I love dynamic functionality in a strongly typed language because it offers us
the best of both worlds. In C# (or any of
the main .NET languages) we now have
the dynamic type that provides a host of dynamic features for
the static C# language. One place where I've found dynamic to be incredibly useful is in building extensible types or types that expose traditionally non-object data (like dictionaries) in easier to use and more readable syntax. I wrote about a couple of these for accessing old school ADO.NET DataRows and DataReaders more easily for example. These classes are dynamic wrappers that provide easier syntax and auto-type conversions which greatly simplifies code clutter and increases clarity in existing code. ExpandoObject in .NET 4.0 Another great use case for dynamic objects is
the ability to create extensible objects - objects that start out with a set of static members and then can add additional properties and even methods dynamically.
The .NET 4.0 framework actually includes an ExpandoObject class which provides a very dynamic object that allows you to add properties and methods on
the fly and then access them again. For example with ExpandoObject you can do stuff like this:dynamic expand = new ExpandoObject();
expand.Name = "Rick";
expand.HelloWorld = (Func<string, string>) ((string name) =>
{
return "Hello " + name;
});
Console.WriteLine(expand.Name);
Console.WriteLine(expand.HelloWorld("Dufus"));
Internally ExpandoObject uses a Dictionary like structure and interface to store properties and methods and then allows you to add and access properties and methods easily. As cool as ExpandoObject is it has a few shortcomings too:
It's a sealed type so you can't use it as a base class
It only works off 'properties' in
the internal Dictionary - you can't expose existing type data
It doesn't serialize to XML or with DataContractSerializer/DataContractJsonSerializer
Expando - A truly extensible Object
ExpandoObject is nice if you just need a dynamic container for a dictionary like structure. However, if you want to build an extensible object that starts out with a set of strongly typed properties and then allows you to extend it, ExpandoObject does not work because it's a sealed class that can't be inherited.
I started thinking about this very scenario for one of my applications I'm building for a customer. In this system we are connecting to various different user stores. Each user store has
the same basic requirements for username, password, name etc. But then each store also has a number of extended properties that is available to each application. In
the real world scenario
the data is loaded from
the database in a data reader and
the known properties are assigned from
the known fields in
the database. All unknown fields are then 'added' to
the expando object dynamically.
In
the past I've done this very thing with a separate property - Properties - just like I do for this class. But
the property and dictionary syntax is not ideal and tedious to work with.
I started thinking about how to represent these extra property structures. One way certainly would be to add a Dictionary, or an ExpandoObject to hold all those extra properties. But wouldn't it be nice if
the application could actually extend an existing object that looks something like this as you can with
the Expando object:public class User : Westwind.Utilities.Dynamic.Expando
{
public string Email { get; set; }
public string Password { get; set; }
public string Name { get; set; }
public bool Active { get; set; }
public DateTime? ExpiresOn { get; set; }
}
and then simply start extending
the properties of this object dynamically? Using
the Expando object I describe later you can now do
the following:[TestMethod]
public void UserExampleTest()
{
var user = new User();
// Set strongly typed properties
user.Email = "
[email protected]";
user.Password = "nonya123";
user.Name = "Rickochet";
user.Active = true;
// Now add dynamic properties
dynamic duser = user;
duser.Entered = DateTime.Now;
duser.Accesses = 1;
// you can also add dynamic props via indexer
user["NickName"] = "AntiSocialX";
duser["WebSite"] = "http://www.west-wind.com/weblog";
// Access strong type through dynamic ref
Assert.AreEqual(user.Name,duser.Name);
// Access strong type through indexer
Assert.AreEqual(user.Password,user["Password"]);
// access dyanmically added value through indexer
Assert.AreEqual(duser.Entered,user["Entered"]);
// access index added value through dynamic
Assert.AreEqual(user["NickName"],duser.NickName);
// loop through all properties dynamic AND strong type properties (true)
foreach (var prop in user.GetProperties(true))
{
object val = prop.Value;
if (val == null)
val = "null";
Console.WriteLine(prop.Key + ": " + val.ToString());
}
}
As you can see this code somewhat blurs
the line between a static and dynamic type. You start with a strongly typed object that has a fixed set of properties. You can then cast
the object to dynamic (as I discussed in my last post) and add additional properties to
the object. You can also use an indexer to add dynamic properties to
the object.
To access
the strongly typed properties you can use either
the strongly typed instance,
the indexer or
the dynamic cast of
the object. Personally I think it's kinda cool to have an easy way to access strongly typed properties by string which can make some data scenarios much easier.
