Testing nullability with arrays

app.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

Program.cs

using System;
using static System.Console;

// Goal: test various combinations of nullability and non-nullability with arrays.
// I was having some trouble working out the syntax for nullability with arrays.
// I ended up at: https://www.reddit.com/r/csharp/comments/dd2ljw/nullable_reference_types_and_arrays/
// This test is intended to demonstrate the syntax and behavior of nullable reference types and arrays (obviously).
// The test code is the same for each test. Where the code isn't appropriate (won't compile), the code is just commented out.
// I used scopes so that I could use the same variable names throughout.
// The point isn't that the tests return different results (that's inherent in the code that was written), 
// but in the pattern that is used to make the same(ish) code work correctly for all four combinations.

// There are four tests:
// - Nullable array variable with nullable elements
// - Nullable array variable with non-nullable elements
// - Non-nullable array variable with nullable elements
// - Non-nullable Array variable with non-nullable elements

// Do you have to use and know all four of these combinations to use nullable reference types and arrays?
// Yes. You don't have to learn and remember the combinations.
// You just need to know one main thing. There are two places to put the nullable '?' operator with arrays (using string[] as the example):
// string?[]? stringArray;
// The first `?` is refering to the string elements in the array. The second one is referring to the aray itself.
// We can compare this  to a non-array type like string.
// string? myString;
// In this case, since string is not an array (ignoring the fact that it exposes an idexer of type char), the `?` that it is marked with
// is the same as the second `?` with stringArray, not the first. The first one (in the middle, not the end, of the type definition)
// is unique to arrays.

{
    // Test 1
    string description = "Nullable array variable with nullable elements";
    WriteLine($"**{description}**");
    string?[]? stringArray = null;

    Write($"Test: is {nameof(stringArray)} an object? Answer: ");
    if (stringArray is object)
    {
        WriteLine($"Yes; Length: {stringArray.Length}");
    }
    else
    {
        WriteLine("No");
    }

    stringArray = new string[2];
    stringArray[0] = description;
    stringArray[0] = null;

    Write($"Test: is {nameof(stringArray)}[0] an object? Answer: ");
    if (stringArray[0] is object)
    {
        WriteLine($"Yes; Value: {stringArray[0]}; Length: {stringArray[0]!.Length}");
    }
    else
    {
        WriteLine("No");
    }

    Write($"Test: is {nameof(stringArray)}[1] an object? Answer: ");
    if (stringArray[1] is object)
    {
        WriteLine($"Yes; Value: {stringArray[1]}; Length: {stringArray[1]!.Length}");
    }
    else
    {
        WriteLine("No");
    }

    stringArray = null;

    string? element = stringArray?[0];

    Write($"Test: is {nameof(element)} an object? Answer: ");
    if (element is object)
    {
        element = description;
        WriteLine($"Yes; Value: {element}; Length: {element.Length}");
    }
    else
    {
        WriteLine("No");
    }
}

{
    // Test 2
    string description = "Nullable array variable with non-nullable elements";
    WriteLine($"**{description}**");
    string[]? stringArray = null;

    Write($"Test: is {nameof(stringArray)} an object? Answer: ");
    if (stringArray is object)
    {
        WriteLine($"Yes; Length: {stringArray.Length}");
    }
    else
    {
        WriteLine("No");
    }

    stringArray = new string[2];
    stringArray[0] = description;
    // stringArray[0] = null;

    Write($"Test: is {nameof(stringArray)}[0] an object? Answer: ");
    if (stringArray[0] is object)
    {
        WriteLine($"Yes; Value: {stringArray[0]}; Length: {stringArray[0]!.Length}");
    }
    else
    {
        WriteLine("No");
    }

    Write($"Test: is {nameof(stringArray)}[1] an object? Answer: ");
    if (stringArray[1] is object)
    {
        WriteLine($"Yes; Value: {stringArray[1]}; Length: {stringArray[1]!.Length}");
    }
    else
    {
        WriteLine("No");
    }

    stringArray = null;

    string? element = stringArray?[0];

    Write($"Test: is {nameof(element)} an object? Answer: ");
    if (element is object)
    {
        element = description;
        WriteLine($"Yes; Value: {element}; Length: {element.Length}");
    }
    else
    {
        WriteLine("No");
    }
}

{
    // Test 3
    string description = "Non-nullable array variable with nullable elements";
    WriteLine($"**{description}**");
    string?[] stringArray = new string[2];

    Write($"Test: is {nameof(stringArray)} an object? Answer: ");
    if (stringArray is object)
    {
        WriteLine($"Yes; Length: {stringArray.Length}");
    }
    else
    {
        WriteLine("No");
    }

    stringArray = new string[2];
    stringArray[0] = description;
    stringArray[0] = null;

