using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var test2s = new List<Test2>();

        var test2sLI = new Test2() { TestString = "TestString"};

        test2s.Add(test2sLI);

        var test2s2 = new List<Test2>();

        var test2sLI2 = new Test2() { TestString = "test"};

        test2s2.Add(test2sLI2);

        var tests = new List<Test>();

        var test = new Test() {What = "What", Test2s = test2s};     

        var test2 = new Test() {What = "What", Wut = "Wut"};

        var test3 = new Test() {What = "What", Wut = "Wut", Test2s = test2s2};

        tests.Add(test);

        tests.Add(test2);

        tests.Add(test3);

        var filtered = tests.Where(x => x.Test2s?.FirstOrDefault()?.TestString != "test").ToList();

        //filtered = tests.Where(x => x.Test2s != null && x.Test2s?.FirstOrDefault()?.TestString != "test").ToList();

        foreach(var i in filtered) {

            Console.WriteLine("testing: " + i.What + " and: " + i.Wut);

        }

    }

}

​

public class Test {

    public string What {get;set;}

    public string Wut {get;set;}

    public List<Test2> Test2s {get;set;}

}

​

public class Test2 {

    public string TestString {get;set;} 

}