How Can I Hide And Show The Tree View Left Side Menu Item ?

imho, Windows MDI application structure is kind of a "fossil" these days, and I would encourage you to, instead, consider writing an application with multiple independent Forms. But, the fact remains that a lot of people are still using MDI, and many students are being assigned projects requiring them to write MDI applications.

In the Windows MDI architecture, you should not add Controls to the MDIParentForm directly; they will obscure (be shown in front of) all MDIChild Windows their bounding-boxes intersect with.

Under-the-hood the MdiParentForm hosts another Window of a special Type: "System.Windows.Forms.MdiClient." When you make a new instance of a Form the MDIChild of an MDIParentForm, the Form is added to the MdiClient's ControlCollection.

Consider this: create one MDIChildForm to hold the TreeView, and use its Dock, or Anchor, Properties to position it where you want it to be.

If you defined two Forms, 'ChildForm1, and 'ChildForm2, and Form 'ChildForm1 had a TreeView docked inside it: your MDIParentForm Load event might look like this:

ChildForm1 child1 = new ChildForm1();
ChildForm2 child2 = new ChildForm2();

private void MdiParentForm_Load(object sender, System.EventArgs e)
{
    // hide the close box ?
    child1.ControlBox = false;
    // borderless, without Caption ?
    child1.FormBorderStyle = FormBorderStyle.FixedToolWindow;
    child1.Text = "";

    child1.MdiParent = this;
    child1.Dock = DockStyle.Left;

    child2.MdiParent = this;
    child2.Dock = DockStyle.Right;

    child1.Show();
    child2.Show();     
}