Ref Local and Parameter Reassignment

In C# 7.2, ref locals and parameters cannot be assigned new references, only assigned values which reassign the underlying storage to the given value.

C# 7.3 introduces a new type of expression, a ref-assignment expression.

The syntax is as follows

ref_assignment
    : identifier '=' 'ref' expression

The identifier on the left-hand side of a ref_assignment must be a ref or ref readonly local variable, or an in, ref, or out parameter. Ref assignments allow ref local variables or parameters, which represent references to storage locations, to be assigned different references. This does not include the this reference, even in structs.

The expression on the right-hand side must have a storage location, i.e. it must be an "l-value." This l-value must be "ref-compatible" with the variable being ref-assigned, where ref-compatible means:

• The type of left-hand side and the type of the right-hand side must have an identity conversion between them.
• Writeable ref variables cannot be assigned read-only l-values. Readonly ref variables can be assigned either read-only or writeable l-values.

The result of the ref-assignment expression is itself an "l-value" and the resulting storage location is read-only if and only if the left-hand side of the assignment is a read-only ref variable.

The storage location for the right-hand side must be definitely assigned before it can be used in a ref-assignment expression. In addition, the storage location referenced by an out parameter must be definitely assigned before the out parameter is ref-reassigned to a new reference.

The lifetime for a ref local or parameter is fixed on declaration and cannot be reassigned with ref-reassignment. The lifetime requirements for the right-hand expression are the same as for the right-hand expression in a variable assignment, identical to the lifetime for Span<T> assignments.