VB Migration Partner

KNOWLEDGE BASE - Classes and COM objects


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[PRB] Members in COM libraries might be missing or raise unexpected errors

VB Migration Partner correctly converts all references to COM type libraries, which the migrated VB.NET project can access via COM Interop, an important portion of the .NET Framework.

However, you should be aware that the COM Interop technology has some limitations. First and foremost, not all COM object can be correctly marshaled between the COM and the .NET worlds, thus in some cases invoking a method or setting a property might raise unexpected errors.

Second, some members of the COM library might not be accessible at all from VB.NET. For example, all methods defined in modules (as opposed to classes) aren’t visible to .NET clients. Examples of these libraries are DirectX and OpenGL components. For more details about this issue, read this Microsoft KB article.

In cases like these, the only viable workaround is creating a VB6 class that works as a “wrapper” for the members that aren’t visible to .NET clients. If the wrapper class preserves the member names of the original type library, you can modify the original VB6 project by changing the reference so that it now points to the wrapper instead of the original library.

If you don’t want to modify the original VB6 project, you can perform the replacement by means of a PreProcess pragma that modifies the .vbp file on-the-fly before the migration begins.

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