C# Language Specification

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 ?? ... ?? E_N 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.
  

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