1/27/2011 Generic Base Classes in ASP.Net

ASP.Net pages can inherit from custom classes (as long as they inherit System.Web.UI.Page).  This can be useful to add utility functions or shared (code-behind) behaviors to your pages.  (Note that you could also use Extension methods or HTTP modules)

However, if you try to inherit a generic base class, it won’t work:

public class DataPage<T> : Page {
    public T Data { get; set; }
}

<%@ Page Language="C#" Inherits="DataPage<string>" %>

This code results in a yellow screen of death, with the  parser error, Could not load type 'DataPage<string>'.

This happens because the ASP.Net page parser is unaware of C# generics syntax.  The familiar generics syntax (eg, List<int>) is actually a C# innovation and is not used at all in the actual framework.  The “native” generics syntax, which is used by Reflection, is markedly different: List`1[Int32] (namespaces omitted for brevity).  This name is returned by the Type.AssemblyQualifiedName property.

Since ASP.Net uses reflection APIs to load types, we need to specify generic types using CLR syntax (and with full namespaces).   Therefore, the following page will work:

<%@ Page Language="C#" 
    Inherits="TestSite.DataPage`1[[System.String, mscorlib]]" %>

However, it’s not so simple.  ASP.Net does not call Type.GetType to parse these strings; instead, it loops over every referenced assembly and calls Assembly.GetType on each one.  This is why you don’t need to include the assembly name whenever using the Inherits attribute (which would have been necessary for Type.GetType)  Ordinarily, this is very useful, but here, it comes back to bite you. 
It is not possible to parse a type from one assembly with a generic parameter from a different assembly using Assembly.GetType, unless the generic parameter is in mscorlib.

Therefore, for example, it is not possible to create an ASPX page that inherits DataPage<DataLayer.Product> if DataLayer.Product is in a different assembly than DataPage.  As a workaround, one can create a non-generic class which inherits DataPage<DataLayer.Product>, then make the ASPX page inherit this temporary class.

Next time: MVC magic