Attributes & Reflection

Attributes

Attributes provide a powerful method of associating metadata, or declarative information, with code (assemblies, types, methods, properties, and so forth). After an attribute is associated with a program entity, the attribute can be queried at run time by using a technique called reflection. 

Attributes are used by many tools and frameworks. NUnit uses attributes like [Test] and [TestFixture] that are used by the NUnit test runner. 

ASP.NET MVC uses attributes like [Authorize] 

[System.AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Struct)] //AttributeUsage: restrict usage

public class AuthorAttribute : System.Attribute

{

    private string Name;

    public double Version;

    public AuthorAttribute(string name)

    {

        Name = name;

        Version = 1.0;

    }

}

Reflection

C# reflection refers to the ability of the C# programming language to inspect and manipulate its own metadata and behavior during runtime. With reflection, you can dynamically examine and interact with types, classes, properties, methods, and other members of a program, even if they were not known at compile time. 

using System;

using System.Reflection;

public class MyClass

{

    public void SayHello(string name)

    {

        Console.WriteLine($"Hello, {name}!");

    }

    public int Add(int a, int b)

    {

        return a + b;

    }

}

class Program

{

    static void Main(string[] args)

    {

        // Instantiate MyClass using reflection

        Type myClassType = typeof(MyClass);

        object myClassInstance = Activator.CreateInstance(myClassType);

        // Invoke SayHello method dynamically

        MethodInfo sayHelloMethod = myClassType.GetMethod("SayHello");

        sayHelloMethod.Invoke(myClassInstance, new object[] { "John" });

        // Invoke Add method dynamically

        MethodInfo addMethod = myClassType.GetMethod("Add");

        int result = (int)addMethod.Invoke(myClassInstance, new object[] { 5, 3 });

        Console.WriteLine($"Result of addition: {result}");

    }

}