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 paramater. Ref
assignments allow ref local variables or parameters, which represent
references to storage locations, to be assigned different references.
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 declaration statement.