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  • Call Generic method using runtime type and cast return object

    - by markpirvine
    I'm using reflection to call a generic method with a type determined at runtime. My code is as follows: Type tType = Type.GetType(pLoadOut.Type); MethodInfo method = typeof(ApiSerialiseHelper).GetMethod("Deserialise", new Type[] { typeof(string) }); MethodInfo generic = method.MakeGenericMethod(tType); generic.Invoke(obj, new object[] { pLoadOut.Data }); This works ok. However the generic.Invoke method returns an object, but what I would like is the type determined at runtime. Is this possible with this approach, or is there a better option? Mark

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  • implict type cast in generic method

    - by bitbonk
    why do I get a compiler error in the following code stating: Cannot implicty convert type SpecialNode to T even though T must derive from NodeBase as I defined in the where clause and even though SpecialNode actually derived from NodeBase? public static T GetNode<T>() where T : NodeBase { if (typeof(T) == typeof(SpecialNode)) { return ThisStaticClass.MySpecialNode; // <-- compiler error } if (typeof(T) == typeof(OtherSpecialNode)) { return ThisStaticClass.MyOtherSpecialNode; // <-- compiler error } ... return default(T); }

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  • Correctly assigning value to a Core Data attribute with an integer data-type

    - by Gordon Fontenot
    I'm missing something here, and feeling like an idiot about it. I'm using a UIPickerView in my app, and I need to assign the row number to a 32-bit integer attribute for a Core Data object. To do this, I am using this method: -(void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component { object.integerValue = row; } This is giving me a warning: warning: passing argument 1 of 'setIntegerValue:' makes pointer from integer without a cast What am I mixing up here? --Edit 1-- Ok, so I can get rid of the warning by changing the method to do the following: NSNumber *number = [NSNumber numberWithInteger:row]; object.integerValue = rating; However, I still get a value of 0 for object.integerValue if I use NSLog to print it out. object.integerValue has a max value of 5, so I print out number instead, and then I'm getting a number above 62,000,000. Which doesn't seem right to me, since there are 5 rows. If I NSLog the row variable, I get a number between 0 and 5. So why do I end up with a completely different number after casting the number to NSNumber? --Edit 2-- Ok, so I'm realizing that there is some fundamental idea that I don't understand. I now understand that the 60 million + number can be cast back to the correct 0-5 number by using integerValue. So, it seems my question is how can I save an integer between 0-5 to the attribute if the NSNumber that is returned is over 60 million? Do I need to be using a different data type?

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  • Python mistaking float for string

    - by wrongusername
    I receive TypeError: Can't convert 'float' object to str implicitly while using Gambler.pot += round(self.bet + self.money * 0.1) where pot, bet, and money are all doubles (or at least are supposed to be). I'm not sure if this is yet another Eclipse thing, but how do I get the line to compile? Code where bet and money are initialized: money = 0 bet = 0

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  • How `is_base_of` works?

    - by Alexey Malistov
    Why the following code works? typedef char (&yes)[1]; typedef char (&no)[2]; template <typename B, typename D> struct Host { operator B*() const; operator D*(); }; template <typename B, typename D> struct is_base_of { template <typename T> static yes check(D*, T); static no check(B*, int); static const bool value = sizeof(check(Host<B,D>(), int())) == sizeof(yes); }; //Test sample class Base {}; class Derived : private Base {}; //Exspression is true. int test[is_base_of<Base,Derived>::value && !is_base_of<Derived,Base>::value]; Note that B is private base. Note that operator B*() is const. How does this work? Why this works? Why static yes check(D*, T); is better than static yes check(B*, int); ?

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  • STORED PROCEDURE working in my local test machine cannot be created in production environment.

