What Reflection APIs Are Slow and What Are Not?

Fast and Light Functions:

• typeof
• Object.GetType
• typeof == Object.GetType
• Type equivalence APIs (including typehandle operator overloads)
• get_Module
• get_MemberType
• Some of the IsXX predicate APIs
• New token/handle resolution APIs in the .NET Framework 2.0

Costly Functions:

• GetXX APIs (MethodInfo, PropertyInfo, FieldInfo, and so on
• GetCustomAttributes
• Type.InvokeMember
• Invoke APIs (MethodInfo.Invoke, FieldInfo.GetValue, and so on)
• get_Name (Name property)Activator.CreateInstance

Generally, lightweight functions are fast for one of two reasons: either the reflection infrastructure has the information it needs already stored in super-fast runtime data structures, or the just-in-time (JIT) compiler detects these method calls and code patterns and generates special optimizations to speed things up a bit.

Adopted from Reflection: Dodge Common Performance Pitfalls to Craft Speedy Applications by Joel Pobar.