Null-Coalescing Assignment Operator In C# 8.0

Introduction

C# 8.0 introduces the null-coalescing assignment operator ??=. You can use the ??= operator to assign the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null. In C# 7.3, the null-coalescing operator ?? also had a few requirements that are now required in C# 8.0. We will discuss this also in detail.

Prerequisites

To use the Null-coalescing assignment operator ??=, you'll need to set up your machine to run .NET Core, including the C# 8.0 compiler. The C# 8 compiler is available starting with Visual Studio 2019 version 16.3 or .NET Core 3.0 SDK.

Let's Start

Use null-coalescing assignment operator ??= to assign the value of the right-hand side operand to the left-hand side operand only if the left-hand side operand evaluates to null.

For instance, in the below-given code, I'm initializing a List of int named "numbers" with the null reference. Then I'm using the Null-coalescing assignment operator to assign a new List of int if " the number; value is null.

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = null;
        int? a = null;
        (numbers ??= new List<int>()).Add(5);
        Console.WriteLine(string.Join(" ", numbers));  // output: 5
        numbers.Add(a ??= 0);
        Console.WriteLine(string.Join(" ", numbers));  // output: 5 0
        Console.WriteLine(a);  // output: 0
    }
}

In previous versions of C# like 7.3, we were using other techniques to do the same task as performed by the Null-coalescing assignment operator??. For instance, I'm using the Null-coalescing operator to do the same, but it seems a little bit cheap.

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        List<int> numbers = null;
        int? a = null;
        (numbers = numbers ?? new List<int>()).Add(5);
        Console.WriteLine(string.Join(" ", numbers));  // output: 5
        numbers.Add((a = a ?? 0).Value);
        Console.WriteLine(string.Join(" ", numbers));  // output: 5 0
        Console.WriteLine(a);  // output: 0
    }
}

Null-coalescing operator updated requirements

In C# 7.3 and earlier, the type of the left-hand operand of the ?? operator must be either a reference type or a nullable value type. Beginning with C# 8.0, that requirement is replaced with the following: the type of the left-hand operand of the ?? and ??= operators cannot be a non-nullable value type, for instance.

int? number = null;
number = number ?? 0;

In particular, beginning with C# 8.0, you can use the null-coalescing operators with unconstrained type parameters. For instance, I'm using a Null-coalescing operator with a non-nullable type, and it gives the error.

int number = 5;
number = number ?? 0;

So when I tried to build this, it gave an error.

ErrorCS0019Operator '??' cannot be applied to operands of type 'int' and 'int'

For more new features of C# 8.0, please see MS Docs.