static class Program
    {
//给这个程序主窗体起个名字
        private static string strProcessName = "Form1";
private static string strAppName = "WindowsFormsApplication1";
static int hWnd = 0;
//随便个数字，用来做判断
        const int SW_SHOW = 1;
//指定是那个DLL，或者进程
        [DllImport("user32.dll", EntryPoint = "ShowWindow")]
public static extern int ShowWindow(int hwnd, int nCmdShow);

        [DllImport("user32")]
public static extern int GetWindowText(int hWnd, StringBuilder lpString, int nMaxCount);

        [DllImport("user32")]
public static extern int EnumWindows(CallBack x, int y);

public delegate bool CallBack(int hWnd, int lParam);

/// <summary>
/// 应用程序的主入口点。
/// </summary>
        [STAThread]
static void Main()
        {
//进程数组
            Process[] ps = Process.GetProcessesByName(strAppName);
//声明个委托
            CallBack myCallBack = new CallBack(FineAppWindow);
            EnumWindows(myCallBack, 0);

if (ps.Length > 1)
            {//如果进程里面有这个
                ShowWindow(hWnd, SW_SHOW);
return;
            }
else
            {
                Application.EnableVisualStyles();
                Application.SetCompatibleTextRenderingDefault(false);
                Application.Run(new Form1());
            }
        }

public static bool FineAppWindow(int hwnd, int lParam)
        {
            StringBuilder sb = new StringBuilder(200);
int n = GetWindowText(hwnd, sb, 200);

if (sb.ToString() == strProcessName)
            {
                hWnd = hwnd;
            }
return true;
        }
    }