C# Reflection
Reflection refers to the ability of a program to access, inspect, and modify its own state or behavior.
An assembly contains modules, which in turn contain types, and these types contain members. Reflection provides objects that encapsulate assemblies, modules, and types.
You can use reflection to dynamically create instances of types, bind types to existing objects, or get types from existing objects. You can then invoke the methods of the type or access its fields and properties.
Pros and Cons
Pros:
- Reflection enhances the flexibility and extensibility of the program.
- It reduces coupling and improves adaptability.
- It allows the program to create and control objects of any class without having to hard-code the target class in advance.
Cons:
- Performance issues: Using reflection is essentially an interpreted operation, which is much slower than direct code when accessing fields and methods. Therefore, reflection is mainly used in system frameworks that require high flexibility and extensibility, and it is not recommended for ordinary programs.
- Reflection obscures the internal logic of the program; programmers expect to see the logic of the program in the source code, but reflection bypasses this, leading to maintenance issues. Reflection code is generally more complex than corresponding direct code.
Uses of Reflection
Reflection has the following uses:
It allows viewing attribute information at runtime.
It allows inspecting various types in a collection and instantiating these types.
It allows late binding of methods and properties.
It allows creating new types at runtime and then using these types to perform tasks.
Viewing Metadata
As mentioned in the previous section, reflection can be used to view attribute information.
The MemberInfo object of the System.Reflection class needs to be initialized to discover the attributes associated with a class. To do this, you can define an object of the target class, as follows:
System.Reflection.MemberInfo info = typeof(MyClass);
The following program demonstrates this:
Example
using System;
[AttributeUsage(AttributeTargets.All)]
public class HelpAttribute : System.Attribute
{
public readonly string Url;
public string Topic // Topic is a named parameter
{
get
{
return topic;
}
set
{
topic = value;
}
}
public 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 the above code is compiled and executed, it displays the custom attributes attached to the class MyClass:
HelpAttribute
Example
In this example, we will use the DeBugInfo attribute created in the previous chapter and use reflection to read the metadata in the Rectangle class.
Example
using System;
using System.Reflection;
namespace BugFixApplication
{
// A custom attribute BugFix is assigned to the class and its members
[AttributeUsage(AttributeTargets.Class |
AttributeTargets.Constructor |
AttributeTargets.Field |
AttributeTargets.Method |
[AttributeUsage(AttributeTargets.Property, AllowMultiple = true)]
public class DeBugInfo : System.Attribute
{
private int bugNo;
private string developer;
private string lastReview;
public string message;
public DeBugInfo(int bg, string dev, string d)
{
this.bugNo = bg;
this.developer = dev;
this.lastReview = d;
}
public int BugNo
{
get
{
return bugNo;
}
}
public string Developer
{
get
{
return developer;
}
}
public string LastReview
{
get
{
return lastReview;
}
}
public string Message
{
get
{
return message;
}
set
{
message = value;
}
}
}
[DeBugInfo(45, "Zara Ali", "12/8/2012", Message = "Return type mismatch")]
[DeBugInfo(49, "Nuha Ali", "10/10/2012", Message = "Unused variable")]
class Rectangle
{
// Member variables
protected double length;
protected double width;
public Rectangle(double l, double w)
{
length = l;
width = w;
}
[DeBugInfo(55, "Zara Ali", "19/10/2012", Message = "Return type mismatch")]
public double GetArea()
{
return length * width;
}
[DeBugInfo(56, "Zara Ali", "19/10/2012")]
public 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 attributes 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);
}
}
// Traverse method attributes
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 Ali
Last Reviewed: 10/10/2012
Remarks: Unused variable
Bug No: 45
Developer: Zara Ali
Last Reviewed: 12/8/2012
Remarks: Return type mismatch
Bug No: 55, for Method: GetArea
Developer: Zara Ali
Last Reviewed: 19/10/2012
Remarks: Return type mismatch
Bug No: 56, for Method: Display
Developer: Zara Ali
Last Reviewed: 19/10/2012
Remarks: