Attributes are attached to program entities such as types and methods to provide
information about the entity at runtime using reflection. An example of
attribute usage is the help context menu that visual studio provides for the
methods and types.
We can declare an attribute class like an ordinary class but it needs to be
derived from System.Attribute and need to mark it with the AttributeUsage
attribute. AttributeUsage attribute defines the program entities to which the
attribute can be applied.
Attribute classes have two types of parameter
- Named Parameters
- Positional Parameters
We can use reflection to discover information about a program entity at run
time and to create instance of a type at runtime. Most of the classes and
interfaces needed for reflection are defined in the System.Reflection namespace.
System.Type is the most fundamental to working with reflection functionality in
.NET and it represents a type in the CTS.
There are several ways To obtain the Type object
Once we have an instance of the type we can get the information about the type
using several methods like GetMembers() or GetMethods(). Following are the
methods defined in the Type class.
Following are the main classes defined in the System.Reflection namespace
| Assembly |
Represents an assembly |
| EventInfo |
This class holds information for a
given event. |
| FieldInfo |
This class holds information for a
given field. |
| MemberInfo |
Class is the abstract base class for
classes used to obtain information about all members of a class. |
| MethodInfo |
This class contains information for a
given method. |
| ConstructorInfo |
This class contains information for a
given constructor. |
In the following example we query the type for
the member fields and methods of the type and dynamically invoke the member
method.
class Test
{
public int a;
public void
Hello()
{
Console.WriteLine("Hello.....");
}
}
class Program
{
static void
Main(string[] args)
{
Type
typeObj = typeof(Test);
foreach(MethodInfo
mi in typeObj.GetMethods())
{
Console.WriteLine("Following
are the methods in Test class");
Console.WriteLine("Method
{0}", mi.Name);
}
foreach (FieldInfo
fi in typeObj.GetFields())
{
Console.WriteLine("Following
are the fields in Test class");
Console.WriteLine("Field
{0}", fi.Name);
}
object o = Activator.CreateInstance(typeObj);
MethodInfo method =
typeObj.GetMethod("Hello");
Console.Write("\t");
method.Invoke(o,
null);
Console.ReadLine();
}
}