The Insert() function in String Class will insert a String in a specified index in the String instance.

string string.Insert(int ind,string str)

Parameters:

ind - The index of the specified string to be inserted.

str - The string to be inserted.

Returns:

String - The result string.

Exceptions:

System.ArgumentOutOfRangeException: startIndex is negative or greater than the length of this instance

System.ArgumentNullException : If the argument is null.

example:

"This is Test".Insert(8,"Insert ") returns "This is Insert Test"

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); 
        }
    }
}