To access
the 'dynamically added' properties you can use either
the indexer on
the strongly typed object, or property syntax on
the dynamic cast.
Using
the dynamic type allows all three modes to work on both strongly typed and dynamic properties.
Finally you can iterate over all properties, both dynamic and strongly typed if you chose. Lots of flexibility.
Note also that by default
the Expando object works against
the (this) instance meaning it extends
the current object. You can also pass in a separate instance to
the constructor in which case that object will be used to iterate over to find properties rather than this.
Using this approach provides some really interesting functionality when use
the dynamic type. To use this we have to add an explicit constructor to
the Expando subclass:public class User : Westwind.Utilities.Dynamic.Expando
{
public string Email { get; set; }
public string Password { get; set; }
public string Name { get; set; }
public bool Active { get; set; }
public DateTime? ExpiresOn { get; set; }
public User() : base()
{ }
// only required if you want to mix in seperate instance
public User(object instance)
: base(instance)
{
}
}
to allow
the instance to be passed. When you do you can now do:[TestMethod]
public void ExpandoMixinTest()
{
// have Expando work on Addresses
var user = new User( new Address() );
// cast to dynamicAccessToPropertyTest
dynamic duser = user;
// Set strongly typed properties
duser.Email = "
[email protected]";
user.Password = "nonya123";
// Set properties on address object
duser.Address = "32 Kaiea";
//duser.Phone = "808-123-2131";
// set dynamic properties
duser.NonExistantProperty = "This works too";
// shows default value Address.Phone value
Console.WriteLine(duser.Phone);
}
Using
the dynamic cast in this case allows you to access *three* different 'objects':
The strong type properties,
the dynamically added properties in
the dictionary and
the properties of
the instance passed in! Effectively this gives you a way to simulate multiple inheritance (which is scary - so be very careful with this, but you can do it).
How Expando works
Behind
the scenes Expando is a DynamicObject subclass as I discussed in my last post. By implementing a few of DynamicObject's methods you can basically create a type that can trap 'property missing' and 'method missing' operations. When you access a non-existant property a known method is fired that our code can intercept and provide a value for. Internally Expando uses a custom dictionary implementation to hold
the dynamic properties you might add to your expandable object.
Let's look at code first.
The code for
the Expando type is straight forward and given what it provides relatively short. Here it is.using System;
using System.Collections.Generic;
using System.Linq;
using System.Dynamic;
using System.Reflection;
namespace Westwind.Utilities.Dynamic
{
/// <summary>
/// Class that provides extensible properties and methods. This
/// dynamic object stores 'extra' properties in a dictionary or
/// checks
the actual properties of
the instance.
///
/// This means you can subclass this expando and retrieve either
/// native properties or properties from values in
the dictionary.
///
/// This type allows you three ways to access its properties:
///
/// Directly: any explicitly declared properties are accessible
/// Dynamic: dynamic cast allows access to dictionary and native properties/methods
/// Dictionary: Any of
the extended properties are accessible via IDictionary interface
/// </summary>
[Serializable]
public class Expando : DynamicObject, IDynamicMetaObjectProvider
{
/// <summary>
/// Instance of object passed in
/// </summary>
object Instance;
/// <summary>
/// Cached type of
the instance
/// </summary>
Type InstanceType;
PropertyInfo[] InstancePropertyInfo
{
get
{
if (_InstancePropertyInfo == null && Instance != null)
_InstancePropertyInfo = Instance.GetType().GetProperties(BindingFlags.Instance | BindingFlags.Public | BindingFlags.DeclaredOnly);
return _InstancePropertyInfo;
}
}
PropertyInfo[] _InstancePropertyInfo;
/// <summary>
/// String Dictionary that contains
the extra dynamic values
/// stored on this object/instance
/// </summary>
/// <remarks>Using PropertyBag to support XML Serialization of
the dictionary</remarks>
public PropertyBag Properties = new PropertyBag();
//public Dictionary<string,object> Properties = new Dictionary<string, object>();
/// <summary>
/// This constructor just works off
the internal dictionary and any
/// public properties of this object.
///
/// Note you can subclass Expando.
/// </summary>
public Expando()
{
Initialize(this);
}
/// <summary>
/// Allows passing in an existing instance variable to 'extend'.
/// </summary>
/// <remarks>
/// You can pass in null here if you don't want to
/// check native properties and only check
the Dictionary!
