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A null coalescing operator, in C#, is an operator that is used to check whether the value of a variable is null. It is represented by the symbol "??".
The null coalescing operator allows for the selection of the first non-null value from a pair of values. It is used to set the default value for variables of nullable value type or reference type. It can be used to build an expression with simplified syntax that is logically equivalent to an expression using an if statement or ternary operator in a more compact form. The expression containing the null-coalescing operator has minimal source code and provides better readability.
In applications such as those related to database and XML data, variables can occur in an undefined state, which implies that they are not set to any proper value. A null-coalescing operator is used to check such a variable (of nullable type) for null. If the variable is null, the null-coalescing operator is used to supply the default value while assigning to a variable of non-nullable type.
The null-coalescing operator is a binary operator that is used in a conditional expression of the form, “a??b”, where the expression in the left-hand operand, "a", must be nullable type or reference type. If "a" is not evaluated as null, it returns the result of the evaluation of "a". Otherwise, the expression in the right-hand operand, "b", is evaluated to obtain the result of the entire expression.
For example, the null-coalescing operator can be used in a property of an object for returning a custom default value to avoid returning a null value.
The assignment of a nullable value type variable to a non-nullable type results in a compiler error, and the use of explicit cast for such an assignment can result in an exception. A null-coalescing operator is used in such assignments to avoid the compiler error and the exception.
The operands used in the expression containing the null-coalescing operator must evaluate to a matching type or should be implicitly converted to a common type.
The operations in an expression that uses the null-coalescing operator are grouped from right to left.