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