I have a C++ struct like this:

struct TESTSTRUCT
{
   DWORD dwTest;
   WCHAR wszTest[ 256 ];
};

used in an interface like this:

virtual HRESULT Test(TESTSTRUCT *test) = 0;

The C# COM server interface is defined as:

void Test(ref TESTSTRUCT test);

and the struct is defined as:

[StructLayout(LayoutKind.Sequential,CharSet=CharSet.Unicode)]
public struct TESTSTRUCT
{
   public UInt32 dwTest;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst=255)] public String
sTest;
}

According to Microsoft, this is the correct way to handle fixed length
arrays in structures, but it does not seem to work.  I'm trying to use
this as an in/out parameter, and values are making it into the C#
server just fine, but any modifications that are made there are not
reflected in the C++ client.  If I remove the array from the struct,
everything works fine, so assume my interface definition is ok, it's
just my structure that is somehow wrong.  When I look at the type
library for the C# assembly, the struct definition looks identical to
the C++ version, yet it still will not marshal correctly.

Any ideas?