.NET Reflection Dynamically Create, Find and Invoke Types

.NET ReflectionDynamically Create, Find and Invoke Types Abhishek Biswas

.NET Reflection • Features • Create and Extend types, modules & assemblies at runtime • Read type metadata at runtime • Invoke type methods at runtime • Language Independent • Advantages • Single location for type information and code • Code is contained within type information • Every .NET object can be queried for its type

Metadata Components

Reflection at Work Assembly assm = appDomain.Load(buffer); Type[] types = assm.GetTypes(); foreach (Type type in types) {Console.WriteLine(type.FullName); } MethodInfomyMethod = assm.GetType("HW3.hashKey"). GetMethod("GenHash"); object obj = Activator.CreateInstance (assm.GetType("HW3.hashKey")); myMethod.Invoke(obj, null);

GetType() Function Breakdown • Member of System.Object • Parent of all .NET classes • Available on every .NET class & simple type • Returns System.Type object • Type Identity • Types have unique identity across any assembly • Types can be compared for identity • if ( a.GetType() == b.GetType() ) { … };

System.Type • Access to meta-data for any .NET type • Allows drilling down into all facets of a type • Category: Simple, Enum, Struct or Class • IsValueType, IsInterface, IsClass • IsNotPublic, IsSealed • IsAbstract • Methods and Constructors, Parameters and Return • MemberInfo: GetMembers(), FindMembers() • FieldInfo: GetFields(), PropertyInfo: GetProperties() • GetConstructors(), GetMethods(), GetEvents() • Fields and Properties, Arguments and Attributes • Events, Delegates and Namespaces

Invoking Methods • Dynamic Invocation through Reflection • Support for late binding • MethodInfo.Invoke() • FieldInfo.SetValue() • PropertyInfo.SetValue()

Creating a New Type/Module/Assembly Namespace “System.Reflection.Emit” Dim aName As New AssemblyName("DynamicAssemblyExample") Dim ab As AssemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(aName, AssemblyBuilderAccess.RunAndSave) Dim mb As ModuleBuilder = ab.DefineDynamicModule(aName.Name, aName.Name & ".dll") Dim tb As TypeBuilder = mb.DefineType("MyDynamicType", TypeAttributes.Public)

Adding Field and Constructor Dim fbNumber As FieldBuilder = tb.DefineField("m_number“, GetType(Integer), FieldAttributes.Private) Dim parameterTypes() As Type = { GetType(Integer) } Dim ctor1 As ConstructorBuilder = tb.DefineConstructor( MethodAttributes.Public, CallingConventions.Standard, parameterTypes) Dim ctor1IL As ILGenerator = ctor1.GetILGenerator() ctor1IL.Emit(OpCodes.Ldarg_0) ctor1IL.Emit(OpCodes.Call,GetType(Object).GetConstructor(Type.EmptyTypes)) ctor1IL.Emit(OpCodes.Ldarg_0) ctor1IL.Emit(OpCodes.Ldarg_1) ctor1IL.Emit(OpCodes.Stfld, fbNumber) ctor1IL.Emit(OpCodes.Ret)

Adding Method Dim meth As MethodBuilder = tb.DefineMethod("MyMethod", MethodAttributes.Public, GetType(Integer), New Type(){GetType(Integer)}) Dim methIL As ILGenerator = meth.GetILGenerator() methIL.Emit(OpCodes.Ldarg_0) methIL.Emit(OpCodes.Ldfld, fbNumber) methIL.Emit(OpCodes.Ldarg_1) methIL.Emit(OpCodes.Mul) methIL.Emit(OpCodes.Ret)

Finishing Dim t As Type = tb.CreateType() ab.Save(aName.Name & ".dll") Dim mi As MethodInfo = t.GetMethod("MyMethod") Dim pi As PropertyInfo = t.GetProperty("Number")

Why? • Build classes dynamically from script-like code • ASP.NET, Regular Expressions do just that ! • Generate code from visual development tools • e.g. build interfaces, base classes from UML • Create dynamic wrappers for existing code • Transfer code-chunks to remote machines • Distributed processing scenarios

References http://www.dcl.hpi.uni-potsdam.de/teaching/componentVl05/slides/Net_VL2_02_Reflection.pdf http://msdn.microsoft.com/en-us/library/system.reflection.emit.assemblybuilder%28v=VS.100%29.asp

.NET Reflection Dynamically Create, Find and Invoke Types