How to use C# string Insert

using System;
using System.Windows.Forms;

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string str = "This is CSharp Test";
            string insStr = "Insert ";
            string strRes = str.Insert(15, insStr);
            MessageBox.Show(strRes); 
        }
    }
}

When you execute this C# program you will get the message box 

showing "This is CSharp Insert Test"