Q: How to open child form with a button on another child form in side the mdi parent form I have 3 forms MainForm, Child1 and Child2 . MainForm is the mdi container. At runtime child 1 is automatically loaded inside the MainForm. I am  trying to open the form child2 from a button on child1 but i cannot seem to do it. I did everything i could. Please help.
I tried using the hide show and activate methods but nothing worked. 
The child1 and child2 forms are separately built. 
public partial class Form1 : Form
    {
        public child1 c1;
        public child2 c2;

        public void LoadChild2()
        {
            if (c2 == null)
            {
                c2 = new child2() ;
                c2.MdiParent = this;
                c2.FormClosed += c2_FormClosed;
                c2.FormBorderStyle = FormBorderStyle.None;
                c2.Dock = DockStyle.Fill;
                c2.Show();
            }
            else { c2.Activate(); }


        }

        void c2_FormClosed(object sender, FormClosedEventArgs e)
        {
            c2 = null;

        }
        public void LoadChild1()
        {
            if (c1 == null)
            {

                c1 = new child1();
                c1.MdiParent = this;
                c1.FormClosed += c1_FormClosed;
                c1.FormBorderStyle = FormBorderStyle.None;
                c1.Dock = DockStyle.Fill;
                c1.Show();
            }
            else { c1.Activate(); }

        }

        void c1_FormClosed(object sender, FormClosedEventArgs e)
        {
            c1 = null;
            //throw new NotImplementedException();
        }
        public Form1()
        {
            InitializeComponent();
            LoadChild1();

           // c2 = new child2();
          //  c1 = new child1();

       // c2.MdiParent = this;
       // c1.MdiParent = this;
        //c1.WindowState = FormWindowState.Maximized;
       // c1.FormBorderStyle = FormBorderStyle.None;
        //c1.Dock = DockStyle.Fill;
       // c2.FormBorderStyle = FormBorderStyle.None;
        //c2.Dock = DockStyle.Fill;
       // c2.Show();
       // c1.Show();

        //c1.Activate(); 
    }

     public partial class child1 : Form
{
    //public child2 c2;

    public child1()
    {
        InitializeComponent();   
    }
    private void child1_Load(object sender, EventArgs e)
    {

    }
    private void button1_Click(object sender, EventArgs e)
    {          
        Form1 f = new Form1();      
        f.LoadChild2();
        //child2 c2 = new child2();
        //Form1 f = new Form1();
        //c2.FormBorderStyle = FormBorderStyle.None;
       // c2.Dock = DockStyle.Fill;
        //c2.MdiParent = f;
       // c2.Activate();
       // c2.BringToFront();
       // this.Hide();
        //this.Close();
    }
}

     public partial class child2 : Form
{
    public child2()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {

        //this.Close();
        Form1 f = new Form1();
        f.LoadChild1();
       // child1 c1 = new child1();
       // c1.FormBorderStyle = FormBorderStyle.None;
       // c1.Dock = DockStyle.Fill;

       // c1.Parent = f;
       // c1.Show();
       // c1.Activate();

    }

    private void child2_Load(object sender, EventArgs e)
    {

    }
}


A: Make sure to set the MdiParent property; if spawning the new form from a Click event handler of a button on an existing MdiChild, you can just set it to that child's MdiParent:
// Event handler inside MdiChild form (Child1)
private void spawnNewChildButton_Click(object sender, EventArgs e)
{
    // Child2 is a System.Windows.Forms.Form
    var child2 = new Child2() { MdiParent = MdiParent };
    child2.Show();
}

