C#

private void newWindowType1ToolStripMenuItem_Click(object sender, EventArgs e)
{
   if (!ContainsChildForm(typeof(child1)))
   {
       child1 c = new child1();
       c.MdiParent = this;
       c.Show();
   }
}

private void newWindowType2ToolStripMenuItem_Click(object sender, EventArgs e)
{
   if (!ContainsChildForm(typeof(child2)))
   {
       child2 c = new child2();
       c.MdiParent = this;
       c.Show();
   }
}

private bool ContainsChildForm(Type t)
{
   for (int i = 0; i < this.MdiChildren.Length; i++)
   {
       if (t.IsInstanceOfType(this.MdiChildren[i]))
       {
           return true;
       }
   }
   return false;
}

DigitalKing wrote:

if (!ContainsChildForm(typeof(child1)))

private bool ContainsChildForm(Type t)
{
   for (int i = 0; i < this.MdiChildren.Length; i++)
   {
       if (t.IsInstanceOfType(this.MdiChildren[i]))
       {
           return true;
       }
   }
   return false;
}

private void CreateUniqueChild(Type t)
{
   int index = GetChildIndex(t);
   if (index == -1)
   {
       Form f = (Form)Activator.CreateInstance(t);
       f.MdiParent = this;
       f.Show();
   }
   else
   {
       this.MdiChildren[index].Focus();
   }
}

private int GetChildIndex(Type t)
{
   for (int i = 0; i < this.MdiChildren.Length; i++)
   {
       if (t.IsInstanceOfType(this.MdiChildren[i]))
       {
           return i;
       }
   }
   return -1;
}

private void newWindowType1ToolStripMenuItem_Click(object sender, EventArgs e)
{
   CreateUniqueChild(typeof(child1));
}

private void newWindowType2ToolStripMenuItem_Click(object sender, EventArgs e)
{
   CreateUniqueChild(typeof(child2));
}

private void newWindowType2ToolStripMenuItem_Click(object sender, EventArgs e)<br />
{<br />
   if (!ContainsChildForm(typeof(child2)))<br />
   {<br />
       child2 c = new child2();<br />
       c.MdiParent = this;<br />
       c.Show();<br />
   }<br />
   else<br />
       c.Activate();<br />
}<br />
<br />
private bool ContainsChildForm(Type t)<br />
{<br />
   for (int i = 0; i < this.MdiChildren.Length; i++)<br />
   {<br />
       if (t.IsInstanceOfType(this.MdiChildren[i]))<br />
       {<br />
           return true;<br />
       }<br />
   }<br />
   return false;<br />
}

private void button1_Click(object sender, EventArgs e)
{
    DialogResult dr = MessageBox.Show("Are you sure you want to close this window?", "Are you sure?", MessageBoxButtons.YesNo, MessageBoxIcon.Question);
    if (dr == DialogResult.Yes)
    {
        this.Close();
    }
    //if anything other than Yes is clicked (like No), then nothing will happen.
}