How to find the first index of a sub-string in a given string, C#

How to find the first index of a sub-string in a given string

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //How to find the first index of a substring in a given string?
            //use question as given string and find the index of the substring "fir".
            //Builin array functions and LINQ are not allowed.
            
            string strMainString = "How to find the first index of a sub-string in a given string?" ;
            string srtSubstring = "fir";
            
            int iMainstrLenght = strMainString.Length;
            int iSubstringLenght = srtSubstring.Length;
            bool bfound = false;
            
            for(int i=0 ;i<=iMainstrLenght-iSubstringLenght;i++)
            {
                int j;
                
                for(j=0;j<iSubstringLenght;j++)
                    if(strMainString[i+j]!=srtSubstring[j])
                       break;
                       
                       if(j==iSubstringLenght)
                     {
                        bfound = true;
                        Console.WriteLine("Index is "+i );
                      }
                        
                       
                       
            }
                     if(!bfound)
                       Console.WriteLine("No substring found" );
                
            
           
            
                    }
    }
}


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