Code List:
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/*****************************************************************
** File Name: frmMain.cs
** Copyright (c) 1999 -2003
** Creator: FirePhoenix
** Created Date: 2004-11-13 15:24
** Modifier:
** Modify Date:
** Description:
** Version:1.0
******************************************************************/

#region Using Directives
using System;
using System.IO ;
using System.Xml ;
using System.Collections ;
using System.Reflection ;
using System.Text ;
using System.Data ;
using System.ComponentModel;
using System.Windows.Forms;
using System.Drawing;
using System.Threading ;
#endregion

namespace WindowsApplication4
{
/// <summary>
/// Copy Large File
/// </summary>
public class frmMain : System.Windows.Forms.Form
{
#region
private System.Windows.Forms.ProgressBar progressBar1;
private System.Windows.Forms.Button btnCopy;
/// <summary>
/// 必需的设计器变量。
/// </summary>
private System.ComponentModel.Container components = null;

public frmMain()
{
//
// Windows 窗体设计器支持所必需的
//
InitializeComponent();

//
// TODO: 在 InitializeComponent 调用后添加任何构造函数代码
//
}

/// <summary>
/// 清理所有正在使用的资源。
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Initialize Components
/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.progressBar1 = new System.Windows.Forms.ProgressBar();
this.btnCopy = new System.Windows.Forms.Button();
this.SuspendLayout();
//
// progressBar1
//
this.progressBar1.Location = new System.Drawing.Point(8, 16);
this.progressBar1.Name = "progressBar1";
this.progressBar1.Size = new System.Drawing.Size(208, 16);
this.progressBar1.TabIndex = 0;
//
// btnCopy
//
this.btnCopy.Location = new System.Drawing.Point(8, 48);
this.btnCopy.Name = "btnCopy";
this.btnCopy.TabIndex = 1;
this.btnCopy.Text = "Copy";
this.btnCopy.Click += new System.EventHandler(this.btnCopy_Click);
//
// frmMain
//
this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
this.ClientSize = new System.Drawing.Size(232, 77);
this.Controls.Add(this.btnCopy);
this.Controls.Add(this.progressBar1);
this.Name = "frmMain";
this.Text = "Copy File";
this.ResumeLayout(false);

}
#endregion

/// <summary>
/// Entry Point
/// </summary>
[STAThread]
static void Main()
{
Application.Run(new frmMain());
}

#endregion

int totalSize; //Total Size
int position; //Position
const int BUFFER_SIZE = 4096;
byte[] buffer;
Stream stream;

private void btnCopy_Click(object sender, System.EventArgs e)
{
string strFile = "";

OpenFileDialog dlg = new OpenFileDialog();
if ( dlg.ShowDialog() == DialogResult.OK )
{
strFile = dlg.FileName ;
}
else
{
return ;
}

FileStream fs = new FileStream( strFile , FileMode.Open , FileAccess.Read ) ;
totalSize = (int)fs.Length ;
stream = fs;

//Delete file which aready exists.
if ( File.Exists( "c:\\copyedFile.bin" ) )
File.Delete( "c:\\copyedFile.bin" );

//Copy file while larger than 4KB.
if ( totalSize > BUFFER_SIZE )
{
buffer = new byte[ BUFFER_SIZE ];

// Async Invoke
stream.BeginRead( buffer , 0 , BUFFER_SIZE , new AsyncCallback( AsyncCopyFile ) , null );
}
else
{
fs.Close();
}

}

/// <summary>
/// Asynchronously copy file
/// </summary>
/// <param name="ar"></param>
private void AsyncCopyFile( IAsyncResult ar )
{
int readedLength ;

// Lock FileStream
lock( stream )
{
readedLength = stream.EndRead( ar ); // When stream endread, get readed length
}

// Write to disk
FileStream fsWriter = new FileStream( "C:\\copyedFile.bin" , FileMode.Append , FileAccess.Write );
fsWriter.Write( buffer , 0 , buffer.Length );
fsWriter.Close();

// Current stream position
position += readedLength;

// Response UI
MethodInvoker m = new MethodInvoker( SynchProgressBar );
m.BeginInvoke( null , null );

if ( position >= totalSize ) // Read over.
{
stream.Close(); //Close FileStream
return ;
}

// Continue to read and write
lock ( stream )
{
int leftSize = totalSize - position;

if ( leftSize < BUFFER_SIZE )
buffer = new byte[ leftSize ];

stream.BeginRead( buffer , 0 , buffer.Length , new AsyncCallback( AsyncCopyFile ) , null );

}
}

private void SynchProgressBar()
{
this.progressBar1.Maximum = totalSize;
this.progressBar1.Value = position ;
}

}
}