using System;
using System.IO;
string filetosplit=@"C:\temp\data.bin";
string targetpath=@"D:\store";
FileStream fsr = new FileStream(filetosplit, FileMode.Open, FileAccess.Read);
long FileLength=fsr.Length;
byte[] btArr = new byte[FileLength];
fsr.Read(btArr, 0, (int)FileLength);
fsr.Close();
int splitcount=3;
long PartLength=FileLength/splitcount+FileLength%splitcount;
int nCount=(int)Math.Ceiling((double)FileLength/PartLength);
string strFileName=Path.GetFileName(filetosplit);
long byteCount=0;
for(int i=1;i<=nCount;i++,byteCount=(i<nCount?byteCount+PartLength:FileLength-PartLength))
{
  FileStream fsw = new FileStream(targetpath + Path.DirectorySeparatorChar+ strFileName +i, FileMode.Create, FileAccess.Write);
  fsw.Write(btArr, (int)byteCount, (int)(i<nCount?PartLength:FileLength-byteCount));
  fsw.Flush();
  fsw.Close();
}

using System;
using System.IO
string filetosplit=@"C:\temp\data.bin";
string targetpath=@"D:\store";
FileStream fsr = new FileStream(filetosplit, FileMode.Open, FileAccess.Read);
long FileLength=fsr.Length;
byte[] btArr = new byte[FileLength];
fsr.Read(btArr, 0, (int)FileLength);
fsr.Close();
int splitcount=3;
long PartLength=FileLength/splitcount+FileLength%splitcount;
int nCount=(int)Math.Ceiling((double)FileLength/PartLength);
string strFileName=Path.GetFileName(filetosplit);
long byteCount=0;
for(int i=1;i<=nCount;i++,byteCount=(i<nCount?byteCount+PartLength:FileLength-PartLength))
{
  FileStream fsw = new FileStream(targetpath + Path.DirectorySeparatorChar+ strFileName +i, FileMode.Create, FileAccess.Write);
  long bc=byteCount;
  long PartCount=i<nCount?PartLength:FileLength-bc;
  int PartBufferCount=(int)(PartCount<int.MaxValue/32?PartCount:int.MaxValue/32);
  int nc=(int)Math.Ceiling((double)PartCount/PartBufferCount);
  for(int j=1;j<=nc;j++,bc=(j<nCount?bc+PartBufferCount:PartCount-PartBufferCount))
    fsw.Write(btArr, (int)bc, (int)(j<nc?PartBufferCount:PartCount-bc));
  fsw.Flush();
  fsw.Close();
}
fsr.Close();