    - by Marcos Buarque
    Hi, I have an SQL CREATE PROCEDURE statement that runs perfectly in my local SQL Server, but cannot be recreated in production environment. The error message I get in production is Msg 102, Level 15, State 1, Incorrect syntax near '='. It is a pretty big query and I don't want to annoy StackOverflow users, but I simply can't find a solution. If only you could point me out what settings I could check in the production server in order to enable running the code... I must be using some kind of syntax or something that is conflicting with some setting in production. This PROCEDURE was already registered in production before, but when I ran a DROP - CREATE PROCEDURE today, the server was able to drop the procedure, but not to recreate it. I will paste the code below. Thank you! =============== USE [Enorway] GO /****** Object: StoredProcedure [dbo].[Spel_CM_ChartsUsersTotals] Script Date: 03/17/2010 11:59:57 ******/ SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO CREATE PROC [dbo].[Spel_CM_ChartsUsersTotals] @IdGroup int, @IdAssessment int, @UserId int AS SET NOCOUNT ON DECLARE @RequiredColor varchar(6) SET @RequiredColor = '3333cc' DECLARE @ManagersColor varchar(6) SET @ManagersColor = '993300' DECLARE @GroupColor varchar(6) SET @GroupColor = 'ff0000' DECLARE @SelfColor varchar(6) SET @SelfColor = '336600' DECLARE @TeamColor varchar(6) SET @TeamColor = '993399' DECLARE @intMyCounter tinyint DECLARE @intManagersPosition tinyint DECLARE @intGroupPosition tinyint DECLARE @intSelfPosition tinyint DECLARE @intTeamPosition tinyint SET @intMyCounter = 1 -- Table that will hold the subtotals... DECLARE @tblTotalsSource table ( IdCompetency int, CompetencyName nvarchar(200), FunctionRequiredLevel float, ManagersAverageAssessment float, SelfAssessment float, GroupAverageAssessment float, TeamAverageAssessment float ) INSERT INTO @tblTotalsSource ( IdCompetency, CompetencyName, FunctionRequiredLevel, ManagersAverageAssessment, SelfAssessment, GroupAverageAssessment, TeamAverageAssessment ) SELECT e.[IdCompetency], dbo.replaceAccentChar(e.[Name]) AS CompetencyName, (i.[LevelNumber]) AS FunctionRequiredLevel, ( SELECT ROUND(avg(CAST(ac.[LevelNumber] AS float)),0) FROM Spel_CM_AssessmentsData aa INNER JOIN Spel_CM_CompetenciesLevels ab ON aa.[IdCompetencyLevel] = ab.[IdCompetencyLevel] INNER JOIN Spel_CM_Levels ac ON ab.[IdLevel] = ac.[IdLevel] INNER JOIN Spel_CM_AssessmentsEvents ad ON aa.[IdAssessmentEvent] = ad.[IdAssessmentEvent] WHERE aa.[EvaluatedUserId] = @UserId AND aa.[AssessmentType] = 't' AND aa.[IdGroup] = @IdGroup AND ab.[IdCompetency] = e.[IdCompetency] AND ad.[IdAssessment] = @IdAssessment ) AS ManagersAverageAssessment, ( SELECT bc.[LevelNumber] FROM Spel_CM_AssessmentsData ba INNER JOIN Spel_CM_CompetenciesLevels bb ON ba.[IdCompetencyLevel] = bb.