Single Instance Form in a MDI application

private void OpenForm<T>() where T : Form, new()

{

    T frm = (T)new List<Form>(this.MdiChildren).Find(f => f is T) ?? new T() { MdiParent = this };

 

    frm.Show();

    frm.Focus();

}

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OpenForm<NewForm>();

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 另外的一种写法：

 public partial class MDIForm : Form

   {

      private Child1Form mChild1Form = null;

      private Child2Form mChild2Form = null;

 

      public MDIForm()

      {

         InitializeComponent();

      }

 

      private Form ShowOrActiveForm(Form form, Type t)

      {

         if (form == null)

         {

            form = (Form)Activator.CreateInstance(t);

            form.MdiParent = this;

            form.Show();

         }

         else

         {

            if (form.IsDisposed)

            {

               form = (Form)Activator.CreateInstance(t);

               form.MdiParent = this;

               form.Show();

            }

            else

            {

               form.Activate();

            }

         }

         return form;

      }

 

      private void newToolStripButton_Click(object sender, EventArgs e)

      {

         mChild1Form = ShowOrActiveForm(mChild1Form, typeof(Child1Form)) as Child1Form;

      }

 

      private void openToolStripMenuItem_Click(object sender, EventArgs e)

      {

         mChild2Form = ShowOrActiveForm(mChild2Form, typeof(Child2Form)) as Child2Form;

      }

   }

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