How do I instantiate a class given its string name?

Look at Activator.CreateInstance().

myObject = (MyAbstractClass)Activator.CreateInstance("AssemblyName", "TypeName");

or

var type = Type.GetType("MyFullyQualifiedTypeName");
var myObject = (MyAbstractClass)Activator.CreateInstance(type);

I believe this should work:

myObject = (MyAbstractClass)Activator.CreateInstance(null, childClassString);

The null in the first parameter defaults to the current executing assembly. For more reference: MSDN

edit: forgot to cast to MyAbstractClass

I had some difficulty implementing some of the answers here because I was trying to instantiate an object from a different assembly (but in the same solution). So I thought I'd post what I found to work.

First, the Activator.CreateInstance method has several overloads. If you just call Activator.CreateInstance(Type.GetType("MyObj")), that assumes the object is defined in the current assembly, and it returns a MyObj.

If you call it as recommended in the answers here: Activator.CreateInstance(string AssemblyName, string FullyQualifiedObjectName), then it instead returns an ObjectHandle, and you need to call Unwrap() on it to get your object. This overload is useful when trying to call a method defined in a different assembly (BTW, you can use this overload in the current assembly, just leave the AssemblyName parameter null).

Now, I found that the suggestion above to use typeof(ParentNamespace.ChildNamespace.MyObject).AssemblyQualifiedName for AssemblyName actually gave me errors, and I could not get that to work. I'd get System.IO.FileLoadException (could not load file or assembly...).

What I did get to work is as follows:

var container = Activator.CreateInstance(@"AssemblyName",@"ParentNamespace.ChildNamespace.MyObject");
MyObject obj = (MyObject)container.Unwrap();
obj.DoStuff();