Why DateTime.MinValue can't be used as optional parameter in C#

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);

Answer 2

Other answers touch upon why DateTime.MinValue cannot be used, it is not a legal compile time constant. It is a static readonly field, which might very well be constant as far as usage goes, but is not legally constant, nor does it fit the rules for what can be used as a default argument. As for why new DateTime() can be used, see section 10.6.1 of the C# 4.0 Language Specification. Relevant bits:

The expression in a default-argument must be one of the following:

· a constant-expression

· an expression of the form new S() where S is a value type

· an expression of the form default(S) where S is a value type

These result in a zero-initialized instance, basically a bit pattern of all zeros. (See: Section 4.1.2)

However, in this case, I still recommend using a DateTime? value = null as the parameter and default argument, particularly when it's representing a nullable date in a database. MinValue is not the absence of a value. null is.