Object Creational Patterns and Instantiation
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by Brian Mains
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Instantiation

Whenever an object is created, it is instantiated in several ways.  The most common way to instantiate an object is using the new keyword.

Listing 1

DataTable table = new DataTable();

Using some of the design patterns, which we will talk about in a moment, instantiate objects using the new keyword.  However, there are dynamic ways to create an object, such as the Activator class's CreateInstance and CreateInstanceFrom methods.  These methods have various overloads.  One of the overloads is a generic method that takes the type to instantiate.  Another takes a Type object and instantiates it using this type.  If a more dynamic reference is needed, the method can take an assembly name and the name of the type to instantiate.  Note, on some overloads the method returns an ObjectHandle and not an object.  To get the object reference, call the Unwrap method.  The following is a few variations.

Listing 2

MyClass classInfo = Activator.CreateInstance<MyClass>();
 MyClass classInfo = (MyClass)Activator.CreateInstanceFrom(
 "MyComponentLibrary""MyComponentLibrary.MyClass").Unwrap();
 Type type = Type.GetType("MyComponentLibrary.MyClass,MyComponentLibrary");
 MyClass classInfo = (MyClass)Activator.CreateInstance(type);

If the object being instantiated does not have a default parameterless constructor, then there is an overload that takes an object parameter array, representing the values of the objects defined in the constructor.  The types of these objects have to match the parameter types for the specific constructor.  This approach is more synonymous with using the ConstructorInfo object, a part of the Reflection API.  Using Reflection, a type can access the constructor through the GetConstructor() or GetConstructors() methods.  This object contains the definition and parameter types of the constructor, and an object reference can be created using the Invoke method.

Listing 3

MyClass classInfo = null; 
 ConstructorInfo constructor = type(MyClass).GetConstructors()[0]; 
 if (constructor != null) 
 classInfo = (MyClass)constructor.Invoke(new object[] { "Some Name", 1, "Sidney" }); 

There are varying thoughts about the cost that reflection has in an application. Some say that using reflection in an application does have a significant performance hit, while others say that in reality the hit is negligible and is worth the cost to gain some of the added capabilities that reflection has in the .NET Framework.


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