c# 9.0 - Roslyn uses MemberAccessExpressionSyntax instead of QualifiedNameSyntax

Question

In the following code, I'd expect that both references to System.Action type to be represented as a QualifiedNameSyntax but the second one is represented as a MemberAccessExpressionSyntax.

Is that correct? If so, why can't it be a QualifiedNameSyntax?

class Foo
{
    public void M(object o)
    {
        var t = typeof(System.Action); // 1
        switch(o)
        {
            case System.Action: // 2
                break;
        }
    }
}

question from:https://stackoverflow.com/questions/66047461/roslyn-uses-memberaccessexpressionsyntax-instead-of-qualifiednamesyntax

Answer 1

Generally you're only going to get a QualifiedNameSyntax in a Roslyn syntax tree where the only legal thing there is a qualified name; in those cases we're running a restricted parser that will only understand qualified names. Anything else we're running our generic expression parser which will spit out whatever expression is there, and we'll figure out what it actually is during binding. Because consider another case like:

SomeEnum e;
switch (e)
{
     case SomeEnum.Blue: Console.WriteLine("Blue!"); return;
}

In that case the SomeEnum.Blue is absolutely an access to a member. But we don't actually know what "SomeEnum" is until binding, so we just always go with MemberAccessExpression.