How can I create an optional DateTime parameter?

问题:

I have this function that returns a reference type. Now, this function has two optional parameters both of which are instances of the DateTime class. The function is something like this:

public DateTime GetDate(DateTime start = DateTime.MinValue, DateTime end = DateTime.MinValue)
{
    // Method body...
}

The error from VS is:

Default parameter value for 'start' must be a compile-time constant

Of course, the error applies to the second parameter and I perfectly understand what is happening.

What I really want is to know if there is a way to go about this, that is, having optional parameters in the method. Right now, what I have done is to create an overload; I mean, I have created a parameterless function GetDate() and a two-parameter overload of it.

This is not really a problem but I just want to know if there is a way to do it.

回答1:

One workaround is to assign them like this:

public DateTime GetDate(DateTime? start = null, DateTime? end = null){
    start = start ?? DateTime.MinValue;
    end = end ?? DateTime.MinValue;

    Console.WriteLine ("start: " + start);
    Console.WriteLine ("end: " + end);
    return DateTime.UtcNow;
}

Which can be used like this:

void Main()
{
    new Test().GetDate();
    new Test().GetDate(start: DateTime.UtcNow);
    new Test().GetDate(end: DateTime.UtcNow);
    new Test().GetDate(DateTime.UtcNow, DateTime.UtcNow);
}

And works just as expected:

start: 1/01/0001 0:00:00
end: 1/01/0001 0:00:00

start: 8/08/2014 17:30:29
end: 1/01/0001 0:00:00

start: 1/01/0001 0:00:00
end: 8/08/2014 17:30:29

start: 8/08/2014 17:30:29
end: 8/08/2014 17:30:29

Note the named parameter to distinguish between the start and end value.

回答2:

Btw, you don't have to use nullable datetime like all other answers says. You can do it with DateTime as well:

public DateTime GetDate(
     DateTime start = default(DateTime), 
     DateTime end = default(DateTime))
{
     start = start == default(DateTime) ? DateTime.MinValue : start;
     end = end == default(DateTime) ? DateTime.MinValue : end;
}

This is unlikely but it won't work as expected if you actually pass the default datetime value to your function.

回答3:

You can set the DateTime to be nullable and then convert to DateTime.Min if no parameter is provided:

public DateTime GetDate(DateTime? start = null, DateTime? end = null) {
    var actualStart = start ?? DateTime.Min;
    var actualEnd = end ?? DateTime.Min;
}

回答4:

The only way would be to do it like this (a bit more code, but it gives you optional args):

public DateTime GetDate(DateTime? start = null, DateTime? end = null)
{
    // Method body...
    if(start == null)
    {
      start = DateTime.MinValue;
    }

    //same for end
}