    Write($"Test: is {nameof(stringArray)}[0] an object? Answer: ");
    if (stringArray[0] is object)
    {
        WriteLine($"Yes; Value: {stringArray[0]}; Length: {stringArray[0]!.Length}");
    }
    else
    {
        WriteLine("No");
    }

    Write($"Test: is {nameof(stringArray)}[1] an object? Answer: ");
    if (stringArray[1] is object)
    {
        WriteLine($"Yes; Value: {stringArray[1]}; Length: {stringArray[1]!.Length}");
    }
    else
    {
        WriteLine("No");
    }

    // stringArray = null;

    string? element = stringArray?[0];

    Write($"Test: is {nameof(element)} an object? Answer: ");
    if (element is object)
    {
        element = description;
        WriteLine($"Yes; Value: {element}; Length: {element.Length}");
    }
    else
    {
        WriteLine("No");
    }
}

{
    // Test 4
    string description = "Non-nullable Array variable with non-nullable elements";
    WriteLine($"**{description}**");
    string[] stringArray = new string[2];

    Write($"Test: is {nameof(stringArray)} an object? Answer: ");
    if (stringArray is object)
    {
        WriteLine($"Yes; Length: {stringArray.Length}");
    }
    else
    {
        WriteLine("No");
    }

    stringArray = new string[2];
    stringArray[0] = description;
    // stringArray[0] = null;

    Write($"Test: is {nameof(stringArray)}[0] an object? Answer: ");
    if (stringArray[0] is object)
    {
        WriteLine($"Yes; Value: {stringArray[0]}; Length: {stringArray[0]!.Length}");
    }
    else
    {
        WriteLine("No");
    }

    Write($"Test: is {nameof(stringArray)}[1] an object? Answer: ");
    if (stringArray[1] is object)
    {
        WriteLine($"Yes; Value: {stringArray[1]}; Length: {stringArray[1]!.Length}");
    }
    else
    {
        WriteLine("No");
    }

    // stringArray = null;

    string? element = stringArray?[0];

    Write($"Test: is {nameof(element)} an object? Answer: ");
    if (element is object)
    {
        element = description;
        WriteLine($"Yes; Value: {element}; Length: {element.Length}");
    }
    else
    {
        WriteLine("No");
    }
}

Results

**Nullable array variable with nullable elements**
Test: is stringArray an object? Answer: No
Test: is stringArray[0] an object? Answer: No
Test: is stringArray[1] an object? Answer: No
Test: is element an object? Answer: No
**Nullable array variable with non-nullable elements**
Test: is stringArray an object? Answer: No
Test: is stringArray[0] an object? Answer: Yes; Value: Nullable array variable with non-nullable elements; Length: 50
Test: is stringArray[1] an object? Answer: No
Test: is element an object? Answer: No
**Non-nullable array variable with nullable elements**
Test: is stringArray an object? Answer: Yes; Length: 2
Test: is stringArray[0] an object? Answer: No
Test: is stringArray[1] an object? Answer: No
Test: is element an object? Answer: No
**Non-nullable Array variable with non-nullable elements**
Test: is stringArray an object? Answer: Yes; Length: 2
Test: is stringArray[0] an object? Answer: Yes; Value: Non-nullable Array variable with non-nullable elements; Length: 54
Test: is stringArray[1] an object? Answer: No
Test: is element an object? Answer: Yes; Value: Non-nullable Array variable with non-nullable elements; Length: 54
PS D:\git\testapps\nullablearrays> dotnet run
**Nullable array variable with nullable elements**
Test: is stringArray an object? Answer: No
Test: is stringArray[0] an object? Answer: No
Test: is stringArray[1] an object? Answer: No
Test: is element an object? Answer: No
**Nullable array variable with non-nullable elements**
Test: is stringArray an object? Answer: No
Test: is stringArray[0] an object? Answer: Yes; Value: Nullable array variable with non-nullable elements; Length: 50
Test: is stringArray[1] an object? Answer: No
Test: is element an object? Answer: No
**Non-nullable array variable with nullable elements**
Test: is stringArray an object? Answer: Yes; Length: 2
Test: is stringArray[0] an object? Answer: No
Test: is stringArray[1] an object? Answer: No
Test: is element an object? Answer: No
**Non-nullable Array variable with non-nullable elements**
Test: is stringArray an object? Answer: Yes; Length: 2
Test: is stringArray[0] an object? Answer: Yes; Value: Non-nullable Array variable with non-nullable elements; Length: 54
Test: is stringArray[1] an object? Answer: No
Test: is element an object? Answer: Yes; Value: Non-nullable Array variable with non-nullable elements; Length: 54