用C#写的单例模式--很好理解，拿来即可用

很容易的操作。先建两个窗体(form),一个作为另一个的父窗体。

即把父窗体的属性ISMdiContainer设置为TRUE；即可成为父窗体。

然后把这段代码写进父窗体里

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

private void Form1_Load(object sender, EventArgs e)
{
CreateUserMenu();
}
private void ChangeChildren(Form frm)
{
if (this.MdiChildren.Length ==1)
{
frm.Show();
return;
}
foreach (Form children in this.MdiChildren)
{
if (frm == children)
{
children.Activate();
break;
}
else
{
children.Close();
frm.Show();
break;
}
}
}

private void CreateUserMenu()
{
ToolStripMenuItem tool= new ToolStripMenuItem();
tool.Name = "tool";
tool.Image = Image.FromFile(Application.StartupPath + "\\p_w_picpaths\\wallet.png");
this.menuStrip1.Items.Add(tool);
tool.Click += new EventHandler(UpdateTool_Click);
}

private void UpdateTool_Click(object sender, EventArgs e)
{
Form2 f2 = new Form2();
f2.MdiParent = this;
f2.Text = "工具";
ChangeChildren(f2);
}
}
}

有些是事件，不能直接复制，需要稍作修改。