Using Reflection at Runtime in .NET Framework

Reflection is a feature of .Net framework which allows navigating and interrogating the type information of an assembly at runtime. This helps in building dynamic systems with plug-in architecture with least effort. With this feature, existing or newly generated code can be loaded and executed on the fly which can help the developer to alter the code based on the state of the assembly during runtime.

To understand the concept of Reflection, it is essential to know about an Assembly file which represents the unit of execution in .Net. An assembly file includes all the following data that the runtime needs to execute:

Assembly metadata (or manifest): includes name, version, strong name and culture information of the assembly.

Type metadata: includes all namespaces and the types (classes) with their individual properties, methods and constructors, if any.

Code: is the actual Intermediate Language (IL) code that gets converted to machine code during execution.

Resources: includes images, strings, files, etc. used from the code.
Each assembly contains one or more modules that represent containers for type information.

.Net classes for accessing Assembly information at runtime
The .Net framework provides rich set of classes for examining an assembly and modules during runtime. The class, Assembly supports a number of static methods for performing many operations related to assembly and also to get assembly information during runtime. Some of the methods provided include loading assembly into the AppDomain, fetch the name of calling assembly, get its modules, types, etc.
In the context of Reflection, a module is a container within a single assembly containing the types. The class, Module has methods to retrieve and search for types based on certain criteria, get fields in the module, etc.

Other assembly related information like copyright, company information, etc. are usually stored as Attributes. To access these attributes, different attribute classes (like AssemblyCopyRightAttribute, AssemblyCultureAttribute, etc) have been designed to retrieve the information during runtime. To get the individual attribute of an assembly, the method, GetCustomAttributes method of Assembly class can be used.

To access the type information during runtime, the method, GetTypes method of Assembly class can be used. Using the Type class, the different parts of a Type like methods, properties, fields and events can be obtained.
For using all these classes, the namespace, System.Reflection have to be included in the code.

Dynamic loading and execution of assembly

Dynamic code implies the code that does not have early access to it during compilation and is dynamically loaded during runtime. The reflection system allows creating objects dynamically from assemblies that have not been referenced ahead of time. This is slightly risky and cumbersome than writing type-safe compiler-checked code and is required sometimes.

Following are the steps for executing the code (invoking a member of an object of a Type) dynamically:

• Load the assembly (containing the given Type) by calling the Load method of the object instance of Assembly class.

• Create a new Type object of the required Type.

• Create the ConstructorInfo object by calling the GetConstructor method of the Type object with or without arguments.

• By calling the Invoke method of the ConstructorInfo object, a new object of the required type can be created.

• Using a newly created MethodInfo object, the required method can be called using the Invoke method by passing the method name and its argument list.

• Using a newly created PropertyInfo object and calling its GetValue and SetValue methods, individual property of the object can be fetched and set.

While invoking static methods, there is no need to dynamically create an object. The other steps remain the same for invoking the method except that NULL parameter needs to be passed (instead of the object in the non-static case) while calling the Invoke method.

Dynamic code generation

Reflection helps to not only examine and load the assembly in runtime but also define types and create new assemblies that can be for in-memory consumption or serialized to disk for re-use. Many builder classes are designed to build assemblies, modules and types at runtime which can even allow persisting the assemblies to disk. Each of these builder classes is derived from its info class counterpart.

These classes have been included in the namespace, System.Reflection.Emit. Some of the builder classes used for dynamic code generation are AssemblyBuilder, MethodBuilder, ModuleBuilder, ParameterBUilder, TypeBuilder and PropertyBuilder.

Following are the steps to be executed for creating a Type in a module of a dynamically created assembly:

• Create a dynamic assembly by calling the method, DefineDynamicAssembly of the current AppDomain object instance. While doing so, options to whether just run, save or run and save option for the assembly can be specified to the call as per the requirement. This call returns an AssemblyBuilder object.

• Create a new ModuleBuilder object representing the module which can hold the required Type by calling the method, DefineDynamicModule of the AssemblyBuilder object along with the module name and assembly file name.

• Define a new TypeBuilder object by invoking the method, DefineType of the ModuleBuilder object.

• Create members for the newly created Type object using the definition methods of the TypeBuilder object such as DefineConstructor, DefineMethod, DefineProperty and DefineField.

• To finally save the assembly, call the method, Save of the AssemblyBuilder object.
Thus, Reflection is a powerful tool that can be utilized to a great extent for providing software that can be controlled dynamically.

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