模板方法访问静态变量的问题

Question

Suppose I have two classes that both contain a static variable, XmlTag. The second class inherits from the first class. I have a template method that needs to obtain the XmlTag depending on the which type it is using. What would be the best way to accomplish this, without having to create an instance of the type? Here is an example(that won't compile), that should hopefully illustrate what I'm talking about.

class A{
public static readonly string XmlTag = "AClass";
}

class B : A {
public static readonly string XmlTag = "BClass";
}

This method is currently invalid.. Static variables can't be accessed from Type paramaters apparently.

string GetName<T>(T AClass) where T : A
{
    return T.XmlTag;
}

Answer 1

First off, stop thinking of generic methods as templates. They are NOT templates. They have very different behaviour from templates; your expectation that they behave like templates is possibly what is leading you astray in this situation.

Here is a series of articles I wrote about your scenario and why it is illegal.

http://blogs.msdn.com/ericlippert/archive/2007/06/14/calling-static-methods-on-type-variables-is-illegal-part-one.aspx

http://blogs.msdn.com/ericlippert/archive/2007/06/18/calling-static-methods-on-type-variables-is-illegal-part-two.aspx

http://blogs.msdn.com/ericlippert/archive/2007/06/21/calling-static-methods-on-type-variables-is-illegal-part-three.aspx

Note that of course the "dynamic" feature that I hint at in part three is in fact shipping with C# 4.0.

To address your actual question: "what's the best way to do this?" Presumably you have some problem which you believe a mechanism like this would solve. This mechanism doesn't actually exist in C#. It is impossible for us to deduce what problem you are actually trying to solve from the fact that you would like this mechanism to exist. Instead of asking "how can I make this impossible thing work in C#?" instead describe the real problem you have , and we can take a crack at trying to come up with an existing C# mechanism that better solves your real problem.

Answer 2

There's no direct way you could do that without resorting to reflection.

If you really want to do that (I encourage you to consider changing your high level design first):

To get a field value:

var returnValue = typeof(T).GetField("FieldName").GetValue(null);

To get a property:

var returnValue = typeof(T).GetProperty("PropertyName").GetValue(null, null);

To invoke a method:

typeof(T).InvokeMember("MethodName", BindingFlags.Static | BindingFlags.InvokeMethod | BindingFlags.Public | BindingFlags.NonPublic, null, null, null);

Answer 3

Why don't you give the bas class a virtual GetTagName class, and have the inheriting classes override that one?

class A{
public virtual string GetXmlTagName()
  {
   return "AClass";
  }
}

it would solve your immediate symptom but Im not sure about solving your problem. It has a destinct code smell but without knowing more I might be wrong when I state that the notion of XML should exist in a differrent class and that having that in A and B is violating single responsibility principle. Basically I would know from the information Im just affraid of it