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C# - Reflection
C# - Reflection
Reflection objects are used for obtaining type information at runtime. The classes that give access to the metadata of a running program are in the System.Reflection namespace.
The System.Reflection namespace contains classes that allow you to obtain information about the apppcation and to dynamically add types, values, and objects to the apppcation.
Apppcations of Reflection
Reflection has the following apppcations −
It allows view attribute information at runtime.
It allows examining various types in an assembly and instantiate these types.
It allows late binding to methods and properties
It allows creating new types at runtime and then performs some tasks using those types.
Viewing Metadata
We have mentioned in the preceding chapter that using reflection you can view the attribute information.
The MemberInfo object of the System.Reflection class needs to be initiapzed for discovering the attributes associated with a class. To do this, you define an object of the target class, as −
System.Reflection.MemberInfo info = typeof(MyClass);
The following program demonstrates this −
using System;
[AttributeUsage(AttributeTargets.All)]
pubpc class HelpAttribute : System.Attribute {
pubpc readonly string Url;
pubpc string Topic // Topic is a named parameter {
get {
return topic;
}
set {
topic = value;
}
}
pubpc HelpAttribute(string url) // url is a positional parameter {
this.Url = url;
}
private string topic;
}
[HelpAttribute("Information on the class MyClass")]
class MyClass {
}
namespace AttributeAppl {
class Program {
static void Main(string[] args) {
System.Reflection.MemberInfo info = typeof(MyClass);
object[] attributes = info.GetCustomAttributes(true);
for (int i = 0; i < attributes.Length; i++) {
System.Console.WriteLine(attributes[i]);
}
Console.ReadKey();
}
}
}
When it is compiled and run, it displays the name of the custom attributes attached to the class MyClass −
HelpAttribute
Example
In this example, we use the DeBugInfo attribute created in the previous chapter and use reflection to read metadata in the Rectangle class.
using System;
using System.Reflection;
namespace BugFixApppcation {
//a custom attribute BugFix to be assigned to a class and its members
[AttributeUsage(
AttributeTargets.Class |
AttributeTargets.Constructor |
AttributeTargets.Field |
AttributeTargets.Method |
AttributeTargets.Property,
AllowMultiple = true)]
pubpc class DeBugInfo : System.Attribute {
private int bugNo;
private string developer;
private string lastReview;
pubpc string message;
pubpc DeBugInfo(int bg, string dev, string d) {
this.bugNo = bg;
this.developer = dev;
this.lastReview = d;
}
pubpc int BugNo {
get {
return bugNo;
}
}
pubpc string Developer {
get {
return developer;
}
}
pubpc string LastReview {
get {
return lastReview;
}
}
pubpc string Message {
get {
return message;
}
set {
message = value;
}
}
}
[DeBugInfo(45, "Zara Ap", "12/8/2012", Message = "Return type mismatch")]
[DeBugInfo(49, "Nuha Ap", "10/10/2012", Message = "Unused variable")]
class Rectangle {
//member variables
protected double length;
protected double width;
pubpc Rectangle(double l, double w) {
length = l;
width = w;
}
[DeBugInfo(55, "Zara Ap", "19/10/2012", Message = "Return type mismatch")]
pubpc double GetArea() {
return length * width;
}
[DeBugInfo(56, "Zara Ap", "19/10/2012")]
pubpc void Display() {
Console.WriteLine("Length: {0}", length);
Console.WriteLine("Width: {0}", width);
Console.WriteLine("Area: {0}", GetArea());
}
}//end class Rectangle
class ExecuteRectangle {
static void Main(string[] args) {
Rectangle r = new Rectangle(4.5, 7.5);
r.Display();
Type type = typeof(Rectangle);
//iterating through the attribtues of the Rectangle class
foreach (Object attributes in type.GetCustomAttributes(false)) {
DeBugInfo dbi = (DeBugInfo)attributes;
if (null != dbi) {
Console.WriteLine("Bug no: {0}", dbi.BugNo);
Console.WriteLine("Developer: {0}", dbi.Developer);
Console.WriteLine("Last Reviewed: {0}", dbi.LastReview);
Console.WriteLine("Remarks: {0}", dbi.Message);
}
}
//iterating through the method attribtues
foreach (MethodInfo m in type.GetMethods()) {
foreach (Attribute a in m.GetCustomAttributes(true)) {
DeBugInfo dbi = (DeBugInfo)a;
if (null != dbi) {
Console.WriteLine("Bug no: {0}, for Method: {1}", dbi.BugNo, m.Name);
Console.WriteLine("Developer: {0}", dbi.Developer);
Console.WriteLine("Last Reviewed: {0}", dbi.LastReview);
Console.WriteLine("Remarks: {0}", dbi.Message);
}
}
}
Console.ReadLine();
}
}
}
When the above code is compiled and executed, it produces the following result −
Length: 4.5 Width: 7.5 Area: 33.75 Bug No: 49 Developer: Nuha Ap Last Reviewed: 10/10/2012 Remarks: Unused variable Bug No: 45 Developer: Zara Ap Last Reviewed: 12/8/2012 Remarks: Return type mismatch Bug No: 55, for Method: GetArea Developer: Zara Ap Last Reviewed: 19/10/2012 Remarks: Return type mismatch Bug No: 56, for Method: Display Developer: Zara Ap Last Reviewed: 19/10/2012 Remarks:Advertisements