System.IO.FileStream.BeginWrite C# (CSharp) Method

        public override IAsyncResult BeginWrite(byte[] array, int offset, int numBytes, AsyncCallback callback, object state)
        {
            if (array == null)
                throw new ArgumentNullException(nameof(array));
            if (offset < 0)
                throw new ArgumentOutOfRangeException(nameof(offset), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (numBytes < 0)
                throw new ArgumentOutOfRangeException(nameof(numBytes), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (array.Length - offset < numBytes)
                throw new ArgumentException(SR.Argument_InvalidOffLen);

            if (IsClosed) throw new ObjectDisposedException(SR.ObjectDisposed_FileClosed);
            if (!CanWrite) throw new NotSupportedException(SR.NotSupported_UnwritableStream);

            if (!IsAsync)
                return base.BeginWrite(array, offset, numBytes, callback, state);
            else
                return TaskToApm.Begin(WriteAsyncInternal(array, offset, numBytes, CancellationToken.None), callback, state);
        }

 public static void ApplyIPSPatch(string romname, string patchname)
 {
     // Noobish Noobsicle wrote this IPS patching code
     // romname is the original ROM, patchname is the patch to apply
     FileStream romstream = new FileStream(romname, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite);
     FileStream ipsstream = new FileStream(patchname, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite);
     int lint = (int)ipsstream.Length;
     byte[] ipsbyte = new byte[ipsstream.Length];
     byte[] rombyte = new byte[romstream.Length];
     IAsyncResult romresult;
     IAsyncResult ipsresult = ipsstream.BeginRead(ipsbyte, 0, lint, null, null);
     ipsstream.EndRead(ipsresult);
     int ipson = 5;
     int totalrepeats = 0;
     int offset = 0;
     bool keepgoing = true;
     while (keepgoing == true)
     {
         offset = ipsbyte[ipson] * 0x10000 + ipsbyte[ipson + 1] * 0x100 + ipsbyte[ipson + 2];
         ipson++;
         ipson++;
         ipson++;
         if (ipsbyte[ipson] * 256 + ipsbyte[ipson + 1] == 0)
         {
             ipson++;
             ipson++;
             totalrepeats = ipsbyte[ipson] * 256 + ipsbyte[ipson + 1];
             ipson++;
             ipson++;
             byte[] repeatbyte = new byte[totalrepeats];
             for (int ontime = 0; ontime < totalrepeats; ontime++)
                 repeatbyte[ontime] = ipsbyte[ipson];
             romstream.Seek(offset, SeekOrigin.Begin);
             romresult = romstream.BeginWrite(repeatbyte, 0, totalrepeats, null, null);
             romstream.EndWrite(romresult);
             ipson++;
         }
         else
         {
             totalrepeats = ipsbyte[ipson] * 256 + ipsbyte[ipson + 1];
             ipson++;
             ipson++;
             romstream.Seek(offset, SeekOrigin.Begin);
             romresult = romstream.BeginWrite(ipsbyte, ipson, totalrepeats, null, null);
             romstream.EndWrite(romresult);
             ipson = ipson + totalrepeats;
         }
         if (ipsbyte[ipson] == 69 && ipsbyte[ipson + 1] == 79 && ipsbyte[ipson + 2] == 70)
             keepgoing = false;
     }
     romstream.Close();
     ipsstream.Close();
 }