Form f2 = new Form(); 
        private void button1_Click(object sender, EventArgs e)
        {    
            f2.Show();
        }

------解决方案--------------------
C# code

        Form f2;
        private void toolStripMenuItem1_Click(object sender, EventArgs e) 
        { 
            if(!f2.Created)
            {
               f2=new Form();
               f2.Show();
            }
        }

------解决方案--------------------
单件模式

C# code

public class Form2()
{
  private Form2 f2;
  private Form2()
  {
     //构造函数声明为私有
  }
  public static Form2 GetForm2()
  {
    if(this.f2==null)
    {
      this.f2=new Form2();
    }
    return this.f2;
  }
}

------解决方案--------------------
private void toolStripMenuItem1_Click(object sender, EventArgs e)  
       {  

           if(f2==null||f2.Disposed)
           {Form f2 = new Form(); }
           f2.Show();  
       }

------解决方案--------------------
参考如下：

C# code

private Form f2 = null;
private void toolStripMenuItem1_Click(object sender, EventArgs e)
{
    if (f2 == null || f2.IsDisposed)
    {
        f2 = new Form();
    }
    f2.Show();
}

------解决方案--------------------
 Form f2 = null;
private void toolStripMenuItem1_Click(object sender, EventArgs e)
{
   if (f2 == null || f2.IsDisposed)
   {
       f2 = new Form();
   }
   f2.Show();
}  



------解决方案--------------------
如果是MDI窗体可参考下
private void toolStripMenuItem1_Click(object sender, EventArgs e)  
       {  
           Form f2 = new Form();  
           f2.Show();  
foreach (Form form in this.MdiChildren)           
   {                if (form is Form)
               {
                   form.Focus();
                   return;
               }
   }
           Form f2 = new Form();
           f2.MdiParent = this;
           f2.Show();
     
  }