C# 语言问题：匹配优先级

Question

I'm reading book "C# Language", and hit this example from Page 123-124:

The meaning of a name within a block may differ based on the context in which the name is used.

In the example

using System;
class A { }
class Test
{
  static void Main()
  {
    string A = "hello, world";
    string s = A; // Expression context
    Type t = typeof(A); // Type context
    Console.WriteLine(s); // Writes "hello, world"
    Console.WriteLine(t); // Writes "A"
  }
}  

the name A is used in an expression context to refer to the local variable A and in a type context to refer to the class A.

I'm fine with the visibility of class A. However, here ( Type t = typeof(A) ) class A preceded string A . So, what is the "priority" or "sequence" of matching/choosing a possible "A"?

Answer 1

There's no conflict. typeof only works on class names. To get the Type of an object instance, you use .GetType() .

Answer 2

string A = "hello, world";
string s = A; // Expression context
A a=new A();
Type t = typeof(A); // Type context
Console.WriteLine(s); // Writes "hello, world"
Console.WriteLine(t); // Writes "A"

Here we see one example of an expression context: string s = A . In the expression context the local variable takes precedence over the class.

When a type context is used:

• Inside typeof(A)
• When declaring a variable A a =...
• After the new keyword: new A()

Only the type is considered. Since in that context A referring to a variable would result in invalid grammar its clear that the type is meant and thus the specification allows it.

One case where the rule is a bit annoying is when you want to refer to a static member of the class. For example A.CallStaticMethod() . Here you have an expression context and it refers to the variable A and not the class A .