Code
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace ddd
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            //Form2 fr2 = new Form2();
            //fr2.Name = "fr2";
            //fr2.MdiParent = this;
            //fr2.Show();
            Form3 fr3 = new Form3();
            fr3.Name = "fr3";
            fr3.MdiParent = this;
            fr3.Show();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 fr2 = new Form2();
            fr2.Name = "fr2";
            openForm(fr2);
        }
        private void openForm(Form form)
        {
            if (this.MdiChildren.Length == 0)
            {
                form.MdiParent = this;
                form.Show();
            }
            else
            {
                int j = 0;
                for (int i = 0; i < this.MdiChildren.Length; i++)
                {
                    if (this.MdiChildren[i].Name.Equals(form.Name))
                    {
                        j++;
                    }
                }
                if (j > 0)
                {
                    form.Activate();
                }
                else
                {
                    form.MdiParent = this;
                    form.Show();
                }
            }
        }
    }
}

Code

                    for (int i = 0; i < this.MdiChildren.Length; i++)
                    {
                        if (this.MdiChildren[i].Name.Equals(form.Name))
                        {
                            this.MdiChildren[i].Activate();
                        }
                    }

Code

        private void openForm(Form form)
        {
            string name = form.Name;
            foreach (Form item in this.MdiChildren)
            {
                if (item.Name.Equals(name))
                {
                    item.Activate();
                    return;
                }
            }
            form.MdiParent = this;
            form.Show();
        }