Proposal: Bind to existing variables in patterns

[alrz]

Proposal: Bind to existing variables in patterns

Ability to assign the target expression to existing variables (rel. #8990, #9005):

public static bool Member(this Expression @this, out Expression expr, out string memberName) {
  if(@this is MemberExpression { Expression: var exprTemp, Member: { Name: var memberNameTemp } }) {
    expr = exprTemp;
    memberName = memberNameTemp
    return true;
  } else {
    expr = null;
    memberName = null;
    return false;
  }
}

We can rewrite this method in one line if we could assign the target expression to existing variables inside the pattern.

public static bool Member(this Expression @this, out Expression expr, out string memberName) {
  return @this is MemberExpression { Expression: out expr, Member: { Name: out memberName } }
}

That's it. If the pattern-match failed, variables will be assigned to their default value, therefore, they are definitely assigned before leaving the method and no CS0177 shall be raised.

This is specifically useful with #11293 and #13401.

Note: When we are matching against the static type of a property of another property it could be simplified further using the dot operator:

public static bool Member(this Expression @this, out Expression expr, out string memberName) {
  return @this is MemberExpression { Expression: out expr, Member.Name: out memberName }
}

etc.

[HaloFour]

I agree. With deconstruction following the out parameter route and possibly active patterns as well I think it would make for some succinct one-liners to be able to match and deconstruct directly into existing variables. As pattern variables are no longer readonly I think that the inconsistency concerns are no longer a factor.