Use the Remove method to remove a key/value pair.
using System; using System.Collections.Generic; public class Example { public static void Main() { Dictionary<string, string> openWith = new Dictionary<string, string>(); openWith.Add("A", "a"); openWith.Add("B", "b"); openWith.Add("C", "c"); openWith.Remove("A"); if (!openWith.ContainsKey("A")) { Console.WriteLine("Key \"A\" is not found."); } } }