c# 4.5.1 MarshalAs(UnmanagedType.BStr) bug

I have a program in c# that calls a number of powerbasic created dlls.  Each dll has one exported function that takes an int and a Bstr as input and returns a Bstr.  The following is a simplified example that demonstrates the exact same problem:

Function DllTestCmd(ByVal Cmd As Long,ByVal CmdStr As WString) ThreadSafe Export As WString
    Local c$
    c$ = cmdStr
    Function = c$ + $CrLf + "Ok!"
End Function

Here's the c# declaration:

         [DllImport("DllTest.dll", EntryPoint = "DLLTESTCMD")]
        [return: MarshalAs(UnmanagedType.BStr)]
        private static extern string DLLTESTCMD(int cmd, [MarshalAs(UnmanagedType.BStr)] string commandString);

Calling this function is the following C# code:

         void Button1Click(object sender, EventArgs e)
        {
            unsafe {
                string x = "Hi There";                
                string ret = DLLTESTCMD(1,x);
                label1.Text = ret;                        
            }
        }

If I compile the c# code in dot net 4.0 all is well.  However if I simply change to compile with 4.5.1, the c# program does not return from the dll and simply disappears, no crash info at all, even in debug mode.

If I change the exported function to:

Function DllTestCmd(ByVal Cmd As Long,ByVal CmdStr As String) ThreadSafe Export As WString
    Local c$
    c$ = cmdStr
    Function = c$ + $CrLf + "Ok!"
End Function

(changing the passed string to ansi bstr) and the c# declaration to:

         [DllImport("DllTest.dll", EntryPoint = "DLLTESTCMD")]
        [return: MarshalAs(UnmanagedType.BStr)]
        private static extern string DLLTESTCMD(int cmd, [MarshalAs(UnmanagedType.AnsiBStr)] string commandString);

and then call the dll, it does work.  But it seems that 4.5.1 has some sort of bug inMarshalAs(UnmanagedType.BStr).

I would change everything to the way that it works, but I've also seen some situations in 4.5.1 where it crashed with a debug error saying there is a memory corruption.  That looks like the GC is jumping the gun and moving things around in memory while I'm in the dll.

Thanks,

Russ