Foros del web, para los que viven de bits

private void button1_Click(object sender, System.EventArgs e) {
Form2 childForm = null;
//Barremos las formas que hay en el parent para encontrar la referencia
foreach(Form f in this.MdiChildren) {
if(f is Form2) {
childForm = (Form2) f;
break;
}
}
//Si la encontró ya no necesita la instancia solo la mostramos
if( childForm != null) {
childForm.Show();
childForm.Focus();
}
else {
//No se ha creado por lo tanto generamos la instancia
childForm = new Form2();
childForm.MdiParent = this;
childForm.Show();
childForm.Focus();
}

}

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim childForm As Form2
//Barremos las formas que hay en el parent para encontrar la referencia
For Each f As Form In Me.MdiChildren
If TypeOf f Is Form2 Then
childForm = CType(f, Form2)
Exit For
End If
Next
'Si la encontró ya no necesita la instancia solo la mostramos
If Not (childForm Is Nothing) Then
childForm.Show()
childForm.Focus()
Else
'No se ha creado por lo tanto generamos la instancia
childForm = New Form2
childForm.MdiParent = Me
childForm.Show()
childForm.Focus()
End If
End Sub

private void button1_Click(object sender, System.EventArgs e) {
foreach (Form f in this.OwnedForms) {
if (f is Form2) {
f.Show();
f.Focus();
return;
}
}

Form2 f2 = new Form2();
this.AddOwnedForm(f2);
f2.Owner = this;
f2.Show();

}

private void Form2_Closing(object sender, System.ComponentModel.CancelEventArgs e) {
Owner.RemoveOwnedForm(this);
}

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

For Each f As Form In Me.OwnedForms
If TypeOf f Is Form2 Then
f.Show()
f.Focus()
Exit Sub
End If
Next

Dim f2 As New Form2
Me.AddOwnedForm(f2)
f2.Owner = Me
f2.Show()

End Sub

Private Sub Form2_Closing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
Owner.RemoveOwnedForm(Me)
End Sub