Question

I was writing a method which takes DateTime value as one of it’s parameters. I decided that it’s optional parameter so I went ahead and tried to make DateTime.MinValue as default.

private void test(string something, DateTime testVar = DateTime.MinValue) {

}

However this gives an error that:

Default parameter value for ‘testVar’ must be a compile-time constant.

Using this code seems to work just fine.

private void test(string something, DateTime testVar = new DateTime()) {

}

I was given advice to use DateTime.MinValue instead of new DateTime() as it’s self-documenting. Since new DateTime() is basically the same thing why DateTime.MinValue can’t be used? Also will there be any potential problem if I leave it with new DateTime()?

Answer 1

DateTime.MinValue is defined as:

public static readonly DateTime MinValue

Which is not the same as const. Since a readonly value is not a compile-time constant (i.e. the value is not evaluated at compile-time), it can’t be used.

The reason that using new DateTime() works is because that expression is known at compile-time. It’s the same as writing default(DateTime). For example, result == true in the following expression:

var result = new DateTime() == default(DateTime);