/// </remarks>
/// <param name="instance"></param>
public Expando(object instance)
{
Initialize(instance);
}
protected virtual void Initialize(object instance)
{
Instance = instance;
if (instance != null)
InstanceType = instance.GetType();
}
/// <summary>
/// Try to retrieve a member by name first from instance properties
/// followed by
the collection entries.
/// </summary>
/// <param name="binder"></param>
/// <param name="result"></param>
/// <returns></returns>
public override bool TryGetMember(GetMemberBinder binder, out object result)
{
result = null;
// first check
the Properties collection for member
if (Properties.Keys.Contains(binder.Name))
{
result = Properties[binder.Name];
return true;
}
// Next check for Public properties via Reflection
if (Instance != null)
{
try
{
return GetProperty(Instance, binder.Name, out result);
}
catch { }
}
// failed to retrieve a property
result = null;
return false;
}
/// <summary>
/// Property setter implementation tries to retrieve value from instance
/// first then into this object
/// </summary>
/// <param name="binder"></param>
/// <param name="value"></param>
/// <returns></returns>
public override bool TrySetMember(SetMemberBinder binder, object value)
{
// first check to see if there's a native property to set
if (Instance != null)
{
try
{
bool result = SetProperty(Instance, binder.Name, value);
if (result)
return true;
}
catch { }
}
// no match - set or add to dictionary
Properties[binder.Name] = value;
return true;
}
/// <summary>
/// Dynamic invocation method. Currently allows only for Reflection based
/// operation (no ability to add methods dynamically).
/// </summary>
/// <param name="binder"></param>
/// <param name="args"></param>
/// <param name="result"></param>
/// <returns></returns>
public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result)
{
if (Instance != null)
{
try
{
// check instance passed in for methods to invoke
if (InvokeMethod(Instance, binder.Name, args, out result))
return true;
}
catch { }
}
result = null;
return false;
}
/// <summary>
/// Reflection Helper method to retrieve a property
/// </summary>
/// <param name="instance"></param>
/// <param name="name"></param>
/// <param name="result"></param>
/// <returns></returns>
protected bool GetProperty(object instance, string name, out object result)
{
if (instance == null)
instance = this;
var miArray = InstanceType.GetMember(name, BindingFlags.Public | BindingFlags.GetProperty | BindingFlags.Instance);
if (miArray != null && miArray.Length > 0)
{
var mi = miArray[0];
if (mi.MemberType == MemberTypes.Property)
{
result = ((PropertyInfo)mi).GetValue(instance,null);
return true;
}
}
result = null;
return false;
}
/// <summary>
/// Reflection helper method to set a property value
/// </summary>
/// <param name="instance"></param>
/// <param name="name"></param>
/// <param name="value"></param>
/// <returns></returns>
protected bool SetProperty(object instance, string name, object value)
{
if (instance == null)
instance = this;
var miArray = InstanceType.GetMember(name, BindingFlags.Public | BindingFlags.SetProperty | BindingFlags.Instance);
if (miArray != null && miArray.Length > 0)
{
var mi = miArray[0];
if (mi.MemberType == MemberTypes.Property)
{
((PropertyInfo)mi).SetValue(Instance, value, null);
return true;
}
}
return false;
}
/// <summary>
/// Reflection helper method to invoke a method
/// </summary>
/// <param name="instance"></param>
/// <param name="name"></param>
/// <param name="args"></param>
/// <param name="result"></param>
/// <returns></returns>
protected bool InvokeMethod(object instance, string name, object[] args, out object result)
{
if (instance == null)
instance = this;
// Look at
the instanceType
var miArray = InstanceType.GetMember(name,
BindingFlags.InvokeMethod |
BindingFlags.Public | BindingFlags.Instance);
if (miArray != null && miArray.Length > 0)
{
var mi = miArray[0] as MethodInfo;
result = mi.Invoke(Instance, args);
return true;
}
result = null;
return false;
}
/// <summary>
/// Convenience method that provides a string Indexer
/// to
the Properties collection AND
the strongly typed
/// properties of
the object by name.
///
/// // dynamic
/// exp["Address"] = "112 nowhere lane";
/// // strong
/// var name = exp["StronglyTypedProperty"] as string;
/// </summary>
/// <remarks>
///
The getter checks
the Properties dictionary first
/// then looks in PropertyInfo for properties.
///
The setter checks
the instance properties before
/// checking
the Properties dictionary.
