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System.Delegate.CreateDelegate methods

This article provides supplementary remarks to the reference documentation for this API.

The CreateDelegate methods create a delegate of a specified type.

This method overload is equivalent to calling the CreateDelegate(Type, MethodInfo, Boolean) method overload and specifying true for throwOnBindFailure.

Examples

This section contains two code examples. The first example demonstrates the two kinds of delegates that can be created with this method overload: open over an instance method and open over a static method.

The second code example demonstrates compatible parameter types and return types.

Example 1

The following code example demonstrates the two ways a delegate can be created using this overload of the CreateDelegate method.

Note

There are two overloads of the CreateDelegate method that specify a MethodInfo but not a first argument; their functionality is the same except that one allows you to specify whether to throw on failure to bind, and the other always throws. This code example uses both overloads.

The example declares a class C with a static method M2 and an instance method M1, and two delegate types: D1 takes an instance of C and a string, and D2 takes a string.

A second class named Example contains the code that creates the delegates.

  • A delegate of type D1, representing an open instance method, is created for the instance method M1. An instance must be passed when the delegate is invoked.
  • A delegate of type D2, representing an open static method, is created for the static method M2.
C#
using System;
using System.Reflection;

// Declare three delegate types for demonstrating the combinations
// of static versus instance methods and open versus closed
// delegates.
//
public delegate void D1(C c, string s);
public delegate void D2(string s);
public delegate void D3();

// A sample class with an instance method and a static method.
//
public class C
{
    private int id;
    public C(int id) { this.id = id; }

    public void M1(string s)
    {
        Console.WriteLine("Instance method M1 on C:  id = {0}, s = {1}",
            this.id, s);
    }

    public static void M2(string s)
    {
        Console.WriteLine("Static method M2 on C:  s = {0}", s);
    }
}

public class Example2
{
    public static void Main()
    {
        C c1 = new C(42);

        // Get a MethodInfo for each method.
        //
        MethodInfo mi1 = typeof(C).GetMethod("M1",
            BindingFlags.Public | BindingFlags.Instance);
        MethodInfo mi2 = typeof(C).GetMethod("M2",
            BindingFlags.Public | BindingFlags.Static);

        D1 d1;
        D2 d2;
        D3 d3;

        Console.WriteLine("\nAn instance method closed over C.");
        // In this case, the delegate and the
        // method must have the same list of argument types; use
        // delegate type D2 with instance method M1.
        //
        Delegate test =
            Delegate.CreateDelegate(typeof(D2), c1, mi1, false);

        // Because false was specified for throwOnBindFailure
        // in the call to CreateDelegate, the variable 'test'
        // contains null if the method fails to bind (for
        // example, if mi1 happened to represent a method of
        // some class other than C).
        //
        if (test != null)
        {
            d2 = (D2)test;

            // The same instance of C is used every time the
            // delegate is invoked.
            d2("Hello, World!");
            d2("Hi, Mom!");
        }

        Console.WriteLine("\nAn open instance method.");
        // In this case, the delegate has one more
        // argument than the instance method; this argument comes
        // at the beginning, and represents the hidden instance
        // argument of the instance method. Use delegate type D1
        // with instance method M1.
        //
        d1 = (D1)Delegate.CreateDelegate(typeof(D1), null, mi1);

        // An instance of C must be passed in each time the
        // delegate is invoked.
        //
        d1(c1, "Hello, World!");
        d1(new C(5280), "Hi, Mom!");

        Console.WriteLine("\nAn open static method.");
        // In this case, the delegate and the method must
        // have the same list of argument types; use delegate type
        // D2 with static method M2.
        //
        d2 = (D2)Delegate.CreateDelegate(typeof(D2), null, mi2);

        // No instances of C are involved, because this is a static
        // method.
        //
        d2("Hello, World!");
        d2("Hi, Mom!");

        Console.WriteLine("\nA static method closed over the first argument (String).");
        // The delegate must omit the first argument of the method.
        // A string is passed as the firstArgument parameter, and
        // the delegate is bound to this string. Use delegate type
        // D3 with static method M2.
        //
        d3 = (D3)Delegate.CreateDelegate(typeof(D3),
            "Hello, World!", mi2);

        // Each time the delegate is invoked, the same string is
        // used.
        d3();
    }
}

/* This code example produces the following output:

An instance method closed over C.
Instance method M1 on C:  id = 42, s = Hello, World!
Instance method M1 on C:  id = 42, s = Hi, Mom!

