Single Instance Form in a MDI application

Sometimes, we want to ensure that only one instance of model's child form opens even if user clicks multiple times on the menu option in a MDI application. If form is already opened just set the focus on the form, otherwise create a new instance of the child form. Here is a quick tip to achieve this:

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;
    }
 }

I defined a function ShowOrActiveForm() and added private member variables for each child Form in MDIForm class which I want as single instance in MDI application. Child1Form and Child2Form are derived from Form class.