Using IReflect to expose a type as IDispatch to COM

The CLR supports marshalling of objects that support the IReflect interface as IDispatch COM objects. Similarly, IExpando gets marshalled as IDispatchEx. Here is a sample of a managed type called ManagedIDispatch which is used from VBScript and used in a late-bound way. VBScript just deals with IDispatch, and under the hoods, the CLR routes the calls to the IReflect methods implemented by ManagedIDispatch, and also takes care of providing a stable mapping between method names and dispids.

IReflect.GetMethods gets called every time COM instantiates the managed type. The CLR caches the method names for the given object, and the names are then available to COM from the IDispatch interface. If COM accesses a method name that is not in the cached list, IReflect.GetMethods get called again.

The C# code should be compiled and registered as:

csc /t:library ManagedIDispatch.cs

regasm /codebase ManagedIDispatch.dll

Here is the significant contents of ManagedIDispatch.cs

public class ManagedIDispatch : IReflect {




    public MethodInfo[] GetMethods(BindingFlags bindingAttr) {

        Console.WriteLine("In ManagedIDispatch.GetMethods (_getMethodsDone={0})", _getMethodsDone);

        // dynFoo is a method that is not always present

        if (_getMethodsDone) {

            MethodInfo[] methods = new MethodInfo[_methodInfos.Length + 1];

            _methodInfos.CopyTo(methods, 0);

            methods[_methodInfos.Length] = new MyMethodInfo("dynFoo");

            return methods;

        } else {

            _getMethodsDone = true;

            return _methodInfos;




    public object InvokeMember(string name, BindingFlags invokeAttr, Binder binder, object target, object[] args, ParameterModifier[] modifiers, CultureInfo culture, string[] namedParameters) {

        Console.WriteLine("In ManagedIDispatch.InvokeMember({0} : {1})", name, GetArgs(target, args));

        switch (name) {

            case "[DISPID=0]": return this;

            case "raiseException": throw new Exception("raiseException has been invoked");

            case "foo": return "foo-return";

            case "dynFoo": Debug.Assert(_getMethodsDone);  return "dynFoo-return";


                if (name == ("bar" + _index)) return "bar-return";

                throw Utils.NotImplemented("This is unreachable");




Here is the VBScript used to script the type:

Sub AssertError(errNum, errDescr)
    If Err.Number <> errNum Then
        MsgBox "Error number is incorrect:" & Err.Number & ", expected " & errNum
    End If

    If Err.Description <> errDescr Then
        MsgBox "Error description is incorrect:" & Err.Description & ", expected " & errDescr
    End If

End Sub

c1 = CreateObject("TestFactory")
m = c1.GetManagedObject()

On Error Resume Next

AssertError 438, "Object doesn't support this property or method"

AssertError -2146233088, "" ' 0x80131500 : CLR exception

The output of the script is:

c:\bugs>cscript vbs1.vbs
Microsoft (R) Windows Script Host Version 5.7
Copyright (C) Microsoft Corporation. All rights reserved.

In ManagedIDispatch.GetMethods (_getMethodsDone=False)
In MyMethodInfo.Name.get(foo)
In MyMethodInfo.Name.get(bar0)
In MyMethodInfo.Name.get(raiseException)
In ManagedIDispatch.InvokeMember([DISPID=0] : ManagedIDispatch)
In ManagedIDispatch.InvokeMember(foo : ManagedIDispatch)
In ManagedIDispatch.GetMethods (_getMethodsDone=True)
In MyMethodInfo.Name.get(dynFoo)
In ManagedIDispatch.InvokeMember(dynFoo : ManagedIDispatch)
In ManagedIDispatch.InvokeMember(bar0 : ManagedIDispatch)
In ManagedIDispatch.GetMethods (_getMethodsDone=True)
In MyMethodInfo.Name.get(dynFoo)
In ManagedIDispatch.InvokeMember(raiseException : ManagedIDispatch)


Comments (4)
  1. eaglus says:

    Nice post, but….

    I still can’t find a way to have parameters passed by reference…

    I’ve implemented MyMethodInfo.GetParameters method, and, there i create array of ParameterInfo descendants, override their methods, but, no  methods of my ParameterInfo descendants gets called!

    This is class – descendant of ParameterInfo (excuse me for Delpi.Net)

     TMyParamInfo = class(ParameterInfo)


       constructor Create(…);

    ///NO ONE OF METHODS BELOW CALLED by CCW IDispatch implementation!

       function GetCustomAttributes(AInherit: Boolean): TObjectArr; overload; override;

       function GetCustomAttributes(AType: System.Type;

                                    AInherit: Boolean): TObjectArr; overload; override;

       function IsDefined(AType: System.Type;

                          AInherit: Boolean): Boolean; override;

       function get_Attributes: ParameterAttributes; override;

       function get_ParameterType: System.Type; override;

       function get_DefaultValue: System.Object; override;

       function get_Member: MemberInfo; override;

       function get_Name: string; override;

       function get_Position: Integer; override;



    //They differs by attributes: ParameterAttributes.In/Out/Retval

    function TMyMethodInfo.GetParameters: TParameterInfoArr;


       i: Integer;


     SetLength(Result, Length(FParams));

     for i := 0 to Length(FParams) – 1 do


       Result[i] := TMyParamInfo.Create(…);



    I have no idea, why CCW IDispatch implementation ignores my own parameters definition…

    Here is call log of my appication:

    //IActiveScript.AddNamedItem… TScriptHost added as ‘Host’ item




    TScriptHost.GetMethods, BindingFlags=[Instance, Static, Public]





    TScriptHost.GetMethods, BindingFlags=Instance, Static, Public



    TScriptHost.GetMethods, BindingFlags=Instance, Static, Public


    TMyMethodInfo.GetCustomAttributes System.Runtime.InteropServices.LCIDConversionAttribute

    TScriptHost.InvokeMember: Method1(mumu, 12), [InvokeMethod, OptionalParamBinding]


    TMyMethodInfo.GetCustomAttributes System.Runtime.InteropServices.LCIDConversionAttribute

    TScriptHost.InvokeMember: Method2(mumu, 12, 1245), [InvokeMethod, OptionalParamBinding]

    TScriptHost.InvokeMember: Method1(mumu, 12, 1245), [InvokeMethod, OptionalParamBinding]

    TScriptHost.InvokeMember: Method2(mumu, 12, 1245), [InvokeMethod, OptionalParamBinding]


  2. eaglus says:

    On my post above:

    i will be very glad, if you will help me to find a way, how to set different attributes (In/Out/Retval) for parameters of methods of application objects exposed to script with AddNamedItem

  3. eaglus says:

    No questions more…

    I’ve fixed the problem.

    It was just Delphi.Net bug

  4. The DLR aims to enable dynamic languages like IronPython and IronRuby to access and minipulate objects

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