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)
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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.

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最佳答案

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.

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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.

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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)
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Optional parameters must appear at the end of the parameter list. out parameters must also appear at the end of the parameter list. Your optional parameter is not an out parameter.

Furthermore, you can't use default values for optional parameters other than literal constants and a few weird corner cases.

All facts point in the following direction:

  • Create a secondary overload method.
  • Make the initial method not include the parameter
  • Make the secondary one include the parameter
  • Call your more general method (the one with the parameter) from your more specific one and implement the logic only in the more general one
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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;
}
}
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we can create CONSTANTS class with default values

public const int DEFAULTINT = -9999;

and use them as CONSTANTS.DEFAULTINT as business defaults..

hope it helps,


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