"Deconstruct assignment requires an expression with a type on the right hand side"

Question

The following code:

(object ret, Expression subexpr) = expr switch {

    // instance member
    MemberExpression mexpr when mexpr.Expression is not null =>
        (mexpr, mexpr.Expression),

    // instance method call
    MethodCallExpression callExpr when callExpr.Object is not null =>
        (callExpr, callExpr.Object),

    // include instance method calls, or static extension method calls
    MethodCallExpression callExpr when
            callExpr.Method.GetCustomAttributes(typeof(ExtensionAttribute)).Any() &&
            callExpr.Arguments.FirstOrDefault() is not null =>
        (callExpr, callExpr.Arguments.First()),

    _ => (null, null)
};

does not compile, with:

CS8131: Deconstruct assignment requires an expression with a type on the right-hand-side.

Why is this happening? How can I fix it?

Answer 1

This seems like a simple case of "the compiler can't figure out the type of the switch". If you replace the deconstruction with a simple var x = to let the compiler figure out what type the switch expressions should evaluate to, the error message changes to a much more useful:

No best type was found for the switch expression

The tuples that you return each case are of type:

(MemberExpression, Expression)
(MethodCallExpression, Expression)
(MethodCallExpression, Expression)

The compiler probably can't decide what common ancestor of MemberExpression and MethodCallExpression it should use.

Even a simple "hint" like this,

((object)mexpr, mexpr.Expression),

or:

((Expression)mexpr, mexpr.Expression),

so that the compiler doesn't need to find common ancestors, fixes the problem.

Answer 2

Instead of (null, null), use (default(object), default(Expression)) in order to help the compiler to guess the types of your right-hand-side expression.