Question

I have the following class of Schedule and using .Net Framework 4.0.
this class has two constructors, both having the parameter of interval with a default value of 120 seconds. is there any way to set the default value of interval in a private variable and then assign that variable to interval parameter. I tried doing it, but I get a compile time error saying Default parameter value for 'interval' must be a compile-time constant

public class Schedule
{
    public Delegate Callback { get; set; }
    public object[] Params { get; set; }
    public int Interval { get; set; }

    public Schedule(Delegate callback, int interval = 120)
    {

    }

    public Schedule(Delegate callback, object[] parameters, int interval = 120)
    {

    }
}

Answer 1

No, because the way that optional parameters work is that the compiler of the calling code has the default value baked into it. So a call like this:

new Schedule(MyCallback)

is converted at compile-time into:

new Schedule(MyCallback, 120)

So that’s why it has to be a constant. Now, you can still make that a constant – even a private one, if you want – but it can’t be a normal variable. So this would be okay:

public class Schedule
{
    private const int DefaultInterval = 120;

    public Schedule(Delegate callback, int interval = DefaultInterval)
    {

    }

    public Schedule(Delegate callback, object[] parameters,
                    int interval = DefaultInterval)
    {
         ...
    }
}

If you want a value which may vary at execution time, you could use a nullable type as the parameter, with the null value being the default, replaced by the real default at execution time. For example, here’s a method which allows you to specify a timestamp, but defaults to “now”:

public void Foo(DateTime? timestamp = null)
{
    DateTime realTimestamp = timestamp ?? DateTime.UtcNow;
    ...
}