Prevent duplicate MDI children forms

Question

Is there a way to prevent the opening of a certain form within an MDI container if that said form is already opened?

Answer 1

A method can be implemented using Generics (below C# and VB.net options), which can be useful if different MDI Forms need to be opened.

C#

private void OpenMDI<T>(bool multipleInstances)
    where T : Form, new()
{
    if (multipleInstances == false)
    {
        // Look if the form is open
        foreach (Form f in this.MdiChildren)
        {
            if (f.GetType() == typeof(T))
            {
                // Found an open instance. If minimized, maximize and activate
                if (f.WindowState == FormWindowState.Minimized)
                {
                    f.WindowState = FormWindowState.Maximized;
                }

                f.Activate();
                return;
            }
        }
    }

    T newForm = new T();
    newForm.MdiParent = this;
    newForm.Show();
}

Use it as follows (indicate false in multipleInstances to prevent them)

OpenMDI<Form2>(false);

---

VB.NET

Public Sub Open_MDI(Of T As {New, Form})(bMultipleInstances As Boolean)
    If bMultipleInstances = False Then
        For Each f As Form In Me.MdiChildren
            If TypeOf f Is T Then
                If (f.WindowState = FormWindowState.Minimized) Then
                    f.WindowState = FormWindowState.Maximized;
                End If

                f.Activate()
                Exit Sub
            End If
        Next
    End If

    Dim myChild As New T()
    myChild.MdiParent = Me
    myChild.Show()
End Sub

Use it as follows (indicate False for bMultipleInstances to prevent them)

Open_MDI(Of Form2)(False)