How do I create an instance of a class in an ASP.NET application

Question

How do you go about creating an instance of an object when given the class name as a string in an ASP.NET v2 application? For example, I've got a class called SystemLog defined in the app_code section of the application. The class is defined within the Reports namespace. To create an instance of the object, I do something like this:

Dim MyObject As New Global.Reports.SystemLog

However, I want to create this object using a string to define the type. The type name is stored in a SQL database as a string. I thinks it's probably something to do with Activator.CreateInstance(AssemblyName, TypeName) but what I don't know is what to pass in those strings. What is the assembly name of an ASP.NET web app?

Help!

Thanks, Rob.

PS. I don't want a hard coded Select statement :-)

Answer 1

string typeName = "Your Type Name Here";
Type t = Type.GetType(typeName);
object o = Activator.CreateInstance(t);

This will give you an instanciated type. If will be up to you to cast it to the correct type and call your appropriate methods.

If you need to create a type that doesn't have a parameterless constructor there is an overload on CreateInstance that takes a params of objects to pass to a constructor. More info at this MSDN article.

Answer 2

Thanks - that worked but only by luck in that the object being created is in the same assembly as the Activator.CreateInstance call. If I was attempting to create it from outside the same assembly, I expect it would get a lot harder!

Cheers, Rob.

Answer 3

You can use this to get it from a particular assembly:

Assembly assembly = Assembly.Load("myAssembly");

Type ObjectType = assembly.GetType("Type name here");

then.....object o = Activator.CreateInstance(ObjectType);

Answer 4

The following is able to create type, even if it's from another assembly:

public object CreateInstance(string typeName) {
   var type = AppDomain.CurrentDomain.GetAssemblies()
              .SelectMany(a => a.GetTypes())
              .FirstOrDefault(t => t.FullName == typeName);

   return type.CreateInstance();
}