C# Definition:

[ComImport, Guid("0000000c-0000-0000-C000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IStream
{
      void Read([Out, MarshalAs(UnmanagedType.LPArray, SizeParamIndex=1)] byte[] pv, uint cb, out uint pcbRead);
      void Write([MarshalAs(UnmanagedType.LPArray, SizeParamIndex=1)] byte[] pv, uint cb, out uint pcbWritten);
      void Seek(long dlibMove, uint dwOrigin, out long plibNewPosition);
      void SetSize(long libNewSize);
      void CopyTo(IStream pstm, long cb, out long pcbRead, out long pcbWritten);
      void Commit(uint grfCommitFlags);
      void Revert();
      void LockRegion(long libOffset, long cb, uint dwLockType);
      void UnlockRegion(long libOffset, long cb, uint dwLockType);
      void Stat(out STATSTG pstatstg, uint grfStatFlag);
      void Clone(out IStream ppstm);
}

Alternative Managed Definition:

System.Runtime.InteropServices.ComTypes.IStream in the .NET Framework 2.0.

User-Defined Types:

STATSTG

Notes:

The Seek and CopyTo methods are documented as allowing NULL to be passed instead of output parameters, if the caller does not care what the result was. If you have consumers of your IStream that use it in this manner, you will need to replace the affected definitions with IntPtr values and use Marshal.WriteInt64() to assign them:

      void Seek(long dlibMove, uint dwOrigin, IntPtr plibNewPosition);

      if (plibNewPosition!= IntPtr.Zero)
        Marshal.WriteInt64(plibNewPosition, newStreamPosition);