[IdCompetencyLevel] INNER JOIN Spel_CM_Levels bc ON bb.[IdLevel] = bc.[IdLevel] INNER JOIN Spel_CM_AssessmentsEvents bd ON ba.[IdAssessmentEvent] = bd.[IdAssessmentEvent] WHERE ba.[EvaluatedUserId] = @UserId AND ba.[AssessmentType] = 's' AND ba.[IdGroup] = @IdGroup AND bb.[IdCompetency] = e.[IdCompetency] AND bd.[IdAssessment] = @IdAssessment ) AS SelfAssessment, ( SELECT ROUND(avg(CAST(cc.[LevelNumber] AS float)),0) FROM Spel_CM_AssessmentsData ca INNER JOIN Spel_CM_CompetenciesLevels cb ON ca.[IdCompetencyLevel] = cb.[IdCompetencyLevel] INNER JOIN Spel_CM_Levels cc ON cb.[IdLevel] = cc.[IdLevel] INNER JOIN Spel_CM_AssessmentsEvents cd ON ca.[IdAssessmentEvent] = cd.[IdAssessmentEvent] WHERE ca.[EvaluatedUserId] = @UserId AND ca.[AssessmentType] = 'g' AND ca.[IdGroup] = @IdGroup AND cb.[IdCompetency] = e.[IdCompetency] AND cd.[IdAssessment] = @IdAssessment ) AS GroupAverageAssessment, ( SELECT ROUND(avg(CAST(dc.[LevelNumber] AS float)),0) FROM Spel_CM_AssessmentsData da INNER JOIN Spel_CM_CompetenciesLevels db ON da.[IdCompetencyLevel] = db.[IdCompetencyLevel] INNER JOIN Spel_CM_Levels dc ON db.[IdLevel] = dc.[IdLevel] INNER JOIN Spel_CM_AssessmentsEvents dd ON da.[IdAssessmentEvent] = dd.[IdAssessmentEvent] WHERE da.[EvaluatedUserId] = @UserId AND da.[AssessmentType] = 'm' AND da.[IdGroup] = @IdGroup AND db.[IdCompetency] = e.[IdCompetency] AND dd.[IdAssessment] = @IdAssessment ) AS TeamAverageAssessment FROM Spel_CM_AssessmentsData a INNER JOIN Spel_CM_AssessmentsEvents c ON a.[IdAssessmentEvent] = c.[IdAssessmentEvent] INNER JOIN Spel_CM_CompetenciesLevels d ON a.[IdCompetencyLevel] = d.[IdCompetencyLevel] INNER JOIN Spel_CM_Competencies e ON d.[IdCompetency] = e.[IdCompetency] INNER JOIN Spel_CM_Levels f ON d.[IdLevel] = f.[IdLevel] -- This will link with user's assigned functions INNER JOIN Spel_CM_FunctionsCompetenciesLevels g ON a.[IdFunction] = g.[IdFunction] INNER JOIN Spel_CM_CompetenciesLevels h ON g.[IdCompetencyLevel] = h.[IdCompetencyLevel] AND e.[IdCompetency] = h.[IdCompetency] INNER JOIN Spel_CM_Levels i ON h.[IdLevel] = i.[IdLevel] WHERE (NOT c.[EndDate] IS NULL) AND a.[EvaluatedUserId] = @UserId AND c.[IdAssessment] = @IdAssessment AND a.[IdGroup] = @IdGroup GROUP BY e.[IdCompetency], e.[Name], i.[LevelNumber] ORDER BY e.[Name] ASC -- This will define the position of each element (managers, group, self and team) SELECT @intManagersPosition = @intMyCounter FROM @tblTotalsSource WHERE NOT ManagersAverageAssessment IS NULL IF IsNumeric(@intManagersPosition) = 1 BEGIN SELECT @intMyCounter += 1 END SELECT @intGroupPosition = @intMyCounter FROM @tblTotalsSource WHERE NOT GroupAverageAssessment IS NULL IF IsNumeric(@intGroupPosition) = 1 BEGIN SELECT @intMyCounter += 1 END SELECT @intSelfPosition = @intMyCounter FROM @tblTotalsSource WHERE NOT SelfAssessment IS NULL IF IsNumeric(@intSelfPosition) = 1 BEGIN SELECT @intMyCounter += 1 END SELECT @intTeamPosition = @intMyCounter FROM @tblTotalsSource WHERE NOT TeamAverageAssessment IS NULL -- This will render the final table for the end user. The tabe will flatten some of the numbers to allow them to be prepared for Google Graphics. SELECT SUBSTRING( ( SELECT ( '|' + REPLACE(ma.