An open instance method.
Instance method M1 on C:  id = 42, s = Hello, World!
Instance method M1 on C:  id = 5280, s = Hi, Mom!

An open static method.
Static method M2 on C:  s = Hello, World!
Static method M2 on C:  s = Hi, Mom!

A static method closed over the first argument (String).
Static method M2 on C:  s = Hello, World!
 */

Example 2

The following code example demonstrates compatibility of parameter types and return types.

The code example defines a base class named Base and a class named Derived that derives from Base. The derived class has a static (Shared in Visual Basic) method named MyMethod with one parameter of type Base and a return type of Derived. The code example also defines a delegate named Example that has one parameter of type Derived and a return type of Base.

The code example demonstrates that the delegate named Example can be used to represent the method MyMethod. The method can be bound to the delegate because:

  • The parameter type of the delegate (Derived) is more restrictive than the parameter type of MyMethod (Base), so that it is always safe to pass the argument of the delegate to MyMethod.
  • The return type of MyMethod (Derived) is more restrictive than the parameter type of the delegate (Base), so that it is always safe to cast the return type of the method to the return type of the delegate.

The code example produces no output.

C#
using System;
using System.Reflection;

// Define two classes to use in the demonstration, a base class and
// a class that derives from it.
//
public class Base { }

public class Derived : Base
{
    // Define a static method to use in the demonstration. The method
    // takes an instance of Base and returns an instance of Derived.
    // For the purposes of the demonstration, it is not necessary for
    // the method to do anything useful.
    //
    public static Derived MyMethod(Base arg)
    {
        Base dummy = arg;
        return new Derived();
    }
}

// Define a delegate that takes an instance of Derived and returns an
// instance of Base.
//
public delegate Base Example5(Derived arg);

class Test
{
    public static void Main()
    {
        // The binding flags needed to retrieve MyMethod.
        BindingFlags flags = BindingFlags.Public | BindingFlags.Static;

        // Get a MethodInfo that represents MyMethod.
        MethodInfo minfo = typeof(Derived).GetMethod("MyMethod", flags);

        // Demonstrate contravariance of parameter types and covariance
        // of return types by using the delegate Example5 to represent
        // MyMethod. The delegate binds to the method because the
        // parameter of the delegate is more restrictive than the
        // parameter of the method (that is, the delegate accepts an
        // instance of Derived, which can always be safely passed to
        // a parameter of type Base), and the return type of MyMethod
        // is more restrictive than the return type of Example5 (that
        // is, the method returns an instance of Derived, which can
        // always be safely cast to type Base).
        //
        Example5 ex =
            (Example5)Delegate.CreateDelegate(typeof(Example5), minfo);

        // Execute MyMethod using the delegate Example5.
        //
        Base b = ex(new Derived());
    }
}

The functionality of these two overloads is the same except that one allows you to specify whether to throw on failure to bind, and the other always throws.

The delegate type and the method must have compatible return types. That is, the return type of method must be assignable to the return type of type.

firstArgument, the second parameter to these overloads, is the first argument of the method the delegate represents. If firstArgument is supplied, it is passed to method every time the delegate is invoked; firstArgument is said to be bound to the delegate, and the delegate is said to be closed over its first argument. If method is static (Shared in Visual Basic), the argument list supplied when invoking the delegate includes all parameters except the first; if method is an instance method, then firstArgument is passed to the hidden instance parameter (represented by this in C#, or by Me in Visual Basic).

If firstArgument is supplied, the first parameter of method must be a reference type, and firstArgument must be compatible with that type.

Important

If method is static (Shared in Visual Basic) and its first parameter is of type Object or ValueType, then firstArgument can be a value type. In this case firstArgument is automatically boxed. Automatic boxing does not occur for any other arguments, as it would in a C# or Visual Basic function call.

If firstArgument is a null reference and method is an instance method, the result depends on the signatures of the delegate type type and of method:

  • If the signature of type explicitly includes the hidden first parameter of method, the delegate is said to represent an open instance method. When the delegate is invoked, the first argument in the argument list is passed to the hidden instance parameter of method.
  • If the signatures of method and type match (that is, all parameter types are compatible), then the delegate is said to be closed over a null reference. Invoking the delegate is like calling an instance method on a null instance, which is not a particularly useful thing to do.

