What do two question marks together mean in C#?

In C#, two question marks (??) together form the null-coalescing operator. It's used to provide a concise way to handle null values by providing a default value when the left operand is null.

The general syntax is:

csharp

expression1 ?? expression2

If expression1 is not null, the result is the value of expression1. If expression1 is null, the result is the value of expression2.

Here's an example:

csharp

int? nullableNumber = null;
int result = nullableNumber ?? 5;

Console.WriteLine(result); // Output: 5

In this example, nullableNumber is a nullable integer that has a value of null. The null-coalescing operator is used to provide a default value of 5 when nullableNumber is null. As a result, the value of result becomes 5.

If nullableNumber had a non-null value, result would have taken that value instead:

csharp

int? nullableNumber = 10;
int result = nullableNumber ?? 5;

Console.WriteLine(result); // Output: 10

In this case, since nullableNumber has a value of 10, the value of result becomes 10, and the default value (5) is not used.

The null-coalescing operator is particularly useful when you want to provide a default value for nullable types or when dealing with potentially null expressions.