﻿ The null coalescing operator
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# The null coalescing operator

The ?? operator is called the null coalescing operator.

null-coalescing-expression:
conditional-or-expression
conditional-or-expression ?? null-coalescing-expression

A null coalescing expression of the form a ?? b requires a to be of a nullable type or reference type. If a is non-null, the result of a ?? b is a; otherwise, the result is b. The operation evaluates b only if a is null.

The null coalescing operator is right-associative, meaning that operations are grouped from right to left. For example, an expression of the form a ?? b ?? c is evaluated as a ?? (b ?? c). In general terms, an expression of the form E1 ?? E2 ?? ... ?? EN returns the first of the operands that is non-null, or null if all operands are null.

The type of the expression a ?? b depends on which implicit conversions are available on the operands. In order of preference, the type of a ?? b is A0, A, or B, where A is the type of a (provided that a has a type), B is the type of b (provided that b has a type), and A0 is the underlying type of A if A is a nullable type, or A otherwise. Specifically, a ?? b is processed as follows:

· If A exists and is not a nullable type or a reference type, a compile-time error occurs.

· If b is a dynamic expression, the result type is dynamic. At run-time, a is first evaluated. If a is not null, a is converted to dynamic, and this becomes the result. Otherwise, b is evaluated, and this becomes the result.

· Otherwise, if A exists and is a nullable type and an implicit conversion exists from b to A0, the result type is A0. At run-time, a is first evaluated. If a is not null, a is unwrapped to type A0, and this becomes the result. Otherwise, b is evaluated and converted to type A0, and this becomes the result.

· Otherwise, if A exists and an implicit conversion exists from b to A, the result type is A. At run-time, a is first evaluated. If a is not null, a becomes the result. Otherwise, b is evaluated and converted to type A, and this becomes the result.

· Otherwise, if b has a type B and an implicit conversion exists from a to B, the result type is B. At run-time, a is first evaluated. If a is not null, a is unwrapped to type A0 (if A exists and is nullable) and converted to type B, and this becomes the result. Otherwise, b is evaluated and becomes the result.

· Otherwise, a and b are incompatible, and a compile-time error occurs.

Conditional operator

The ?: operator is called the conditional operator. It is at times also called the ternary operator.

conditional-expression:
null-coalescing-expression
null-coalescing-expression ? expression : expression

A conditional expression of the form b ? x : y first evaluates the condition b. Then, if b is true, x is evaluated and becomes the result of the operation. Otherwise, y is evaluated and becomes the result of the operation. A conditional expression never evaluates both x and y.

The conditional operator is right-associative, meaning that operations are grouped from right to left. For example, an expression of the form a ? b : c ? d : e is evaluated as a ? b : (c ? d : e).

The first operand of the ?: operator must be an expression that can be implicitly converted to bool, or an expression of a type that implements operator true. If neither of these requirements is satisfied, a compile-time error occurs.

The second and third operands, x and y, of the ?: operator control the type of the conditional expression.

· If x has type X and y has type Y then

o If an implicit conversion (§6.1) exists from X to Y, but not from Y to X, then Y is the type of the conditional expression.

o If an implicit conversion (§6.1) exists from Y to X, but not from X to Y, then X is the type of the conditional expression.

o Otherwise, no expression type can be determined, and a compile-time error occurs.

· If only one of x and y has a type, and both x and y, of areimplicitly convertible to that type, then that is the type of the conditional expression.

· Otherwise, no expression type can be determined, and a compile-time error occurs.

The run-time processing of a conditional expression of the form b ? x : y consists of the following steps:

· First, b is evaluated, and the bool value of b is determined:

o If an implicit conversion from the type of b to bool exists, then this implicit conversion is performed to produce a bool value.

o Otherwise, the operator true defined by the type of b is invoked to produce a bool value.

· If the bool value produced by the step above is true, then x is evaluated and converted to the type of the conditional expression, and this becomes the result of the conditional expression.

· Otherwise, y is evaluated and converted to the type of the conditional expression, and this becomes the result of the conditional expression.

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