using System;

using System.Collections.Generic;

​

#nullable enable

public static class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var list = new List<string?> { "a", null, "b" };

        var results = ToNonNullDictionaryBlue(list, value => value.ToUpperInvariant()!);

        Console.WriteLine(string.Join(", ", results));

    }

    //Takes a list and a value selector function. Returns a dictionary populated only with non-null values.

    private static IDictionary<T1, T2> ToNonNullDictionaryBlue<T1, T2>(

     IEnumerable<T1> list,

     Func<T1, T2?> valueSelector

    )

     where T1 : notnull

    {

     var dictionary = new Dictionary<T1, T2>();

     foreach (var item in list)

     {

      var value = valueSelector(item);

      if (value != null)

      {

       dictionary[item] = value;

      }

     }

     return dictionary;

    }

}

​