If firstArgument is a null reference and method is static, the result depends on the signatures of the delegate type type and of method:

  • If the signature of method and type match (that is, all parameter types are compatible), the delegate is said to represent an open static method. This is the most common case for static methods. In this case, you can get slightly better performance by using the CreateDelegate(Type, MethodInfo) method overload.
  • If the signature of type begins with the second parameter of method and the rest of the parameter types are compatible, then the delegate is said to be closed over a null reference. When the delegate is invoked, a null reference is passed to the first parameter of method.

Example

The following code example shows all the methods a single delegate type can represent: closed over an instance method, open over an instance method, open over a static method, and closed over a static method.

The code example defines two classes, C and F, and a delegate type D with one argument of type C. The classes have matching static and instance methods M1, M3, and M4, and class C also has an instance method M2 that has no arguments.

A third class named Example contains the code that creates the delegates.

  • Delegates are created for instance method M1 of type C and type F; each is closed over an instance of the respective type. Method M1 of type C displays the ID properties of the bound instance and of the argument.
  • A delegate is created for method M2 of type C. This is an open instance delegate, in which the argument of the delegate represents the hidden first argument on the instance method. The method has no other arguments. It is called as if it were a static method.
  • Delegates are created for static method M3 of type C and type F; these are open static delegates.
  • Finally, delegates are created for static method M4 of type C and type F; each method has the declaring type as its first argument, and an instance of the type is supplied, so the delegates are closed over their first arguments. Method M4 of type C displays the ID properties of the bound instance and of the argument.
C#
using System;
using System.Reflection;

// Declare a delegate type. The object of this code example
// is to show all the methods this delegate can bind to.
//
public delegate void D(C1 c);

// Declare two sample classes, C1 and F. Class C1 has an ID
// property so instances can be identified.
//
public class C1
{
    private int id;
    public int ID { get { return id; } }
    public C1(int id) { this.id = id; }

    public void M1(C1 c)
    {
        Console.WriteLine("Instance method M1(C1 c) on C1:  this.id = {0}, c.ID = {1}",
            this.id, c.ID);
    }

    public void M2()
    {
        Console.WriteLine("Instance method M2() on C1:  this.id = {0}",
            this.id);
    }

    public static void M3(C1 c)
    {
        Console.WriteLine("Static method M3(C1 c) on C1:  c.ID = {0}", c.ID);
    }

    public static void M4(C1 c1, C1 c2)
    {
        Console.WriteLine("Static method M4(C1 c1, C1 c2) on C1:  c1.ID = {0}, c2.ID = {1}",
            c1.ID, c2.ID);
    }
}

public class F
{
    public void M1(C1 c)
    {
        Console.WriteLine("Instance method M1(C1 c) on F:  c.ID = {0}",
            c.ID);
    }

    public static void M3(C1 c)
    {
        Console.WriteLine("Static method M3(C1 c) on F:  c.ID = {0}", c.ID);
    }

    public static void M4(F f, C1 c)
    {
        Console.WriteLine("Static method M4(F f, C1 c) on F:  c.ID = {0}",
            c.ID);
    }
}

public class Example
{
    public static void Main()
    {
        C1 c1 = new C1(42);
        C1 c2 = new C1(1491);
        F f1 = new F();

        D d;

        // Instance method with one argument of type C1.
        MethodInfo cmi1 = typeof(C1).GetMethod("M1");
        // Instance method with no arguments.
        MethodInfo cmi2 = typeof(C1).GetMethod("M2");
        // Static method with one argument of type C1.
        MethodInfo cmi3 = typeof(C1).GetMethod("M3");
        // Static method with two arguments of type C1.
        MethodInfo cmi4 = typeof(C1).GetMethod("M4");

        // Instance method with one argument of type C1.
        MethodInfo fmi1 = typeof(F).GetMethod("M1");
        // Static method with one argument of type C1.
        MethodInfo fmi3 = typeof(F).GetMethod("M3");
        // Static method with an argument of type F and an argument
        // of type C1.
        MethodInfo fmi4 = typeof(F).GetMethod("M4");

        Console.WriteLine("\nAn instance method on any type, with an argument of type C1.");
        // D can represent any instance method that exactly matches its
        // signature. Methods on C1 and F are shown here.
        //
        d = (D)Delegate.CreateDelegate(typeof(D), c1, cmi1);
        d(c2);
        d = (D)Delegate.CreateDelegate(typeof(D), f1, fmi1);
        d(c2);

