Описание тега null-coalescing

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The concept of setting a default value if a condition evaluates to null.

In C#, the ?? operator is known as the null coalescing operator. It returns the first operand unless it's null, in which case it returns the second operand:

string foo = null;
Console.WriteLine(foo ?? "bar"); // Outputs "bar"

foo = "baz";
Console.WriteLine(foo ?? "bar"); // Outputs "baz"

There are a few equivalent ideas in other languages but not all languages have an equivalent operator. As of Perl 5.10, one can use the // operator to set a default value if the condition evaluates to undef.