using System;

using System.Reflection;

using System.Linq;

using System.Text.Json;

​

#nullable enable

​

//https://stackoverflow.com/questions/66018737/multidimensional-arrays-nullable-reference-types-and-type-conversion

//https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/arrays#1721-general

// ... the rank_specifiers are read from left to right before the final non-array element type.

// Example: The type in T[][,,][,] is a single-dimensional array of three-dimensional arrays of two-dimensional arrays of int. end example

​

//https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references#generics

​

public static partial class ArrayExtensions

{

    public static T[]?[,] ToArrayOfNullable<T>(this T [,][] a) => a;

}

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Environment version: {0} ({1}), {2}\n", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version, Environment.OSVersion);

​

        TestSolution2();

    }

​

    static void TestSolution2()

    {

        var nullableArray = (new string?[1,2][]).ToArrayOfNullable();

        nullableArray[0, 0] = null; // No warning

        nullableArray[0, 0] = new string? [] { null }; // No warning

        nullableArray.Dump();

    }

​

    static void TestSolution1()

    {

        var f1 = ArrayExtensions.Create2D<string []>(1, 1); // No error

        f1[0, 0] = null; // Warning as expected

        f1[0, 0] = new string [] { null }; // Warning as expected

        var f2 = ArrayExtensions.Create2D<string? []>(1, 1);

        f2[0, 0] = null; // Warning as expected

        f2[0, 0] = new string?  [] { null }; // No warning as expected

​

        var f3 = ArrayExtensions.Create2D<string? []?>(1, 1);

        f3[0, 0] = null; // No warning as expected

        f3[0, 0] = new string? [] { null }; // No warning as expected

​

        string ?[]?[,] f4 = ArrayExtensions.Create2D<string []?>(1, 1);

        f4[0, 0] = null; // No warning

        f4[0, 0] = new string? [] { null }; // Warning

        f4[0, 0] = new string [] { "" }; // No warning as expected

​

        Console.WriteLine(f3.GetType() == f1.GetType());

​

        string? [][][] a = new string? [][][] { new string? [][] { new string? [] { null } } };

        a[0][0][0] = null;

        string? [,][] b = new string? [,][] { { new string? [] { null } } };

        b[0,0][0] = null;

    }

}

​

public static partial class ArrayExtensions

{

    public static T[,] Create2D<T>(int count1, int count2) => new T[count1, count2];

}

​