Late Binding using Reflection in C#

The two main terms appearing in the above topic are Late Binding and reflection. So let us first define these two terms. The binding of methods and objects during run time is called Late Binding or Dynamic Binding. Reflection is the ability of an assembly to inspect its metadata. Metadata contains the information of the data within the assembly. 
Reflection is used to implement late binding as it allows you to use code that is not available at compile time. We will see the examples of this in the following code segments.

Example 1 : In this program, we make use of late binding because we do not have the knowledge of the class that has to be instantiated at compile time. Declare an object named executing of class Assembly and load the current Assembly using GetExecutingAssembly method. Next, we have to find the type of the class that has to be instantiated later i.e. Student. To do this use the method GetType on the object of Assembly class and store the type in another object called studentType. GetType method expects a String argument which is the complete name of the Class, LateBinding.Student in this case. Since the type is unknown at this point in the program we use the class Type to declare studentType. Now we create an instance of the studentType using the CreateInstance method that uses the type as a parameter. The Activator class contains methods to create types of objects locally or remotely or obtain references to existing remote objects. Next, create a MethodInfo object and use the GetMethod method to store the information. Pass the name of the method as an argument to GetMethod. To invoke GetDetails, use the MethodInfo object to call the Invoke method and pass studentObject as a parameter. And finally, display the details using a String det and also define the class.

C#

// C# program to show the Late
// Binding using Reflection
using System;
using System.Reflection;

namespace LateBinding {

class Program {

    // Main Method
    static void Main(string[] args)
    {
        // Declare Instance of class Assembly
        // Call the GetExecutingAssembly method
        // to load the current assembly
        Assembly executing = Assembly.GetExecutingAssembly();

        // To find the type of the Class Student
        Type studentType = executing.GetType("LateBinding.Student");

        // Create an Instance of the Student type
        object studentObject = Activator.CreateInstance(studentType);

        // Store the info of the method in an object
        // of class MethodInfo
        MethodInfo getMethod = studentType.GetMethod("GetDetails");

        // To store the parameters required
        // by Method GetDetails
        String[] param = new String[2];
        param[0] = "1";
        param[1] = "Lisa";

        // To display the result of the method
        String det = (String)getMethod.Invoke(studentObject, param);
        Console.WriteLine("Student Details : ");
        Console.WriteLine("Roll Number - Name \n{0}", det);

    } // end Main

} // end Program


public class Student {

    public String GetDetails(String RollNumber, String Name)
    {
        return RollNumber + " - " + Name;
    }

} // end Student

}

Output:

Student Details : 
Roll Number - Name 
1 - Lisa

Note: In the code above, if you do not define the class and build the code, it will build successfully since the binding is not done at compile time. But when you run it you will get an error as it will only be encountered at run time. The error message will be as follows;

Unhandled Exception: System.ArgumentNullException: Value cannot be null. 
Parameter name: type 
at System.Activator.CreateInstance(Type type, Boolean nonPublic) 
at System.Activator.CreateInstance(Type type) 
at LateBinding.Program.Main(String[] args) in m:\Documents\Visual Studio 2012\Projects\LateBinding\LateBinding\Program.cs:line 19 

Example 2: Another similar example is the code given below that has a Shape class and we are late binding the method shapeName. 

C#

// C# program to show the Late
// Binding using Reflection
using System;
using System.Reflection;

namespace Geometry {

// class Shape
public class Shape {

    // Function that gives the name of the
    // shape based on the number of sides
    // the number of sides is a string that
    // contains the value
    public String shapeName(String sideNumber)
    {
        if (sideNumber == "0" || sideNumber == "1" || sideNumber == "2")
            return "Not Valid";
        else if (sideNumber == "3")
            return "Triangle";
        else if (sideNumber == "4")
            return "Quadrilateral";
        else
            return "Polygon";
    }

} // end Shape

class Program {

    // Main Method
    static void Main(string[] args)
    {
        // Object of Assembly class
        Assembly executable = Assembly.GetExecutingAssembly();

        // To find the type of the class
        Type shapeType = executable.GetType("Geometry.Shape");

        // To create an instance of Shape class
        // without prior information about it
        object shapeObject = Activator.CreateInstance(shapeType);

        // To find the info about the method
        MethodInfo shapeNameMethod = shapeType.GetMethod("shapeName");

        // Prepare parameters for the method
        String[] param = new String[1];
        param[0] = "4";

        // To invoke the method using Invoke
        String sName = (String)shapeNameMethod.Invoke(shapeObject, param);

        // To  display the name of the shape
        Console.WriteLine("Name of the Shape is {0}", sName);

    } // end Main

} // end Program

}

Output:

Name of the Shape is Quadrilateral

Note: The disadvantage of Late Binding is that if you make any spelling error in the name of a method or class, the compiler will not identify it at compile time and therefore the code will build successfully. In most practical scenarios, Early Binding is used as opposed to Late Binding.