How to get limited number of outputs from the query c#

You could also try this syntax...

var key = dict.FirstOrDefault(v => v.Value == "Peter").Key;

Edit: Added code for ease of understanding / copy-pasting... Rextester - http://rextester.com/AIAKRZ95654

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

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {

            Dictionary<int, string> dict = new Dictionary<int, string>();

            dict.Add(1, "Jack");
            dict.Add(2, "Peter");
            dict.Add(3, "Chris");
            dict.Add(4, "Peter");

            var key = dict.FirstOrDefault(v => v.Value == "Peter").Key;

            Console.WriteLine(key);
        }
    }
}

UPDATE: Please note when using FirstOrDefault() keyword, ?.key is not required, worst case it will return 0. The ?.key is required when using First(). Because of this confusion Flater voted this answer down, and deleted his comments from the comment section below. (He probably should have acknowledge and mentioned this factor to give other programmers a heads up / caution sign about this subtle difference between First() and FirstOrDefault())