[CompetencyName],' ','+')) FROM @tblTotalsSource ma ORDER BY ma.[CompetencyName] DESC FOR XML PATH('') ), 2, 1000) AS 'CompetenciesNames', SUBSTRING( ( SELECT ( ',' + REPLACE(ra.[FunctionRequiredLevel]*10,' ','+')) FROM @tblTotalsSource ra FOR XML PATH('') ), 2, 1000) AS 'FunctionRequiredLevel', SUBSTRING( ( SELECT ( ',' + CAST(na.[ManagersAverageAssessment]*10 AS nvarchar(10))) FROM @tblTotalsSource na FOR XML PATH('') ), 2, 1000) AS 'ManagersAverageAssessment', SUBSTRING( ( SELECT ( ',' + CAST(oa.[GroupAverageAssessment]*10 AS nvarchar(10))) FROM @tblTotalsSource oa FOR XML PATH('') ), 2, 1000) AS 'GroupAverageAssessment', SUBSTRING( ( SELECT ( ',' + CAST(pa.[SelfAssessment]*10 AS nvarchar(10))) FROM @tblTotalsSource pa FOR XML PATH('') ), 2, 1000) AS 'SelfAssessment', SUBSTRING( ( SELECT ( ',' + CAST(qa.[TeamAverageAssessment]*10 AS nvarchar(10))) FROM @tblTotalsSource qa FOR XML PATH('') ), 2, 1000) AS 'TeamAverageAssessment', SUBSTRING( ( SELECT ( '|t++' + CAST([FunctionRequiredLevel] AS varchar(10)) + ',' + @RequiredColor + ',0,' + CAST(ROW_NUMBER() OVER(ORDER BY CompetencyName) - 1 AS varchar(2)) + ',9') FROM @tblTotalsSource FOR XML PATH('') ), 2, 1000) AS 'FunctionRequiredAverageLabel', SUBSTRING( ( SELECT ( '|t++' + CAST([ManagersAverageAssessment] AS varchar(10)) + ',' + @ManagersColor + ',' + CAST(@intManagersPosition AS varchar(2)) + ',' + CAST(ROW_NUMBER() OVER(ORDER BY CompetencyName) - 1 AS varchar(2)) + ',9') FROM @tblTotalsSource FOR XML PATH('') ), 2, 1000) AS 'ManagersLabel', SUBSTRING( ( SELECT ( '|t++' + CAST([GroupAverageAssessment] AS varchar(10)) + ',' + @GroupColor + ',' + CAST(@intGroupPosition AS varchar(2)) + ',' + CAST(ROW_NUMBER() OVER(ORDER BY CompetencyName) - 1 AS varchar(2)) + ',9') FROM @tblTotalsSource FOR XML PATH('') ), 2, 1000) AS 'GroupLabel', SUBSTRING( ( SELECT ( '|t++' + CAST([SelfAssessment] AS varchar(10)) + ',' + @SelfColor + ',' + CAST(@intSelfPosition AS varchar(2)) + ',' + CAST(ROW_NUMBER() OVER(ORDER BY CompetencyName) - 1 AS varchar(2)) + ',9') FROM @tblTotalsSource FOR XML PATH('') ), 2, 1000) AS 'SelfLabel', SUBSTRING( ( SELECT ( '|t++' + CAST([TeamAverageAssessment] AS varchar(10)) + ',' + @TeamColor + ',' + CAST(@intTeamPosition AS varchar(2)) + ',' + CAST(ROW_NUMBER() OVER(ORDER BY CompetencyName) - 1 AS varchar(2)) + ',10') FROM @tblTotalsSource FOR XML PATH('') ), 2, 1000) AS 'TeamLabel', (Count(src.[IdCompetency]) * 30) + 100 AS 'ControlHeight' FROM @tblTotalsSource src SET NOCOUNT OFF GO

