c# - Factory vs instance constructors

Question

I can't think of any reasons why one is better than the other. Compare these two implementations:

public class MyClass
{
    public MyClass(string fileName)
    {
        // some code...
    }
}

as opposed to:

public class MyClass
{
    private MyClass(){}

    public static MyClass Create(string fileName)
    {
       // some code...
    }
}

There are some places in the .Net framework that use a static method to create instances. At first I was thinking, it registers it's instances to keep track of them, but regular constructors could do the same thing through the use of private static variables.

What is the reasoning behind this style?

Answer 1

Note: What you have is not a static constructor, it's a static function that creates the instance rather than calling the instance constructor yourself. A static constructor is a different thing entirely.

The factory pattern is a classic example of using a function (static or not) to instantiate a type rather than using the constructor directly. Note that the actual instance constructor will get called no matter what, but the static function provides a layer of indirection that allows it to return an instance of any type that either is or inherits from the return type, rather than only instances that are the return type.

For example:

public abstract class BaseClass
{
    public static BaseClass Create(int parameter)
    {
        if (parameter == 1)
        {
            return new Class1();
        }
        else
        {
            return new Class2();
        }
    }
}

internal class Class1 : BaseClass
{
    //code here ...
}

internal class Class2 : BaseClass
{
    //code here ...
}

This allows you to hide Class1 and Class2 from external assemblies while still allowing the consumer to deal with something specialized.