string sourceFile = @"c:\temp\New Text Document.txt"; 
string destinationFile = @"c:\temp\test.txt"; 
bool isrewrite=true; // true=覆盖已存在的同名文件,false则反之
System.IO.File.Copy(sourcePath, targetPath, isrewrite); 

string sourceFile = @"c:\temp\New Text Document.txt"; 
string destinationFile = @"c:\temp\test.txt"; 
FileInfo file = new FileInfo(sourceFile); 
if (file.Exists) 
{ 
    // true is overwrite 
    file.CopyTo(destinationFile, true); 
} 

#region 拷贝操作
private void button3_Click(object sender, EventArgs e)
{
    OpenFileDialog ofd = new OpenFileDialog();
    DialogResult res = ofd.ShowDialog();
    if (res == DialogResult.OK) 
    {
        if (!string.IsNullOrEmpty(ofd.FileName))
        {
            //1.创建读入文件流对象
            FileStream streamRead = new FileStream(ofd.FileName, FileMode.Open);
            //2.创建1个字节数组，用于接收文件流对象读操作文件值
            byte[] data = http://www.mamicode.com/new byte[1024 * 1024];//1M
            int length = 0;
            SaveFileDialog sfd = new SaveFileDialog();
            DialogResult sres = sfd.ShowDialog();
            if (sres == DialogResult.OK)
            {
                if (!string.IsNullOrEmpty(ofd.FileName))
                {
                    FileStream streamWrite = new FileStream(sfd.FileName, FileMode.Create);
                    do
                    {
                        //3.文件流读方法的参数1.data-文件流读出数据要存的地方，2. 0--从什么位置读，3. data.Length--1次读多少字节数据
                        //3.1 Read方法的返回值是一个int类型的，代表他真实读取 字节数据的长度，
                        length = streamRead.Read(data, 0, data.Length);//大文件读入时候，我们定义字节长度的可能会有限，如果文件超大，要接收文件流对象的Read()方法，会返回读入的实际长度
                        //加密 和解密
                        for (int i = 0; i < length; i++) 
                        {
                             data[i] = (byte)(255 - data[i]);
                        }
                            streamWrite.Write(data, 0, length);
                    } while (length == data.Length); //如果实际写入长度等于我们设定的长度，有两种情况1.文件正好是我们设定的长度2.文件超大只上传了截取的一部分
                }
            }
        }
    }
}
#endregion