public class SelectForm : System.Windows.Forms.Form
{

private static SelectForm SelFormInstance;

public static void SelectForm GetInstance()
{
   if(SelFormInstance == null)
   {
       SelFormInstance = new SelectForm ();
   }
   return SelFormInstance ;
}

// 点确定时，窗口被CLOSE了。
public void btnOK_Click(object sender, EventArgs e)
{
   this.close();
}
}

public class SelectForm : System.Windows.Forms.Form
{

private static SelectForm SelFormInstance;

public static void SelectForm GetInstance()
{
   if(SelFormInstance == null)
   {
       SelFormInstance = new SelectForm ();
       SelFormInstance.Close += onclose;
   }
   return SelFormInstance ;
}

   static void onclose(object sender, EventArgs e)
   {
        SelFormInstance = null;
   }

// 点确定时，窗口被CLOSE了。
public void btnOK_Click(object sender, EventArgs e)
{
   this.close();
}
}

------解决方案--------------------
this.Dispose();

------解决方案--------------------
Main方法执行完了肯定就释放了.

你所谓的"存在",A窗体CLSOE后B窗体还可以访问么？
你自己试试在Ａ窗体添加一个静态全局变量,看可不可以访问咯.

结论是---静态是首次调用后,便永远存在了(if ...Main.Run ...).