/// </remarks>
/// <param name="key"></param>
///
/// <returns></returns>
public object this[string key]
{
get
{
try
{
// try to get from properties collection first
return Properties[key];
}
catch (KeyNotFoundException ex)
{
// try reflection on instanceType
object result = null;
if (GetProperty(Instance, key, out result))
return result;
// nope doesn't exist
throw;
}
}
set
{
if (Properties.ContainsKey(key))
{
Properties[key] = value;
return;
}
// check instance for existance of type first
var miArray = InstanceType.GetMember(key, BindingFlags.Public | BindingFlags.GetProperty);
if (miArray != null && miArray.Length > 0)
SetProperty(Instance, key, value);
else
Properties[key] = value;
}
}
/// <summary>
/// Returns and
the properties of
/// </summary>
/// <param name="includeProperties"></param>
/// <returns></returns>
public IEnumerable<KeyValuePair<string,object>> GetProperties(bool includeInstanceProperties = false)
{
if (includeInstanceProperties && Instance != null)
{
foreach (var prop in this.InstancePropertyInfo)
yield return new KeyValuePair<string, object>(prop.Name, prop.GetValue(Instance, null));
}
foreach (var key in this.Properties.Keys)
yield return new KeyValuePair<string, object>(key, this.Properties[key]);
}
/// <summary>
/// Checks whether a property exists in
the Property collection
/// or as a property on
the instance
/// </summary>
/// <param name="item"></param>
/// <returns></returns>
public bool Contains(KeyValuePair<string, object> item, bool includeInstanceProperties = false)
{
bool res = Properties.ContainsKey(item.Key);
if (res)
return true;
if (includeInstanceProperties && Instance != null)
{
foreach (var prop in this.InstancePropertyInfo)
{
if (prop.Name == item.Key)
return true;
}
}
return false;
}
}
}
Although
the Expando class supports an indexer, it doesn't actually implement IDictionary or even IEnumerable. It only provides
the indexer and Contains() and GetProperties() methods, that work against
the Properties dictionary AND
the internal instance.
The reason for not implementing IDictionary is that a) it doesn't add much value since you can access
the Properties dictionary directly and that b) I wanted to keep
the interface to class very lean so that it can serve as an entity type if desired. Implementing these IDictionary (or even IEnumerable) causes LINQ extension methods to pop up on
the type which obscures
the property interface and would only confuse
the purpose of
the type. IDictionary and IEnumerable are also problematic for XML and JSON Serialization -
the XML Serializer doesn't serialize IDictionary<string,object>, nor does
the DataContractSerializer.
The JavaScriptSerializer does serialize, but it treats
the entire object like a dictionary and doesn't serialize
the strongly typed properties of
the type, only
the dictionary values which is also not desirable. Hence
the decision to stick with only implementing
the indexer to support
the user["CustomProperty"] functionality and leaving iteration functions to
the publicly exposed Properties dictionary.
Note that
the Dictionary used here is a custom PropertyBag class I created to allow for serialization to work. One important aspect for my apps is that whatever custom properties get added they have to be accessible to AJAX clients since
the particular app I'm working on is a SIngle Page Web app where most of
the Web access is through JSON AJAX calls. PropertyBag can serialize to XML and one way serialize to JSON using
the JavaScript serializer (not
the DCS serializers though).
The key components that make Expando work in this code are
the Properties Dictionary and
the TryGetMember() and TrySetMember() methods.
The Properties collection is public so if you choose you can explicitly access
the collection to get better performance or to manipulate
the members in internal code (like loading up dynamic values form a database).
Notice that TryGetMember() and TrySetMember() both work against
the dictionary AND
the internal instance to retrieve and set properties. This means that user["Name"] works against native properties of
the object as does user["Name"] = "RogaDugDog".
What's your Use Case?
This is still an early prototype but I've plugged it into one of my customer's applications and so far it's working very well.
The key features for me were
the ability to easily extend
the type with values coming from a database and exposing those values in a nice and easy to use manner. I'm also finding that using this type of object for ViewModels works very well to add custom properties to view models. I suspect there will be lots of uses for this - I've been using
the extra dictionary approach to extensibility for years - using a dynamic type to make
the syntax cleaner is just a bonus here.
What can you think of to use this for?
Resources
Source Code and Tests (GitHub)
Also integrated in Westwind.Utilities of
the West Wind Web Toolkit
West Wind Utilities NuGet© Rick Strahl, West Wind Technologies, 2005-2012Posted in CSharp .NET Dynamic Types
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