memset(&ofn, 0, sizeof (ofn));

   // Geez, I wish C# had a Marshal.MemSet() method

There is no Marshal.MemSet method to zero-out the buffer prior to use, so you might consider the following hack

   // Geez, I wish C# had a Marshal.MemSet() method

Removed SetLastError attribute from bottom 2 signatures as memset does not use this API.

[DllImport("msvcrt.dll", EntryPoint = "memset", CallingConvention = CallingConvention.Cdecl, SetLastError = false)]

public static extern IntPtr MemSet(IntPtr dest, int c, int byteCount);

<DllImport("msvcrt.dll", EntryPoint := "memset", CallingConvention := CallingConvention.Cdecl, SetLastError := False)> _

Public Shared Function MemSet(dest As IntPtr, c As Integer, byteCount As Integer) As IntPtr

You can use the memset - [Function]

<DllImport("MSVCRT.DLL", EntryPoint:="memset", CharSet:=CharSet.Auto, CallingConvention:=CallingConvention.Cdecl)>

Public Shared Sub memset(<[In](), Out()> ByVal dest As IntPtr, _

memset(ptrValue, 0, sizeVariant)

     [DllImport("coredll.dll", EntryPoint = "memset", SetLastError = false)]

     private static extern void memset(IntPtr dest, int c, int size);

           memset(gc.AddrOfPinnedObject(), 0, Marshal.SizeOf(o));