Instantiate a System.Type from a type definition string

Question

In the web.config file, we see a lot of strings following this pattern:

    type="System.Web.WebPages.Razor.Configuration.RazorWebSectionGroup, 
System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, 
PublicKeyToken=31BF3856AD364E35"

I want to read this information from the web.config (which I know how to read as a string), and I want to instantiate a type from this string.

Is there some way to do this?

Update

I guess I could just do

Activator.CreateInstance(System.GetType(stringTypeName))

Please confirm?

Answer 1

There are several overloads of Activator.CreateInstance that enable you to do this. However, you will have to split the string into a type name and assembly name manually.

Update: Your own take on this is also correct (although the method is Type.GetType() ).

Answer 2

It's simple to create an instance of a type using Activator.CreateInstance(Type) .

Edit For some reason I though you could just pass the type name - you can't; thanks Jon for pointing this out.

But you need to get the Type from the type name. If the type name is always fully-qualified (at least down to the assembly name) - then you can simply use:

Type t = Type.GetType(typeName);

However, that throws an exception if the type can't be found. You might be better off with:

Type t = Type.GetType(typeName, false);

And then:

object result = null;
if(t != null)
  result = Activator.CreateInstance(t);

End edit

However, in some cases that type might not have a default constructor, in which case you either have to skip it (you either catch an exception from Activator.CreateInstance , or do a reflection search for the constructor first), or find a way to build the dependant types as well ( Activator.CreateInstance supports constructors with parameters too - @Jon's answer includes a link).

I don't believe this simple (Type) overload I suggest here works with types with constructors that have all-optional parameters, either:

public class MyClass {
   public MyClass(string p1 = null, int p2 = 10, ...) { }
}

//...

var o = Activator.CreateInstance(typeof(MyClass)); //<-- will fail

Because that's not a default constructor - it's up to a compiler to bind such constructors as if they were defaults, by pulling out all the default values into a call to the 'expanded' version.