using System.Collections.Generic;

using System.Collections.ObjectModel;

using System.Linq;

​

​

public class Struct {

    public int f1 {get; set;}

    public bool f2 {get; set;}

}

​

public class Program

{

    public static void Main()

    {

        var dick = func();

    }

    public static Dictionary<int, ReadOnlyCollection<Struct>> func() {

        var list = (new List<Struct> { new Struct { f1 = 10, f2 = false}, new Struct { f1 = 10, f2 = true}, new Struct { f1 = 11, f2 = false}, new Struct { f1 = 11, f2 = true} });

        var groupped = from l in list

            group  l by l.f1 into lgroup

            select new { key = lgroup.Key, value = lgroup.Select(x=> x) };

        return groupped.ToDictionary(x=>x.key, x=>x.value.ToList().AsReadOnly());

    }

}