EN CSharp Operator Null Coalescing

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Value at null

Handbook C# · Practical Guide

?? selects a spare value only at null; ??= writes it only if the left part is still null.

Simple example (Example.cs)

Save this example separately as Example.cs. Run it independently of the other examples on this page.

Example.cs:

// PROJECT-EXAMPLE
string? name = null;
name ??= "Player";
System.Console.WriteLine(name);

Result:

Player

Syntax

value ?? fallback / value ??= fallback

Example 1

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        string? name = null;
        Console.WriteLine(name ?? "Unknown");
        name ??= "Archer";
        Console.WriteLine(name);
    }
}

Example 2

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        List<int>? values = null;
        (values ??= []).Add(3);
        Console.WriteLine(values.Count);
    }
}

Official source: Microsoft Learn.