Default parameter for value must be a compile time constant?

This is my method signature. While trying to pass end as an optional parameter it gives me this error. What should I do to resolve this? Why isn't DateTime.MinValue a constant?

public static void DatesToPeriodConverter(DateTime start, DateTime end = DateTime.MinValue,
                                          out string date, out string time)

Solution 1:

DateTime.MinValue is not a const, because the language doesn't like const on DateTime. One option is to use DateTime? instead, i.e.

public static void DatesToPeriodConverter(DateTime start, DateTime? end = null,
     out string date, out string time)
{
    var effectiveEnd = end ?? DateTime.MinValue;
    // ...
}

However, you will still have the issue of having non-default parameters after default parameters - you may need to re-order them to use that as a default.

Solution 2:

Use regular method overloads instead:

public static void DatesToPeriodConverter(DateTime start, out string date, out string time)
{
    DatesToPeriodConverter(start, DateTime.MinValue, out date, out time);  
}

public static void DatesToPeriodConverter(DateTime start, DateTime end, out string date, out string time) 
{ }

Atlernatively, default(DateTime) is the same as DateTime.MinValue and is compile time constant, but I tend to err away from using this style (there's no guarantee in future that default(DateTime) will equal DateTime.MinValue):

public static void DatesToPeriodConverter(DateTime start, DateTime end = default(DateTime), out string date, out string time)

Or as Marc suggests, use DateTime? which allows a null default value.

Solution 3:

You can try doing it this way:

public static void DatesToPeriodConverter(DateTime start, DateTime? end , out string date, out string time)
{
    if(!end.HasValue){
        end = DateTime.MinValue;
    }
}

Solution 4:

Change a type of the parameter end to a Nullable and use null as a default value:

public static void DatesToPeriodConverter(DateTime start, DateTime? end = null, out string date, out string time)

or use default(DateTime) as a default value:

public static void DatesToPeriodConverter(DateTime start, DateTime end = default(DateTime), out string date, out string time)

Solution 5:

You are correct. Default parameter for value must be a compile time constant. Dynamically calculated value is not accepted by compiler against optional parameter. The reason behind this may be that it is not definite that the dynamic value you are providing would give some valid value.