using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Dictionary<ClassA, List<ClassB>> MyDict = Enumerable.Range(0,10).Select(i => i).ToDictionary(

        i => new ClassA(){ Name = i.ToString(), Id = i},

        i => new List<ClassB>(){ 

            new ClassB(){ Tag = i.ToString(), Text = i.ToString() },

            new ClassB(){ Tag = (i+10).ToString(), Text = (i+10).ToString() }

        });

        IEnumerable<KeyValuePair<ClassA,List<ClassB>>> q = MyDict.ToDictionary(k => k.Key, k => k.Value.Where(b => b.Tag == "10").ToList()).Where(kv => kv.Value.Any());

        Console.WriteLine(q.First().Value.First().Tag);

    }

}

​

public class ClassA {

    public string Name;

    public int Id;

 }

​

public class ClassB {

    public string Tag;

    public string Text;

 }