Purpose of Activator.CreateInstance with example?

Can someone explain Activator.CreateInstance() purpose in detail?

Solution 1:

Say you have a class called MyFancyObject like this one below:

class MyFancyObject
{
 public int A { get;set;}
}

It lets you turn:

String ClassName = "MyFancyObject";

Into

MyFancyObject obj;

Using

obj = (MyFancyObject)Activator.CreateInstance("MyAssembly", ClassName))

and can then do stuff like:

obj.A = 100;

That's its purpose. It also has many other overloads such as providing a Type instead of the class name in a string. Why you would have a problem like that is a different story. Here's some people who needed it:

• Createinstance() - Am I doing this right?
• C# Using Activator.CreateInstance
• Creating an object without knowing the class name at design time

Solution 2:

Well i can give you an example why to use something like that. Think of a game where you want to store your level and enemies in an XML file. When you parse this file, you might have an element like this.

<Enemy X="10" Y="100" Type="MyGame.OrcGuard"/>

what you can do now is, create dynamically the objects found in your level file.

foreach(XmlNode node in doc)
   var enemy = Activator.CreateInstance(null, node.Attributes["Type"]);

This is very useful, for building dynamic enviroments. Of course its also possible to use this for Plugin or addin scenarios and alot more.

Solution 3:

My good friend MSDN can explain it to you, with an example

Here is the code in case the link or content changes in the future:

using System;

class DynamicInstanceList
{
    private static string instanceSpec = "System.EventArgs;System.Random;" +
        "System.Exception;System.Object;System.Version";

    public static void Main()
    {
        string[] instances = instanceSpec.Split(';');
        Array instlist = Array.CreateInstance(typeof(object), instances.Length);
        object item;
        for (int i = 0; i < instances.Length; i++)
        {
            // create the object from the specification string
            Console.WriteLine("Creating instance of: {0}", instances[i]);
            item = Activator.CreateInstance(Type.GetType(instances[i]));
            instlist.SetValue(item, i);
        }
        Console.WriteLine("\nObjects and their default values:\n");
        foreach (object o in instlist)
        {
            Console.WriteLine("Type:     {0}\nValue:    {1}\nHashCode: {2}\n",
                o.GetType().FullName, o.ToString(), o.GetHashCode());
        }
    }
}

// This program will display output similar to the following: 
// 
// Creating instance of: System.EventArgs 
// Creating instance of: System.Random 
// Creating instance of: System.Exception 
// Creating instance of: System.Object 
// Creating instance of: System.Version 
// 
// Objects and their default values: 
// 
// Type:     System.EventArgs 
// Value:    System.EventArgs 
// HashCode: 46104728 
// 
// Type:     System.Random 
// Value:    System.Random 
// HashCode: 12289376 
// 
// Type:     System.Exception 
// Value:    System.Exception: Exception of type 'System.Exception' was thrown. 
// HashCode: 55530882 
// 
// Type:     System.Object 
// Value:    System.Object 
// HashCode: 30015890 
// 
// Type:     System.Version 
// Value:    0.0 
// HashCode: 1048575

Solution 4:

You can also do this -

var handle = Activator.CreateInstance("AssemblyName", 
                "Full name of the class including the namespace and class name");
var obj = handle.Unwrap();