花了我一上午的时间，找来找去找不到答案，又是急性子，只好自己动手了，见笑，也请大家多提意见，谢了。

parent.cs

------------------------------------------------------------

using System;

using System.Drawing;

using System.Collections;

using System.ComponentModel;

using System.Windows.Forms;

namespace testagain

{

///

/// Summary description for parent.

///

public class parent : System.Windows.Forms.Form

{

private System.Windows.Forms.MainMenu mainMenu1;

private System.Windows.Forms.MenuItem m_FileMenu;

private System.Windows.Forms.MenuItem m_OpenFile;

private System.Windows.Forms.MenuItem m_Close;

private System.Windows.Forms.MdiClient mdiClient1;

private System.Windows.Forms.MenuItem menuItem1;

///

/// Required designer variable.

///

private System.ComponentModel.Container components = null;

public parent()

{

//

// Required for Windows Form Designer support

//

InitializeComponent();

testform f_test = new testform(this);

f_test.Show();

//

// TODO: Add any constructor code after InitializeComponent call

//

}

public static void Main()

{

Application.Run(new parent());

}

///

/// Clean up any resources being used.

///

protected override void Dispose( bool disposing )

{

if( disposing )

{

if(components != null)

{

components.Dispose();

}

}

base.Dispose( disposing );

}

#region Windows Form Designer generated code

///

/// Required method for Designer support - do not modify

/// the contents of this method with the code editor.

///

private void InitializeComponent()

{

this.mainMenu1 = new System.Windows.Forms.MainMenu();

this.m_FileMenu = new System.Windows.Forms.MenuItem();

this.m_OpenFile = new System.Windows.Forms.MenuItem();

this.m_Close = new System.Windows.Forms.MenuItem();

this.menuItem1 = new System.Windows.Forms.MenuItem();

this.mdiClient1 = new System.Windows.Forms.MdiClient();

this.SuspendLayout();

//

// mainMenu1

//

this.mainMenu1.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {

this.m_FileMenu,

this.menuItem1});

//

// m_FileMenu

//

this.m_FileMenu.Index = 0;

this.m_FileMenu.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {

this.m_OpenFile,

this.m_Close});

this.m_FileMenu.Text = "文件(&F)";

//

// m_OpenFile

//

this.m_OpenFile.Index = 0;

this.m_OpenFile.Shortcut = System.Windows.Forms.Shortcut.CtrlO;

this.m_OpenFile.Text = "Dns探测";

this.m_OpenFile.Click += new System.EventHandler(this.m_OpenFile_Click);

//

// m_Close

//

this.m_Close.Index = 1;

this.m_Close.Shortcut = System.Windows.Forms.Shortcut.CtrlX;

this.m_Close.Text = "关闭";

this.m_Close.Click += new System.EventHandler(this.m_Close_Click);

//

// menuItem1

//

this.menuItem1.Index = 1;

this.menuItem1.Text = "关于...(&A)";

this.menuItem1.Click += new System.EventHandler(this.menuItem1_Click);

//

// mdiClient1

//

this.mdiClient1.Dock = System.Windows.Forms.DockStyle.Fill;

this.mdiClient1.Name = "mdiClient1";

this.mdiClient1.TabIndex = 0;

//

// parent

//

this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);

this.ClientSize = new System.Drawing.Size(536, 309);

this.Controls.AddRange(new System.Windows.Forms.Control[] {

this.mdiClient1});

this.IsMdiContainer = true;

this.Menu = this.mainMenu1;

this.Name = "parent";

this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;

this.Text = "MDI窗体操作演示_父窗体";

this.ResumeLayout(false);

}

#endregion

private void menuItem1_Click(object sender, System.EventArgs e)

{

Form f_activeform = this.ActiveMdiChild;

if (f_activeform != null)

{

if (f_activeform.Name != null && f_activeform.Name != "parent")

{

f_activeform.Close();

}

}

child child_form = new child(this);

child_form.Show();

}

private void m_Close_Click(object sender, System.EventArgs e)

{

this.Close();

}

private void m_OpenFile_Click(object sender, System.EventArgs e)

{

Form f_activeform = this.ActiveMdiChild;

if (f_activeform != null)

{

if (f_activeform.Name != null && f_activeform.Name != "parent")

{

f_activeform.Close();

}

}

testform f_test = new testform(this);

f_test.Show();

}

}

}