Why does Type.IsByRef for type String return false if String is a reference type?

Question

According to this a string (or String) is a reference type.

Yet given:

Type t = typeof(string);

then

if (t.IsByRef) ...    

returns false

why?

Edit: After some quick testing, I'm obviously misunderstanding the purpose of IsByRef... as even using a class name in place of 'string' ,returns false as well. I'm writing a generic class and want to test if one the types passed in when the generic is instantiate is a value or reference type. How does one test for this?

Answer 1

There are "reference types" -- for which we have !type.IsValueType -- and then there are types that represent references to anything -- whether their targets are value types or reference types.

When you say void Foo(ref int x), the x is said to be "passed by reference", hence ByRef.
Under the hood, x is a reference of the type ref int, which would correspond to typeof(int).MakeReferenceType().

Notice that these are two different kinds of "reference"s, completely orthogonal to each other.

(In fact, there's a third kind of "reference", System.TypedReference, which is just a struct.
There's also a fourth type of reference, the kind that every C programmer knows -- the pointer, T*.)

Answer 2

You should use IsValueType instead:

bool f = !typeof (string).IsValueType; //return true;

As for IsByRef, the purpose of this property is to determine whether the parameter is passed into method by ref or by value.

Example you have a method which a is passed by ref:

public static void Foo(ref int a)
{
}

You can determine whether a is pass by reference or not:

  bool f = typeof (Program).GetMethod("Foo")
                                 .GetParameters()
                                 .First()
                                 .ParameterType
                                 .IsByRef;   //return true