如何为没有具体类型但有接口的参数赋予默认值

Question

I have this function doSomething. 99% of the time it take the same arg. I would like to create a default argument. But the type of the argument is a interface

    public static void doSomething(Itype arg=new classWhichImplementItype())
    {
        //do something
    }

Default paramter value for arg must be a compile time constant.

How can I do

Answer 1

Default arguments must be compile-time constants. So, use overloads:

public static void doSomething()
{
    doSomething(new classWitchImplementItype());
}

public static void doSomething(Itype arg)
{
    //do something
}

In recent versions of C#, you can also use the arrow syntax for the overload:

public static void doSomething() => doSomething(new classWitchImplementItype());

Answer 2

Try this:

public static void doSomething(Itype arg=null)
    {
          if (arg==null) arg=new classWitchImplementItype();

        //do something
    }