楼上的方法是不是有点太复杂了。呵……

楼上的楼上只是说的MDI窗口形式吧？

是的，上面的代码是MDI中判断窗体是否已经存在的方法，如果不是MDI窗体，则调用窗体设全局变量(Flag)，并将该全局变量做为参数传递到被调用窗体，被调用窗体初始化时设置Flag为true,关闭设置为false

//只允许打开一个应用程序实例
//需要添加的命名空间
using System.Runtime.InteropServices;
using System.Diagnostics;
using System.Reflection;
public static Process RunningInstance()
{
Process current = Process.GetCurrentProcess();
Process[] processes = Process.GetProcessesByName (current.ProcessName);

//Loop through the running processes in with the same name
foreach (Process process in processes)
{
//Ignore the current process
if (process.Id != current.Id)
{
//Make sure that the process is running from the exe file.
if (Assembly.GetExecutingAssembly().Location.Replace("/","\\") ==
current.MainModule.FileName)
{
//Return the other process instance.
return process;
}
}
}

//No other instance was found, return null.
return null;
}

public static void HandleRunningInstance(Process instance)
{
//Make sure the window is not minimized or maximized
ShowWindowAsync (instance.MainWindowHandle , WS_SHOWNORMAL);

//Set the real intance to foreground window
SetForegroundWindow (instance.MainWindowHandle);
}

[DllImport("User32.dll")]

private static extern bool ShowWindowAsync(
IntPtr hWnd, int cmdShow);
[DllImport("User32.dll")] private static extern bool
SetForegroundWindow(IntPtr hWnd);
private const int WS_SHOWNORMAL = 1;
}

static void Main()
{
//Get the running instance.
Process instance = RunningInstance();
if (instance == null)
{
//There isn't another instance, show our form.
frmMediaPlayer objFromMediaPlayer=new frmMediaPlayer();
Application.Run (objFromMediaPlayer);
}
else
{
//There is another instance of this process.
HandleRunningInstance(instance);
}

}
这个是通用的，我感觉看明白不难，不过代码倒是挺多的
不知道楼主要的是不是这个

鼎鼎

用个单件模式就行了
比如
class Form2 : form
{
private static bool _isExisted = false;
public static FormShow()
{
if( ! _isExisted)
{
_isExisted = true ;
new Form2().show();

}
}
}

打开新窗体时，设置打开的窗体为对话框模式不就可以避免了？

我也遇到这种问题，各位大侠出手帮忙一下，谢谢。
上面的代码好像都可以，可能与我定义的窗体有关吧
我是想打开普通窗体不是子窗体啊。
谢谢。