Reflection in the .NET framework

Lesson 1: Understanding Reflection

Code in CLR is packaged within assemblies. The meta data within the assembly contains type information for all classes, structures, delegates and interfaces in assembly. The assembly is a logical container for the different parts the CLR needs to execute code

An assembly does not need to be contained in a single file, e.g. click-once applications are frequently split into separate files so that only the parts required are pulled down.

Modules are containers for types within an individual assembly. It can be a container within a single assembly, or more usually part of a multi-file assembly. Visual Studio does not offer support for multiple modules - must use command line tools.

Examining Assembly

Assembly class supports several methods to create instances of itself. Some such as GetCallingAssembly and GetExecutingAssembly return instance of a particular assembly. Others such as GetAssembly and Load allow access to assemblies not currently loaded.

Once have instance of Assembly can interrogate its properties, e.g.

Assembly a = Assembly.GetExecutingAssembly();



Assembly class also supports loading assembly to interrogate its information. Normally when loading assembly there is work involved with preparing it for execution, if you do not expect to execute its code then calling ReflectionOnlyLoad results in faster load times - but some of the methods (e.g. CreateInstance) on the assembly returned by this call will be inoperative.

Each assembly contains one or more modules that represent containers for type information.

Assembly a = Assembly.GetExecutingAssembly();

foreach (Module m in a.GetModules())

Lesson 2: Assembly Attributes

Common Attributes

Typically add assembly attributes to AssemblyInfo file, e.g.

[assebly: AssemblyCompany("Company Name")]

Getting Assembly Attributes

Main mechanism is GetCustomAttributes method on the Assembly. To get all attributes...

Assembly a = Assembly.GetExecutingAssembly();

foreach (Attribute attr in a.GetCustomAttributes(false))

or a specific attribute...

Assembly a = Assembly.GetExecutingAssembly();

object[] attrs = a.GetCustomAttributes(typeof(AssemblyDescriptionAttribute), false))


Lesson 3: Reflection Types

Getting Types

Can get type objects in number of ways:

The Type class represents a single type. With it you can get information about the type, e.g.

Type t = typeof(String);




Can also call GetCustomAttributes on a type. Differs to call on assembly as the boolean parameter is ignored on assemblies, but with types instruct the runtime as to whether to ignore inherited attributes - a type can be derived from other types, an assembly cannot.

Note the type of an object remains fixed. Even if cast to another or to one of the interfaces it implements, the Name property for its type will still reflect the underlying type it was created as.

Enumerating class members

The type class provides methods for getting different parts of a type, including methods, properties and fields. Each is represented by an appropriately named class, for example the class representing a field is called FieldInfo, that for a property is PropertyInfo. All these classes derive from MemberInfo. For example to find the name for all the properties

foreach (PropertyInfo prop in t.GetProperties())

Can also gain access to nested types via GetNestedType or GetNestedTypes methods. Note this method returns a Type object instead of a specialised class.

Can iterate through all parts of a type via GetMembers call. The MemberInfo object returned contains MemberType property that allows the different parts to be differentiated.

foreach (MemberInfo member in t.GetMembers())
if (member.MemberType == MemberTypes.Property)
PropertyInfo p = (PropertyInfo)member; Console.WriteLine(p.PropertyType.Name);

Method Body

Iterating through type information provides a view onto the structure of a type. Does not provide view onto the code. This is access via the MethodBody container. This contains local variables and actual IL that are compiled into machine code at runtime. Gain access via the GetMethodBody method on a MethodBase instance (ConstructorInfo or MethodInfo class).

Binding Flags

Controls how members of a type are retrieved via the GetMembers method. The BindingFlags enumeration contains:

Combine these values together and pass into GetMembers override.

Lesson 4: Writing Dynamic Code

Using Dynamic Code

Reflection allows creation of objects dynamically, even from assemblies not referenced ahead of time.

To create objects...

// gather type information...
Assembly assembly = Assembly.LoadFile("mscorlib.dll");

Type hashType = assembly.GetType("System.Collections.Hashtable");

// get constructor
ConstructorInfo ctor = hashType.GetConstructor(Type.EmptyType);

// Now invoke the constructor
object newHash = ctor.Invoke(new object[] {});

Once have instance of object can use reflection to execute its methods

MethodInfo meth = hashType.GetMethod("Add");

meth.Invoke(newHash, new object[] { "Hi", "Hello"});

Invoking static methods

When dynamically calling static methods can skip creation of object..

Type conoleType = typeof(Console);

MethodInfo writeLineMethod = consoleType.GetMethod("WriteLine", new Type[] {typeof(string});

writeLineMethod.Invoke(null, new object[] {"Hello"});

because the above is a static method the first argument to Invoke is null.

Lesson 5: Creating code at runtime

System.Reflection.Emit contains builder classes to construct assemblies, types, methods, etc.

To build code at runtime it has to be encapsulated like any other - create assembly, then module within assembly, then types within the module.

Each builder class derives from the Info class counterpart, so AssemblyBuilder derives from Assembly, MethodBuilder from MethodInfo and TypeBuilder from Type.

Creating Assembly and Module

First ask AppDomain to create a dynamic assembly via the DefineDynamicAssembly method.

AssemblyName tempName = new AssemblyName();

tempName.Name = "MyTempAssembly";

AssemblyBuilder assemBldr = AppDomain.CurentDomain.DefineDynamicAssembly(tempName, AssemblyBuilderAccess.RunAndSave);

The AssemblyBuilderAccess specifies what can be done with the new assembly - ReflectionOnly, Run, Save, RunAndSave.

Having gained an AssemblyBuilder object it is possible to create a MethodBuilder providing it with a name and file name

ModuleBuilder modBldr = assemblyBldr.DefineDynamicModule("MainMod", "MyTempAssembly.dll");

Using the ModuleBuilder it is possible to create dynamic types.

Defining Types

Use the DefineType method on the ModuleBuilder providing a name and TypeAttributes value.

TypeBuilder typeBldr = modBldr.DefineType("MyNewType", TypeAttributes.Public | TypeAttributes.Class);

Overloaded variants of DefineType allow the base class and interfaces to be implemented to be specified.

Creating Members

The TypeBuilder allows any elements of the type to be defined. First thing to do is create the constructor...

ConstructorBuilder ctorBldr =typeBldr.DefineDefaultConstructor(MethodAttributes.Public);

To write code get the ILGenerator object from it and insert IL code, for example if the constructor is going to do nothing but return:

ILGenerator codeGen = ctorBld.GetILGenerator();


The Emit call inserts a new line of IL code into the constructor. The OpCodes class exposes each of the operations available in IL as a separate static method.

Persisting to disk

Once written to disk any other process can attempt to load it and treat it exactly like any other assembly.