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The null-coalescing operator ?? returns the value of its left-hand operand if it isn’t null; otherwise, it evaluates the right-hand operand and returns its result.

The ?? operator doesn’t evaluate its right-hand operand if the left-hand operand evaluates to non-null.

int? myValue = null;
int result = myValue ?? -1;

//result = -1;

Syntax:

p ?? q

Here, p is the left and q is the right operand of ?? operator. The value of p can be nullable type, but the value of q must be non-nullable type. If the value of p is null, then it returns the value of q. Otherwise, it will return the value of p.

Important Points:

• The ?? operator is used to check null values and you can also assign a default value to a variable whose value is null(or nullable type).
• You are not allowed to overload ?? operator.
• It is right-associative.
• In ?? operator, you can use throw expression as a right-hand operand of ?? operator which makes your code more concise.
• You are allowed to use ?? operator with value types and reference types.

The null-coalescing assignment operator ??= assigns the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null.

The ??= operator doesn’t evaluate its right-hand operand if the left-hand operand evaluates to non-null.

int? myValue = null; 
myValue ??= 0; 

//myValue = 0;