您必须按如下方式设置窗体的WindowState属性：
在Windows窗体中：

private void button1_Click(object sender, EventArgs e)
{
    this.WindowState = FormWindowState.Minimized;
}

在WPF中：

private void button1_Click(object sender, RoutedEventArgs e)
{
    this.WindowState = WindowState.Minimized;
}

窗体窗口状态属性
http://msdn.microsoft.com/en-us/library/system.windows.forms.form.windowstate%28v=VS.90%29.aspx

public FormWindowState WindowState { get; set; }

例如-

var form = new Form();
form.WindowState = FormWindowState.Maximized;
form.WindowState = FormWindowState.Minimized;
form.WindowState = FormWindowState.Normal;

但是，如果您在主窗体（或任何窗体）上的代码后面，请执行以下操作-

WindowState = FormWindowState.Maximized;

如果您使用的是WindowsForms，则必须更改WindowState属性：）

private void button4_Click(object sender, EventArgs e)
    {
        if (this.WindowState == FormWindowState.Normal) 
        {
            this.WindowState = FormWindowState.Maximized;
        }
        else
        {
            this.WindowState = FormWindowState.Normal;
        }