        Console.WriteLine("\nAn instance method on C1 with no arguments.");
        // D can represent an instance method on C1 that has no arguments;
        // in this case, the argument of D represents the hidden first
        // argument of any instance method. The delegate acts like a
        // static method, and an instance of C1 must be passed each time
        // it is invoked.
        //
        d = (D)Delegate.CreateDelegate(typeof(D), null, cmi2);
        d(c1);

        Console.WriteLine("\nA static method on any type, with an argument of type C1.");
        // D can represent any static method with the same signature.
        // Methods on F and C1 are shown here.
        //
        d = (D)Delegate.CreateDelegate(typeof(D), null, cmi3);
        d(c1);
        d = (D)Delegate.CreateDelegate(typeof(D), null, fmi3);
        d(c1);

        Console.WriteLine("\nA static method on any type, with an argument of");
        Console.WriteLine("    that type and an argument of type C1.");
        // D can represent any static method with one argument of the
        // type the method belongs and a second argument of type C1.
        // In this case, the method is closed over the instance of
        // supplied for the its first argument, and acts like an instance
        // method. Methods on F and C1 are shown here.
        //
        d = (D)Delegate.CreateDelegate(typeof(D), c1, cmi4);
        d(c2);
        Delegate test =
            Delegate.CreateDelegate(typeof(D), f1, fmi4, false);

        // This final example specifies false for throwOnBindFailure
        // in the call to CreateDelegate, so the variable 'test'
        // contains Nothing if the method fails to bind (for
        // example, if fmi4 happened to represent a method of
        // some class other than F).
        //
        if (test != null)
        {
            d = (D)test;
            d(c2);
        }
    }
}

/* This code example produces the following output:

An instance method on any type, with an argument of type C1.
Instance method M1(C1 c) on C1:  this.id = 42, c.ID = 1491
Instance method M1(C1 c) on F:  c.ID = 1491

An instance method on C1 with no arguments.
Instance method M2() on C1:  this.id = 42

A static method on any type, with an argument of type C1.
Static method M3(C1 c) on C1:  c.ID = 42
Static method M3(C1 c) on F:  c.ID = 42

A static method on any type, with an argument of
    that type and an argument of type C1.
Static method M4(C1 c1, C1 c2) on C1:  c1.ID = 42, c2.ID = 1491
Static method M4(F f, C1 c) on F:  c.ID = 1491
*/

Compatible parameter types and return type

The parameter types and return type of a delegate created using this method overload must be compatible with the parameter types and return type of the method the delegate represents; the types do not have to match exactly.

A parameter of a delegate is compatible with the corresponding parameter of a method if the type of the delegate parameter is more restrictive than the type of the method parameter, because this guarantees that an argument passed to the delegate can be passed safely to the method.

Similarly, the return type of a delegate is compatible with the return type of a method if the return type of the method is more restrictive than the return type of the delegate, because this guarantees that the return value of the method can be cast safely to the return type of the delegate.

For example, a delegate with a parameter of type Hashtable and a return type of Object can represent a method with a parameter of type Object and a return value of type Hashtable.

Determine the methods a delegate can represent

Another useful way to think of the flexibility provided by the CreateDelegate(Type, Object, MethodInfo) overload is that any given delegate can represent four different combinations of method signature and method kind (static versus instance). Consider a delegate type D with one argument of type C. The following describes the methods D can represent, ignoring the return type since it must match in all cases:

  • D can represent any instance method that has exactly one argument of type C, regardless of what type the instance method belongs to. When CreateDelegate is called, firstArgument is an instance of the type method belongs to, and the resulting delegate is said to be closed over that instance. (Trivially, D can also be closed over a null reference if firstArgument is a null reference.)

  • D can represent an instance method of C that has no arguments. When CreateDelegate is called, firstArgument is a null reference. The resulting delegate represents an open instance method, and an instance of C must be supplied each time it is invoked.

  • D can represent a static method that takes one argument of type C, and that method can belong to any type. When CreateDelegate is called, firstArgument is a null reference. The resulting delegate represents an open static method, and an instance of C must be supplied each time it is invoked.

  • D can represent a static method that belongs to type F and has two arguments, of type F and type C. When CreateDelegate is called, firstArgument is an instance of F. The resulting delegate represents a static method that is closed over that instance of F. Note that in the case where F and C are the same type, the static method has two arguments of that type. (In this case, D is closed over a null reference if firstArgument is a null reference.)