You receive a "System.Runtime.InteropServices.COMException" error message when you access a COM component in Visual Basic (814912)

SYMPTOMS

You may access a Component Object Model (COM) object from managed code and then call a method that has parameters of the type IUnKnown. When you do this by using Microsoft Visual Basic 2005 in Microsoft Visual Studio 2005 or by using Microsoft Visual Basic .NET in Microsoft Visual Studio .NET, you may receive the following error message:

Unhandled exception: System.Runtime.InteropServices.COMException: Type mismatch, occurred at Microsoft.VisualBasic.CompilerServices.LateBinding.LateGet(Object o, Type objType, String name, Object[] args, String[] paramnames, Boolean[] CopyBack) at MyRepro.Module1.Main().

CAUSE

When you use a late-binding mechanism in Visual Studio 2005 or in Visual Studio .NET, the runtime cannot marshal an object that has a value of NOTHING. In Visual Studio 2005 or in Visual Studio .NET, if an object variable contains NOTHING when it is encountered, the object variable is not changed and no instance is automatically created. If the object variable is set to NULL in a program that is written in Microsoft Visual C# .NET, this reproduces the same behavior. In the Microsoft .NET Framework, late binding is implemented by using the InvokeMember method of the Type class.

WORKAROUND

When you use the late-binding mechanism, you can avoid this behavior if you pass the parameters as UnKnownWrapper to methods that have parameters of type IUnKnown. The UnKnownWrapper constructor initializes a new instance of the UnknownWrapper class with the object that will be wrapped.

The following code sample wraps the invar object, the inoutvar object, and the outvar object that the marshaler must marshal as a COM variant type VT_UNKNOWN.

' This is the code sample. 

'MyRepro is being imported.
Imports MyRepro

Module Module1

' This method must be named "Main".
    Sub Main()	

'This variable of type Object can hold data of any type.
								Dim CObj11 As Object

	'AppObj holds object of type MyReproApp.
        Dim AppObj As MyReproApp

	'The following Object variables can hold data of any type and can be used as parameters to method M_UNKNOWN.
        Dim invar As Object
        Dim inoutvar As Object
        Dim outvar As Object
        Dim retval As Object
        Dim COMNothing As Object
        
        COMNothing = New Runtime.InteropServices.UnknownWrapper(Nothing)

        invar = COMNothing
        inoutvar = COMNothing
        outvar = COMNothing

        AppObj = New MyReproApp()
        CObj1 = AppObj.NewReproObj

        retval = CObj1.Repro(invar, inoutvar, outvar)

    End Sub

End Module

MORE INFORMATION

Steps to reproduce the behavior

If you have a COM component that is registered on your system, follow these steps:

Start Visual Studio 2005 or Visual Studio .NET. Create a new Visual Basic 2005 or Visual Basic .NET console application.

In the Module1.vb file, replace the existing code with the following code.

' This is a code sample. 

'MyRepro is being imported.
Imports MyRepro

Module Module1

' This method must be named "Main".
    Sub Main()	

'This variable of type Object can hold data of any type.
								Dim CObj11 As Object

	'AppObj holds object of type MyReproApp.
     			Dim AppObj As MyReproApp

	'The following Object variables can hold data of any type and can be used as parameters to method M_UNKNOWN.
        Dim invar As Object
        Dim inoutvar As Object
        Dim outvar As Object
        Dim retval As Object   

        AppObj = New MyReproApp()
        CObj1 = AppObj.NewReproObj

        retval = CObj1.Repro(invar, inoutvar, outvar)

    End Sub

End Module

In Solution Explorer, right-click References.
Click Add Reference.
In the Add Reference dialog box, click the COM tab.
In the list, click to select the COM component that you have already registered, and then click OK.
On the Debug menu, click Start.

REFERENCES

For more information about default marshaling for objects, visit the following Microsoft Developer Network (MSDN) Web site:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpcondefaultmarshalingforobjects.asp