Type, MemberInfo, and discovery

Type, MemberInfo, and discovery

Reflection is programming against metadata: the tables the compiler wrote into the assembly. typeof(T), obj.GetType(), and assembly.GetTypes() hand you Type objects; from there, GetProperties, GetMethods, GetFields, GetConstructors walk the member tables.

var t = typeof(Order);
foreach (var p in t.GetProperties(BindingFlags.Public | BindingFlags.Instance))
    Console.WriteLine($"{p.Name}: {p.PropertyType.Name}");

The cost model (measured repeatedly across .NET versions): first-time metadata resolution populates caches — GetProperty/GetMethod are the expensive part, not GetValue on an already-resolved member. Consequences:

• Resolve once. Store the MethodInfo/PropertyInfo in a static ConcurrentDictionary keyed by (Type, MemberName) and reuse.
• BindingFlags are a performance control, not decoration: fewer candidates = less work.
• assembly.GetTypes() can THROW (ReflectionTypeLoadException) when dependencies are missing — in plugin code, use GetExportedTypes or handle the loader exception's Types array, which contains nulls.

Generics interact with discovery in a way that surprises people: typeof(List<>) is an open generic type (no type arguments); you close it with MakeGenericType(typeof(int)). Attribute lookup on open generics reads the generic type definition's attributes, not the closed one's.