Aggregate

CSharp · Console

/Program.cs

using System;
using System.Linq;
					
public class Program
{
	public static void Main()
	{
		// Organize the strings into a collection, say, an array 
        // (Set, List etc. will do as well)
        string[] lines = {
           "vJrwpWtwJgWrhcsFMMfFFhFp",
           "jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL",
           "PmmdzqPrVvPwwTWBwg",
        };

        // Query the collection above with a help of Linq
        string commonString = string.Concat(lines
            .Select(line => line.AsEnumerable()) // we deal with IEnumerable<char>, not string
            .Aggregate((s, a) => s.Intersect(a))
            .OrderBy(c => c) // In case you want common characters being ordered
        );
		
		Console.Write(commonString); 
	}
}