Category: Expression Trees

Previously I discussed how to work around the variance limitations of the Delegate.CreateDelegate method. CreateDelegate has quite a few overloads, and acts differently based on the parameters which are passed. It also allows you to create open instance or closed static delegates. Without going into too much detail:

• Closed delegate: an instance is bound to the delegate along with the method. In practice this means when calling the delegate, it will always operate on the same instance.
• Open delegate: not bound to a specific target object until invocation time. An instance on which the method should operate is passed as first argument during invocation.

For now, I will only discuss creating open instance delegates. These allow you to invoke an instance method, while passing a first argument, specifying the instance on which it should be invoked.

Divide your programs into methods that perform one identifiable task. Keep all of the operations in a method at the same level of abstraction. – Kent Beck’s Smalltalk Best Practice Patterns

In an attempt to better follow this principle, better known as the Single Level of Abstraction (SLAP) principle, I’ve split up the CreateDelegate method into two helper methods. One to create ‘simple’ delegates, and one to create open instance delegates. Later, an additional third method could be added to create closed static delegates.

By ‘simple’ delegates I mean delegates of which the signature matches that of MethodInfo. More particularly, the method has the same amount of arguments than the delegate.

public static TDelegate CreateDelegate<TDelegate>(
    MethodInfo method,
    object instance = null,
    CreateOptions options = CreateOptions.None )
    where TDelegate : class
{ ... }

By default, this method results in a simple CreateDelegate call. Instead of having to pass the delegate type as an argument a generic approach is used, eliminating the need to cast the returned delegate. When the options argument is set to CreateOptions.Downcasting, this method behaves like the CreateDowncastingDelegate method from my previous post. Downcasts from the delegate argument types and return type to the required types of the method are generated where needed.

The helper method to create open instance delegates looks as follows:

/// <summary>
///   Creates a delegate of a specified type that represents a method
///   which can be executed on an instance passed as parameter.
/// </summary>
/// <typeparam name = "TDelegate">
///   The type for the delegate. This delegate needs at least one (first) type parameter
///   denoting the type of the instance which will be passed.
///   E.g. Action<ExampleObject, object>,
///        where ExampleObject denotes the instance type and object denotes
///        the desired type of the first parameter of the method.
/// </typeparam>
/// <param name = "method">The MethodInfo describing the method of the instance type.</param>
/// <param name = "options">Options which specify what type of delegate should be created.</param>
public static TDelegate CreateOpenInstanceDelegate<TDelegate>(
    MethodInfo method,
    CreateOptions options = CreateOptions.None )
    where TDelegate : class
    { ... }

The comments indicate clearly what TDelegate should look like. Of course, no instance can be passed, clearly specifying it needs to be specified in the delegate type. Furthermore, the passed MethodInfo is required to be an instance method. All these measures are meant to clarify this specific usage of CreateDelegate. Additionally, similar to passing CreateOptions.Downcasting to the other helper method, it allows you to break the variance safety of the ordinary CreateDelegate method.

Both helper functions can be found in DelegateHelper.cs. I also added MethodInfo extension functions to the reflection extensions, resulting in a more concise syntax.

Example usage:

MethodInfo toUpperMethod = typeof( string ).GetMethod( "ToUpper", new Type[] { } );
Func<string, string> toUpper = toUpperMethod.CreateOpenInstanceDelegate<Func<string, string>>();
string upper = toUpper( "test" ); // Will result in "TEST".

Using reflection, .NET allows you to create delegates based on MethodInfo objects. Calling a delegate is a lot more performant than using Invoke as already discussed by Jon Skeet. His results showed a delegate invocation is 600 times as fast. Plenty of reason to go through the extra effort of creating and caching a delegate. However, in a real world scenario when adding behavior to an existing class by using reflection, you’ll quickly encounter several limitations when using CreateDelegate. This post shows you where, and how to easily work around them.

In my previous article I discussed covariance and contravariance. One of the limitations of CreateDelegate is it only allows you to create delegates according to those rules. This makes perfect sense, but isn’t always desirable in practical use cases.

How would you go about creating a delegate for any method which is attributed with CallThis when the exact type of the argument is unknown? You do know the method’s signature matches Action, and only the correct type will ever be passed to the delegate.

class CallThisAttribute : Attribute { }
class SomeClass
{
    [CallThis]
    public void SomeMethod( AnyType type ) { ... }
}

...

SomeClass instance = new SomeClass();
MethodInfo methodToCall = instance.GetType().GetAttributedMethod( typeof( CallThisAttribute ) );

// The following would work, but during reflection we don't know about AnyType.
Action<AnyType> action
    = Delegate.CreateDelegate( typeof( Action<AnyType> ), instance, methodToCall );

// The following throws an ArgumentException, since the method can only be called with AnyType.
Action<object> unkownArgument
    = Delegate.CreateDelegate( typeof( Action<object> ), instance, methodToCall );

Creating this delegate isn’t impossible. This is how you would go about creating it ordinarily.

SomeClass instance = new SomeClass();
Action<object> unknownArgument = o => instance.SomeMethod( (AnyType)o );
unknownArgument( new AnyType() );  // This works ...
unknownArgument( 10 );  // ... but this will throw an InvalidCastException.

Can this downcast be generated at runtime? Yes, and the easiest approach seems to be by using expression trees. Instead of passing the type of the required delegate to create as an argument, I opted to use a generic approach. Its usage looks as follows:

Action<object> unknownArgument
    = DelegateHelper.CreateDowncastingDelegate<Action<object>>( instance, methodToCall );

Any possible delegate type can be passed. All parameters of the method, and return value are matched with those of the delegate, and casts are created where necessary.

public static TDelegate CreateDowncastingDelegate<TDelegate>( object instance, MethodInfo method )
{
    MethodInfo delegateInfo = MethodInfoFromDelegateType( typeof( TDelegate ) );

    // Create delegate original and converted arguments.
    var delegateTypes = delegateInfo.GetParameters().Select( d => d.ParameterType );
    var methodTypes = method.GetParameters().Select( m => m.ParameterType );
    var delegateArgumentExpressions
        = CreateDelegateArgumentExpressions( delegateTypes, methodTypes );

    // Create method call.
    Expression methodCall = Expression.Call(
        instance == null ? null : Expression.Constant( instance ),
        method,
        delegateArgumentExpressions.ConvertedArguments );

    // Convert return type when necessary.
    Expression convertedMethodCall
        = delegateInfo.ReturnType == method.ReturnType
              ? methodCall
              : Expression.Convert( methodCall, delegateInfo.ReturnType );

    return Expression.Lambda<TDelegate>(
        convertedMethodCall,
        delegateArgumentExpressions.OriginalArguments
        ).Compile();
}

I’m still not entirely satisfied with the name of the method. CreateDowncastingDelegate refers to the fact that downcasts are generated where necessary. Its usage looks more like an upcasted delegate however. If anyone can come up with a better name, be sure to let me know.

The entire source code can be found in the FCL Extension library. In there you can also find another method which will most likely be the subject of my next post.