Issue converting C# marshal interface code to VB.NET

Question

I'm attempting to convert a project from C# to VB.NET to enable drag and dropping of Outlook messages into our program.

The original source can be found here on CodeProject.

I'm 95% of the way there but having trouble implementing the IStorage interface , a portion of which is outlined below.

public interface IStorage
{
    [return: MarshalAs(UnmanagedType.Interface)]
    IStream CreateStream([In, MarshalAs(UnmanagedType.BStr)] string pwcsName,
                         [In, MarshalAs(UnmanagedType.U4)] int grfMode,
                         [In, MarshalAs(UnmanagedType.U4)] int reserved1,
                         [In, MarshalAs(UnmanagedType.U4)] int reserved2);                
}

I've converted it to the following in VB.NET:

<MarshalAs(UnmanagedType.Interface)>
Function CreateStream(<[In], MarshalAs(UnmanagedType.BStr)> ByVal pwcsName As String,
                      <[In], MarshalAs(UnmanagedType.U4)> ByVal grfMode As Integer,
                      <[In], MarshalAs(UnmanagedType.U4)> ByVal reserved1 As Integer,
                      <[In], MarshalAs(UnmanagedType.U4)> ByVal reserved2 As Integer) As IStream

However, it results in the following error:

Attribue 'MarshalAsAttribute' cannot be applied to 'CreateStream' because the attribute is not valid on this declaration type.

I'm a little out of my depth here and would really appreciate any pointers you may have in helping me solve this issue.

Many thanks.

Answer 1

You can apply the attribute to the return value in this way:

Function CreateStream(<[In], MarshalAs(UnmanagedType.BStr)> ByVal pwcsName As String,
            <[In], MarshalAs(UnmanagedType.U4)> ByVal grfMode As Integer,
            <[In], MarshalAs(UnmanagedType.U4)> ByVal reserved1 As Integer,
            <[In], MarshalAs(UnmanagedType.U4)> ByVal reserved2 As Integer) As <MarshalAs(UnmanagedType.Interface)> IStream