function SLUpdateMemoryHash(hCtx: HSECLIB; 
                            const pData: Pointer;
                            nDataSize: Cardinal): SLSTATUS ; stdcall ;

function DoHashFile( hCtx : HSECLIB;
                        hFileIn: THandle ;
			hFileOut: THandle;
                        pBuffer: Pointer ;
                        nSize : Cardinal  ): SLSTATUS ;
var
 	processed : DWORD ;
begin
 	ReadFile( hFileIn, pBuffer^, nSize, processed, Nil );

	Result := SLUpdateMemoryHash( hCtx, pBuffer, nSize ) ;//Как выполнить подобное в C#

	WriteFile( hFileOut, pBuffer^, nSize, processed, Nil );	
end;

[StructLayout(LayoutKind.Sequential)]
public struct HSECLIB__
{
       public int unused;
}

[DllImport("SecLibrary.dll", CallingConvention = CallingConvention.StdCall)]
static extern uint SLUpdateMemoryHash(HSECLIB__ hCtx, IntPtr bytePointer, uint nDataSize);

class Sender
{
    public Sender()
    {
        HSECLIB__ hSecLib2 = new HSECLIB__();
        string sFileIn = @"e:\text.txt";
        string sFileOut = @"e:\text.sign";
        FileStream fsIn = new FileStream(sFileIn, FileMode.Open);
        FileStream fsOut = new FileStream(sFileOut, FileMode.Create);
        long bufSize = fsIn.Length;
        byte[] byteBuf = new byte[bufSize];

        int countRead = fsIn.Read(bytePointer, 0, bufSize);    

        IntPtr unmanagedPointer = Marshal.AllocHGlobal(byteBuf.Length);
        Marshal.Copy(bytePointer, 0, unmanagedPointer, byteBuf.Length);

        int rez = SLUpdateMemoryHash(hSecLib2, unmanagedPointer, bufSize); //В чем ошибка?
        //if (rez != 0) { WriteLog("Ошибка SLUpdateMemoryHash"); }

        byte[] outBytes = new byte[bufSize];
        Marshal.Copy(unmanagedPointer, outBytes, 0, byteBuf.Length);

        fsOut.Write(bytePointer, 0, outBytes.Length);
        fsOut.Flush();
        fsOut.Close();
        fsIn.Close();
    }
}

        FileStream fsIn = new FileStream(sFileIn, FileMode.Open);
        long bufSize = fsIn.Length;
        byte[] byteBuf = new byte[bufSize];
        int countRead = fsIn.Read(byteBuf, 0, bufSize);