This property getter has a constant time complexity.
If this member is a type not nested in another type, this property returns null.
If this member is an application type nested in another type, this property returns the parent type.
Notice that for third-party nested types, the property IType.IType.IsNested is set to true, but the parent/nested relation is not resolved. Hence for a third-party nested types, null is returned by this property.

Reference

IMember Interface
IMember Members