using System;

using System.Collections.Generic;

using System;

using System.Linq;

using System.Xml;                   

using System.Xml.Linq;

using System.Collections; 

using System.Collections.Generic;

using System.Data;

using System.Data.DataSetExtensions;

using System.Data.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Xml.Serialization; 

using System.IO;

using System.Globalization;

using System.Diagnostics;

using System.Collections.Concurrent;

public class Program

{

    public static void Main()

    {

        List<string> searchFor = new List<string>() { "123111117", "123456123", "12159785" };

        foreach(var s in searchFor) {

           var r = FindMatch(s);

           if(r.code == null) {

               Console.WriteLine(string.Format("No match for {0}", s));

           } else {

               Console.WriteLine( string.Format( "{0} is {1}", s, r.name ) );

           }

        }

    }

    public static List<Lookup> lookup =new List<Lookup>() {

    new Lookup {

        code = "12345",

        name = "abc"

    },

    new Lookup {

        code = "123",

        name = "def"

    },

    new Lookup {

        code = "123456",

        name = "ghi"

    }

};

​

    public static Lookup FindMatch(string valueToSearchInLinq) {

        // find the first result or null if no match found

        var result = lookup.Select(x=> new {x, count = valueToSearchInLinq.StartsWith(x.code)? x.code.Length : -1}) 

                           .OrderByDescending(x=>x.count);      

        return result.All(x=>x.count <0)? new Lookup() : result.FirstOrDefault().x;

    }

}

​

public struct Lookup {

    public string code;

    public string name;

}

​

​