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  • var in C# - Why can't it be used as a member variable?

    - by David Neale
    Why is it not possible to have implicitly-typed variables at a class level within C# for when these variables are immediately assigned? ie: public class TheClass { private var aList = new List<string>(); } Is it just something that hasn't been implemented or is there a conceptual/technical reason for why it hasn't been done?

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  • Why aren't operator conversions implicitly called for templated functions? (C++)

    - by John Gordon
    I have the following code: template <class T> struct pointer { operator pointer<const T>() const; }; void f(pointer<const float>); template <typename U> void tf(pointer<const float>); void g() { pointer<float> ptr; f(ptr); tf(ptr); } When I compile the code with gcc 4.3.3 I get a message (aaa.cc:17: error: no matching function for call to ‘tf(pointer<float>&)’) indicating that the compiler called 'operator pointer<const T>' for the non-templated function f(), but didn't for the templated function tf(). Why and is there any workaround short of overloading tf() with a const and non-const version? Thanks in advance for any help.

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  • Why `is_base_of` works with private inheritance?

    - by Alexey Malistov
    Why the following code works? typedef char (&yes)[1]; typedef char (&no)[2]; template <typename B, typename D> struct Host { operator B*() const; operator D*(); }; template <typename B, typename D> struct is_base_of { template <typename T> static yes check(D*, T); static no check(B*, int); static const bool value = sizeof(check(Host<B,D>(), int())) == sizeof(yes); }; //Test sample class B {}; class D : private B {}; //Exspression is true. int test[is_base_of<B,D>::value && !is_base_of<D,B>::value]; Note that B is private base. Note that operator B*() is const. How does this work? Why this works? Why static yes check(D*, T); is better than static yes check(B*, int); ?

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  • Is it guaranteed that new Integer(i) == i in Java?

    - by polygenelubricants
    Consider the following snippet: int i = 99999999; byte b = 99; short s = 9999; Integer ii = Integer.valueOf(9); // should be within cache System.out.println(new Integer(i) == i); // "true" System.out.println(new Integer(b) == b); // "true" System.out.println(new Integer(s) == s); // "true" System.out.println(new Integer(ii) == ii); // "false" It's obvious why the last line will ALWAYS prints "false": we're using == reference identity comparison, and a new object will NEVER be == to an already existing object. The question is about the first 3 lines: are those comparisons guaranteed to be on the primitive int, with the Integer auto-unboxed? Are there cases where the primitive would be auto-boxed instead, and reference identity comparisons are performed? (which would all then be false!)

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  • scala 2.8 implict java collections conversions

    - by nablik
    I have problem with JavaConversions with 2.8 beta: import scala.collection.JavaConversions._ class Utils(dbFile : File, sep: String) extends IUtils { (...) def getFeatures() : java.util.List[String] = csv.attributes.toList } And then exception: [INFO] Utils.scala:20: error: type mismatch; [INFO] found : List[String] [INFO] required: java.util.List[String] [INFO] def getFeatures() : java.util.List[String] = csv.attributes.toList [INFO]

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  • How to cast a character to int in Clojure?

    - by Learning Clojure
    How to cast a character to int in Clojure? I am trying to write a rot 13 in clojure, so I need to have something to cast my char to int. I found something called (int), so I put: (int a) Get: CompilerException java.lang.RuntimeException: Unable to resolve symbol: a in this context, compiling:(NO_SOURCE_PATH:13:1) Then I put: (int 'a) Get: ClassCastException clojure.lang.Symbol cannot be cast to java.lang.Character clojure.lang.RT.intCast (RT.java:1087) Then: (rot13 ''a') Get: ClassCastException clojure.lang.PersistentList cannot be cast to java.lang.Character clojure.lang.RT.intCast (RT.java:1087) And: (rot13 "a") Get: ClassCastException java.lang.String cannot be cast to java.lang.Character clojure.lang.RT.intCast (RT.java:1087) So what is the right way to do it? btw, I always get confused with all these clojure syntax. But I can never find any source only help me with my problem. Any suggestions? Thank you!!

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  • Why does Option not extend the Iterable trait directly?

    - by oxbow_lakes
    Option is implicitly convertible to an Iterable - but why does it not just just implement Iterable directly: def iterator = new Iterator[A] { var end = !isDefined def next() = { val n = if (end) throw new NoSuchElementException() else get end = true n } def hasNext = !end } EDIT: In fact it's even weider than that because in 2.8 Option does declare an iterator method: def iterator: Iterator[A] = if (isEmpty) Iterator.empty else Iterator.single(this.get)

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  • In SQLCMD mode, should CONNECT be an implicit batch separator?

