Usage of Activator.CreateInstance()

Question

What is actually the difference between the below two code snippets?

Object o = Activator.CreateInstance(typeof(StringBuilder));

StringBuilder sb = (StringBuilder) o;

and

StringBuilder sb = new StringBuilder();

Answer 1

From a practical point of view. There is no difference.

However, from a technical point of view. The first will incur a substantial performance penalty.

First, from the Activator.CreateInstance , since this is a Reflection call.

Then another performance hit when you cast object to StringBuilder .

From a design point of view however. Activator.CreateInstance takes Type as a parameter...

This means you can do the following...

public IStringBuilder ActivateStringBuilder(Type builderType)
{
     return (IStringBuilder) Activator.CreateInstance(builderType);
}

Ignoring that there is no such thing as IStringBuilder the above code allows you to, at runtime, change the behavior of the code, by passing in different Type s that implement IStringBuilder .

This is the basis for Dependency Injection (although, we tend to use much more complicated mechanisms to get around the performance issues I pointed out).