member or the structure is going change by some reason, we need to change the reader. The two other solutions use the approach of the structure assignment. In the unsafe implementation we need to compile the project with the /unsafe option. We increase the performance, but we get some security risks.

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• Dynamic Types and DynamicObject References in C#

  - by Rick Strahl

  I've been working a bit with C# custom dynamic types for several customers recently and I've seen some confusion in understanding how dynamic types are referenced. This discussion specifically centers around types that implement IDynamicMetaObjectProvider or subclass from DynamicObject as opposed to arbitrary type casts of standard .NET types. IDynamicMetaObjectProvider types  are treated special when they are cast to the dynamic type. Assume for a second that I've created my own implementation of a custom dynamic type called DynamicFoo which is about as simple of a dynamic class that I can think of:public class DynamicFoo : DynamicObject { Dictionary<string, object> properties = new Dictionary<string, object>(); public string Bar { get; set; } public DateTime Entered { get; set; } public override bool TryGetMember(GetMemberBinder binder, out object result) { result = null; if (!properties.ContainsKey(binder.Name)) return false; result = properties[binder.Name]; return true; } public override bool TrySetMember(SetMemberBinder binder, object value) { properties[binder.Name] = value; return true; } } This class has an internal dictionary member and I'm exposing this dictionary member through a dynamic by implementing DynamicObject. This implementation exposes the properties dictionary so the dictionary keys can be referenced like properties (foo.NewProperty = "Cool!"). I override TryGetMember() and TrySetMember() which are fired at runtime every time you access a 'property' on a dynamic instance of this DynamicFoo type. Strong Typing and Dynamic Casting I now can instantiate and use DynamicFoo in a couple of different ways: Strong TypingDynamicFoo fooExplicit = new DynamicFoo(); var fooVar = new DynamicFoo(); These two commands are essentially identical and use strong typing. The compiler generates identical code for both of them. The var statement is merely a compiler directive to infer the type of fooVar at compile time and so the type of fooExplicit is DynamicFoo, just like fooExplicit. This is very static - nothing dynamic about it - and it completely ignores the IDynamicMetaObjectProvider implementation of my class above as it's never used. Using either of these I can access the native properties:DynamicFoo fooExplicit = new DynamicFoo();// static typing assignmentsfooVar.Bar = "Barred!"; fooExplicit.Entered = DateTime.Now; // echo back static values Console.WriteLine(fooVar.Bar); Console.WriteLine(fooExplicit.Entered); but I have no access whatsoever to the properties dictionary. Basically this creates a strongly typed instance of the type with access only to the strongly typed interface. You get no dynamic behavior at all. The IDynamicMetaObjectProvider features don't kick in until you cast the type to dynamic. If I try to access a non-existing property on fooExplicit I get a compilation error that tells me that the property doesn't exist. Again, it's clearly and utterly non-dynamic. Dynamicdynamic fooDynamic = new DynamicFoo(); fooDynamic on the other hand is created as a dynamic type and it's a completely different beast. I can also create a dynamic by simply casting any type to dynamic like this:DynamicFoo fooExplicit = new DynamicFoo(); dynamic fooDynamic = fooExplicit; Note that dynamic typically doesn't require an explicit cast as the compiler automatically performs the cast so there's no need to use as dynamic. Dynamic functionality works at runtime and allows for the dynamic wrapper to look up and call members dynamically. A dynamic type will look for members to access or call in two places: Using the strongly typed members of the object Using theIDynamicMetaObjectProvider Interface methods to access members So rather than statically linking and calling a method or retrieving a property, the dynamic type looks up - at runtime  - where the value actually comes from. It's essentially late-binding which allows runtime determination what action to take when a member is accessed at runtime *if* the member you are accessing does not exist on the object. Class members are checked first before IDynamicMetaObjectProvider interface methods are kick in. All of the following works with the dynamic type:dynamic fooDynamic = new DynamicFoo(); // dynamic typing assignments fooDynamic.NewProperty = "Something new!"; fooDynamic.LastAccess = DateTime.Now; // dynamic assigning static properties fooDynamic.Bar = "dynamic barred"; fooDynamic.Entered = DateTime.Now; // echo back dynamic values Console.WriteLine(fooDynamic.NewProperty); Console.WriteLine(fooDynamic.LastAccess); Console.WriteLine(fooDynamic.Bar); Console.WriteLine(fooDynamic.Entered); The dynamic type can access the native class properties (Bar and Entered) and create and read new ones (NewProperty,LastAccess) all using a single type instance which is pretty cool. As you can see it's pretty easy to create an extensible type this way that can dynamically add members at runtime dynamically. The Alter Ego of IDynamicObject The key point here is that all three statements - explicit, var and dynamic - declare a new DynamicFoo(), but the dynamic declaration results in completely different behavior than the first two simply because the type has been cast to dynamic. Dynamic binding means that the type loses its typical strong typing, compile time features. You can see this easily in the Visual Studio code editor. As soon as you assign a value to a dynamic you lose Intellisense and you see which means there's no Intellisense and no compiler type checking on any members you apply to this instance. If you're new to the dynamic type it might seem really confusing that a single type can behave differently depending on how it is cast, but that's exactly what happens when you use a type that implements IDynamicMetaObjectProvider. Declare the type as its strong type name and you only get to access the native instance members of the type. Declare or cast it to dynamic and you get dynamic behavior which accesses native members plus it uses IDynamicMetaObjectProvider implementation to handle any missing member definitions by running custom code. You can easily cast objects back and forth between dynamic and the original type:dynamic fooDynamic = new DynamicFoo(); fooDynamic.NewProperty = "New Property Value"; DynamicFoo foo = fooDynamic; foo.Bar = "Barred"; Here the code starts out with a dynamic cast and a dynamic assignment. The code then casts back the value to the DynamicFoo. Notice that when casting from dynamic to DynamicFoo and back we typically do not have to specify the cast explicitly - the compiler can induce the type so I don't need to specify as dynamic or as DynamicFoo. Moral of the Story This easy interchange between dynamic and the underlying type is actually super useful, because it allows you to create extensible objects that can expose non-member data stores and expose them as an object interface. You can create an object that hosts a number of strongly typed properties and then cast the object to dynamic and add additional dynamic properties to the same type at runtime. You can easily switch back and forth between the strongly typed instance to access the well-known strongly typed properties and to dynamic for the dynamic properties added at runtime. Keep in mind that dynamic object access has quite a bit of overhead and is definitely slower than strongly typed binding, so if you're accessing the strongly typed parts of your objects you definitely want to use a strongly typed reference. Reserve dynamic for the dynamic members to optimize your code. The real beauty of dynamic is that with very little effort you can build expandable objects or objects that expose different data stores to an object interface. I'll have more on this in my next post when I create a customized and extensible Expando object based on DynamicObject.© Rick Strahl, West Wind Technologies, 2005-2012Posted in CSharp  .NET   Tweet !function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0];if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src="//platform.twitter.com/widgets.js";fjs.parentNode.insertBefore(js,fjs);}}(document,"script","twitter-wjs"); (function() { var po = document.createElement('script'); po.type = 'text/javascript'; po.async = true; po.src = 'https://apis.google.com/js/plusone.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(po, s); })();

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