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Introduction to .NET Reflection

Introduction

Reflection is ability to find information about types contained in an assembly at run time. Prior to .NET languages like C++ provided such ability in a limited sense. .NET provides a whole new set of APIs to introspect assemblies and objects. All the APIs related to reflection are located under System.Reflection namespace. .NET reflection is a powerful mechanism which not only allows you to inspect type information but also allows you to invoke methods on those types at runtime. Certain reflection APIs also allow creating of assembly in memory dynamically and use it in your code. In this article we will examine the basic and most commonly used features of reflection. Reflection APIs can be used to develop applications like class browsers, add-ons for development IDEs and inelegant editors.

.NET assemblies

Assemblies are the building blocks of any .NET application. All functionality of .NET application is exposed via assemblies. Assemblies form a unit of deployment and versioning. Assemblies contain modules which in turn contain various types (classes, structures, enumerations etc.).

Getting started

The first thing you should do while using reflection classes is to include System.Reflection namespace.

using System.Reflection;

Loading an assembly

Before obtaining any information about types contained in an assembly we must first load the assembly.

Assembly myassembly = Assembly.LoadFrom("employee.dll");

This statement loads an assembly called employee.dll. You can substitute your own path here. Assembly class has a static method called LoadFrom that loads the specified assembly in memory. The method returns an instance of assembly class itself.

obtaining details about types from the assembly

The next step is to obtain a list of various types contained in the assembly.

Types mytypes[] = myassembly.GetTypes();
Type mytype=myassembly.GetType("Company.Employee");

There are two methods to get type information . The method GetTypes returns an array of System.Type objects. The method GetType returns a type object having details of specified object. Note that in our example Company is the namespace. In case your assembly do not contain any namespace you will simply write the type name.

Obtaining type details

The Type class has following properties that gives details about the type under consideration :

  • Name : Gives name of the type
  • FullName : Give fully qualified name of the type
  • Namespace : Gives namespace name
  • IsClass
  • IsInterface
  • IsAbstract
  • IsCOMObject : Indicates if the type is a COM object
  • IsEnum
  • IsSealed
  • IsPublic

All the property names are self-explanatory and need no separate explanation.

obtaining details about methods, properties and fields

Each type may have fields (member variables), properties and methods. The details about each of these types are obtained by following methods of the Type object.

  • GetMembers() : Gives array of MemberInfo objects
  • GetFields() : Gives array of FieldInfo objects
  • GetProperties() : Gives array of PropertyInfo objects
  • GetMethods() : Gives array of MethodInfo objects

Note that you can also get information about specific method, property or field using GetMethod("mymethod"), GetProperty("myprop") or GetField("myfield") methods.

MethodInfo[] mymethods= mytype.GetMethods();
MethodInfo mymethod = mytype.GetMethod("GetSalary");

Following paragraphs list commonly used properties and methods of above objects. The property and method names are self explanatory. You can refer MSDN for more details.

Properties and methods of MethodInfo Object
  • Name
  • IsPrivate
  • IsPublic
  • IsStatic
  • IsConstructor
  • ReturnType
  • GetParameters()
  • Invoke()
Properties and methods of PropertyInfo Object
  • Name
  • CanRead
  • CanWrite
  • PropertyType
  • GetValue()
  • SetValue()
Properties and methods of FieldInfo Object
  • Name
  • FieldType
  • IsPublic
  • IsPrivate
  • IsStatic
  • GetValue()
  • SetValue()

Invoking a method on a type

We have seen how to get information about various types from an assembly. Reflection also allows us to create instances of these types and invoke methods on them. Following code fragment shows just that.

Assembly a=Assembly.LoadFrom("employee.dll"); 
Type t=a.GetType("Company.Employee");
MethodInfo getsalary=t.GetMethod("DisplayMsg");
object obj=Activator.CreateInstance(t);
object[] p=new object[1];
p[0]="Hello bipin";
getsalary.Invoke(obj,p);
Assembly a=Assembly.LoadFrom("employee.dll"); 
Type t=a.GetType("Company.Employee");
MethodInfo getsalary=t.GetMethod("GetSalary");
object obj=Activator.CreateInstance(t);
object[] p=new object[1];
p[0]="bipin";
object retval=getsalary.Invoke(obj,BindingFlags.DefaultBinding,
null,p,null);
Console.WriteLine(retval);

Here, we first obtained type of employee class. We then created an instance of it using Activator.CreateInstance() method.  There are two forms of Invoke() method :

  • If your method is not returning any value then you may use following form
    Invoke ( obj , obj[])
  • If your method is returning some value and you want to trap it use following form :
    obj = Invoke ( obj , bindingflag, binder , parameters, cultureflag )

For both the forms you pass instance of object on which the method will be called and array of objects that contains method parameters.

The second method shown here is with most commonly used values for BindingFlags, Binder and Culture. For more detailed information on the second syntax of Invoke method please refer MSDN.

Summary

Reflection allows a powerful mechanism to introspect your types. The reflection APIs can be found in System.Reflection namespace. The APIs allow you to inspect types as well as create types on the fly and invoke methods on them.


Bipin Joshi is the founder of BinaryIntellect Consulting and conducts professional training programs on ASP.NET in Thane. He is a published author and has authored or co-authored books for Apress and Wrox press. To know more about him click here. To know more about his training programs go here.

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Tags : .NET Framework Components Programming Languages
Posted On : 10 Jun 2001
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