    - by Greg Low
    Hi Folks, I've been working with SQLCMD mode again today and one thing about it always bites me. If I execute a script like: ::CONNECT SERVER1 SELECT @@VERSION; ::CONNECT SERVER2 SELECT @@VERSION; ::CONNECT SERVER3 SELECT @@VERSION; I'm sure I'm not the only person that would be surprised to see all three SELECT commands executed against SERVER3 and none executed against SERVER1 or SERVER2. If you think that's odd behavior, here's where to vote: https://connect.microsoft.com/SQLServer/feedback/details/611144/sqlcmd-connect-to-a-different-server-should-be-an-implicit-batch-separator#detail...(read more)

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  • Java HashSet<Integer> to int array

    - by jackweeden
    I've got a HashSet with a bunch of (you guessed it) integers in it. I want to turn it into an array, but calling hashset.toArray(); returns an array of Object type. This is fine, but is there a better way to cast it to an array of int, other than iterating through every element manually? A method I want to pass it to void doSomething(int[] arr) Won't accept the Object[] array, even if I try casting it like doSomething((int[]) hashSet.toArray());

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  • How are integers casted to bytes in Java?

    - by helixed
    I know Java doesn't allow unsigned types, so I was wondering how it casts an integer to a byte. Say I have an integer a with a value of 255 and I cast the integer to a byte. Is the value represented in the byte 11111111? In other words, is the value treated more as a signed 8 bit integer, or does it just directly copy the last 8 bits of the integer? Thanks, helixed

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  • Fast double -> short conversion with clamping using SSE?

    - by gct
    Is there a fast way to cast double values to shorts (16 bits signed), currently I'm doing something like this: double dval = <sum junk> int16_t sval; if (val > int16_max) { sval = int16_max; } else if (val < int16_min) { sval = int16_min; } else sval = (int16_t)val; I suspect there's a fast way to do this using SSE that will be significantly more efficient.

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  • How do you cast a LinqToSql Table<TEntity> as a Table<IEntity> where TEntity : IEntity?

    - by DanM
    I'm trying to use DbLinq with a SQLite database, but I'm running into a problem when I try to cast an ITable as a Queryable<TEntity>. There is a known bug in DbLinq (Issue 211), which might be the source of my problem, but I wanted to make sure my code is sound and, if it is, find out if there might be something I can do to work around the bug. Here is the generic repository method that attempts to do the cast: public IQueryable<TEntity> GetAll() { return Table.Cast<TEntity>(); // Table is an ITable } This compiles, but if I pass in the interface IPerson for TEntity and the type of the entities in the table is Person (where Person : IPerson), I'm getting this error from DbLinq: S0133: Implement QueryMethod Queryable.Cast. Why am I trying to do this? I have a library project that doesn't know the type of the entity until runtime, but it does know the interface for the entity. So, I'm trying to cast to the interface type so that my library project can consume the data. Questions: Am I attempting an impossible cast or is this definitely a bug in DbLinq? How else could I go about solving my problem?

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  • C#/.NET Little Pitfalls: The Dangers of Casting Boxed Values

