TypeDescriptor.GetProperties() vs Type.GetProperties()

Question

Consider the following code.

Object obj;
PropertyDescriptorCollection A = TypeDescriptor.GetProperties(obj);
PropertyInfo[] B = obj.GetType().GetProperties(); // EDIT*

I'm trying to understand the difference between A and B. From what I understand TypeDescriptor.GetProperties() will return custom TypeDescriptor properties, where as Type.GetProperties() will only return intrinsic "real" properties of the object. Is this right? If obj doesn't have any custom TypeDescriptor properties then it just defaults to also returning the literal intrinsic properties of the object.

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* Original second line of code before EDIT (had wrong return value):

PropertyDescriptorCollection B = obj.GetType().GetProperties();

Answer 1

The TypeDescriptor class is used in designers, so that they can interact with the design-time environment. In particular, designers can override and extend various features of TypeDescriptor, but not Type.

One good example is working with data-bound controls. The DataSource property is of type System.Object, yet at design-time, that property is replaced with a new property that has a richer design-time UI.

Answer 2

obj.GetType().GetProperties() does not return a PropertyDescriptorCollection, it returns a System.Reflection.PropertyInfo[]. The PropertyInfo class does, as you suggest, represent only actual properties created on the object. A PropertyDescriptor is either a custom concrete child of the PropertyDescriptor class (implemented by the type defining the custom descriptor), or is an instance of ReflectPropertyDescriptor that uses the PropertyInfo class to provide dynamic invocation of the property.

So for a class that does not define a custom descriptor, you will functionally get the same objects back, though the PropertyDescriptor is abstracting away the PropertyInfo.