Syntax:

public static string ChangeExtension(
    string path,
    string extension
)

Example:

using System;
using System.IO;

public class CPath{

    public static void Main() {     
        string file1 = @"C:\mydir\myfile.com.extension";
        string file2 = @"C:\mydir\";
        string result;

        result = Path.ChangeExtension(file1, ".old");
    
           // result will conatin  'C:\mydir\myfile.com.old'

        result = Path.ChangeExtension(file1, "");
        
  
        // result will conatin   'C:\mydir\myfile.com.'
        //If you don’t specify an extension, the current extension is remove

        result = Path.ChangeExtension(file2, ".old");
                
      // result will contain 'C:\mydir\.old'
 
   }
}