    - by James Michael Hare
    Starting a new series to parallel the Little Wonders series.  In this series, I will examine some of the small pitfalls that can occasionally trip up developers. Introduction: Of Casts and Conversions What happens when we try to assign from an int and a double and vice-versa? 1: double pi = 3.14; 2: int theAnswer = 42; 3:  4: // implicit widening conversion, compiles! 5: double doubleAnswer = theAnswer; 6:  7: // implicit narrowing conversion, compiler error! 8: int intPi = pi; As you can see from the comments above, a conversion from a value type where there is no potential data loss is can be done with an implicit conversion.  However, when converting from one value type to another may result in a loss of data, you must make the conversion explicit so the compiler knows you accept this risk.  That is why the conversion from double to int will not compile with an implicit conversion, we can make the conversion explicit by adding a cast: 1: // explicit narrowing conversion using a cast, compiler 2: // succeeds, but results may have data loss: 3: int intPi = (int)pi; So for value types, the conversions (implicit and explicit) both convert the original value to a new value of the given type.  With widening and narrowing references, however, this is not the case.  Converting reference types is a bit different from converting value types.  First of all when you perform a widening or narrowing you don’t really convert the instance of the object, you just convert the reference itself to the wider or narrower reference type, but both the original and new reference type both refer back to the same object. Secondly, widening and narrowing for reference types refers the going down and up the class hierarchy instead of referring to precision as in value types.  That is, a narrowing conversion for a reference type means you are going down the class hierarchy (for example from Shape to Square) whereas a widening conversion means you are going up the class hierarchy (from Square to Shape).  1: var square = new Square(); 2:  3: // implicitly convers because all squares are shapes 4: // (that is, all subclasses can be referenced by a superclass reference) 5: Shape myShape = square; 6:  7: // implicit conversion not possible, not all shapes are squares! 8: // (that is, not all superclasses can be referenced by a subclass reference) 9: Square mySquare = (Square) myShape; So we had to cast the Shape back to Square because at that point the compiler has no way of knowing until runtime whether the Shape in question is truly a Square.  But, because the compiler knows that it’s possible for a Shape to be a Square, it will compile.  However, if the object referenced by myShape is not truly a Square at runtime, you will get an invalid cast exception. Of course, there are other forms of conversions as well such as user-specified conversions and helper class conversions which are beyond the scope of this post.  The main thing we want to focus on is this seemingly innocuous casting method of widening and narrowing conversions that we come to depend on every day and, in some cases, can bite us if we don’t fully understand what is going on!  The Pitfall: Conversions on Boxed Value Types Can Fail What if you saw the following code and – knowing nothing else – you were asked if it was legal or not, what would you think: 1: // assuming x is defined above this and this 2: // assignment is syntactically legal. 3: x = 3.14; 4:  5: // convert 3.14 to int. 6: int truncated = (int)x; You may think that since x is obviously a double (can’t be a float) because 3.14 is a double literal, but this is inaccurate.  Our x could also be dynamic and this would work as well, or there could be user-defined conversions in play.  But there is another, even simpler option that can often bite us: what if x is object? 1: object x; 2:  3: x = 3.14; 4:  5: int truncated = (int) x; On the surface, this seems fine.  We have a double and we place it into an object which can be done implicitly through boxing (no cast) because all types inherit from object.  Then we cast it to int.  This theoretically should be possible because we know we can explicitly convert a double to an int through a conversion process which involves truncation. But here’s the pitfall: when casting an object to another type, we are casting a reference type, not a value type!  This means that it will attempt to see at runtime if the value boxed and referred to by x is of type int or derived from type int.  Since it obviously isn’t (it’s a double after all) we get an invalid cast exception! Now, you may say this looks awfully contrived, but in truth we can run into this a lot if we’re not careful.  Consider using an IDataReader to read from a database, and then attempting to select a result row of a particular column type: 1: using (var connection = new SqlConnection("some connection string")) 2: using (var command = new SqlCommand("select * from employee", connection)) 3: using (var reader = command.ExecuteReader()) 4: { 5: while (reader.Read()) 6: { 7: // if the salary is not an int32 in the SQL database, this is an error! 8: // doesn't matter if short, long, double, float, reader [] returns object! 9: total += (int) reader["annual_salary"]; 10: } 11: } Notice that since the reader indexer returns object, if we attempt to convert using a cast to a type, we have to make darn sure we use the true, actual type or this will fail!  If the SQL database column is a double, float, short, etc this will fail at runtime with an invalid cast exception because it attempts to convert the object reference! So, how do you get around this?  There are two ways, you could first cast the object to its actual type (double), and then do a narrowing cast to on the value to int.  Or you could use a helper class like Convert which analyzes the actual run-time type and will perform a conversion as long as the type implements IConvertible. 1: object x; 2:  3: x = 3.14; 4:  5: // if you want to cast, must cast out of object to double, then 6: // cast convert. 7: int truncated = (int)(double) x; 8:  9: // or you can call a helper class like Convert which examines runtime 10: // type of the value being converted 11: int anotherTruncated = Convert.ToInt32(x); Summary You should always be careful when performing a conversion cast from values boxed in object that you are actually casting to the true type (or a sub-type). Since casting from object is a widening of the reference, be careful that you either know the exact, explicit type you expect to be held in the object, or instead avoid the cast and use a helper class to perform a safe conversion to the type you desire. Technorati Tags: C#,.NET,Pitfalls,Little Pitfalls,BlackRabbitCoder

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