C# 4.0: Can I use a TimeSpan as an optional parameter with a default value?

user_164506 0
Viewed 60124

Both of these generate an error saying they must be a compile-time constant:

void Foo(TimeSpan span = TimeSpan.FromSeconds(2.0))
void Foo(TimeSpan span = new TimeSpan(2000))

First of all, can someone explain why these values can't be determined at compile time? And is there a way to specify a default value for an optional TimeSpan object?

timespan c#-4.0 default-value optional-parameters c#
8 Answers

You can work around this very easily by changing your signature.

void Foo(TimeSpan? span = null) {

   if (span == null) { span = TimeSpan.FromSeconds(2); }

   ...

}

I should elaborate - the reason those expressions in your example are not compile-time constants is because at compile time, the compiler can't simply execute TimeSpan.FromSeconds(2.0) and stick the bytes of the result into your compiled code.

As an example, consider if you tried to use DateTime.Now instead. The value of DateTime.Now changes every time it's executed. Or suppose that TimeSpan.FromSeconds took into account gravity. It's an absurd example but the rules of compile-time constants don't make special cases just because we happen to know that TimeSpan.FromSeconds is deterministic.

user_817690

My VB6 heritage makes me uneasy with the idea of considering "null value" and "missing value" to be equivalent. In most cases, it's probably fine, but you might have an unintended side effect, or you might swallow an exceptional condition (for example, if the source of span is a property or variable that should not be null, but is).

I would therefore overload the method:

void Foo()
{
    Foo(TimeSpan.FromSeconds(2.0));
}
void Foo(TimeSpan span)
{
    //...
}
user_3858440

The set of values which can be used as a default value are the same as can be used for an attribute argument. The reason being that default values are encoded into metadata inside of the DefaultParameterValueAttribute.

As to why it can't be determined at compile time. The set of values and expressions over such values allowed at compile time is listed in official C# language spec:

C# 6.0 - Attribute parameter types:

The types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:

  • One of the following types: bool, byte, char, double, float, int, long, sbyte, short, string, uint, ulong, ushort.
  • The type object.
  • The type System.Type.
  • An enum type.
    (provided it has public accessibility and the types in which it is nested (if any) also have public accessibility)
  • Single-dimensional arrays of the above types.

The type TimeSpan does not fit into any of these lists and hence cannot be used as a constant.

JaredPar_232830
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