Set default value for DateTime in optional parameter [duplicate]

问题

How can I set default value for DateTime in optional parameter?

public SomeClassInit(Guid docId, DateTime addedOn = DateTime.Now???)
{
    //Init codes here
}

回答1:

There is a workaround for this, taking advantage of nullable types and the fact that null is a compile-time constant. (It's a bit of a hack though, and I'd suggest avoiding it unless you really can't.)

public void SomeClassInit(Guid docId, DateTime? addedOn = null)
{
    if (!addedOn.HasValue)
        addedOn = DateTime.Now;

    //Init codes here
}

In general, I'd prefer the standard overloading approach suggested in the other answers:

public SomeClassInit(Guid docId)
{
    SomeClassInit(docId, DateTime.Now);
}

public SomeClassInit(Guid docId, DateTime addedOn)
{
    //Init codes here
}

回答2:

I guess that you did not really want addedOn = DateTime.Now because that would suggest you never get any result as everything would be added before 'Now'. :)

A default DateTime can be set like this:

public void SomeClassInit(Guid docId, DateTime addedOn = default(DateTime))

Update
If you deal with SQL Server, do not forget that it doesn't accept default(DateTime) what is 1/1/0001. SQL Server's minimal DateTime is 1/1/1753 (explanation). SQL's DateTime2 accepts 1/1/0001, though.

回答3:

I'd slightly modify LukeH's solution as:

public void SomeClassInit(Guid docId, DateTime? addedOn = null)
{
    DateTime TargetDateTimeProperty = addedOn ?? DateTime.Now;
}

which is shorter and more readable, it seems.

回答4:

Don't use an optional parameter:

public SomeClassInit(Guid docId, DateTime addedOn)
{
    SomeClassInitCore(docId, addedOn);
}

public SomeClassInit(Guid docId)
{
    SomeClassInitCore(docId, null);
}

private SomeClassInitCore(Guid docId, DateTime? addedOn)
{
    // set default value
    if (addedOn.IsNull) addedOn = DateTime.Now;

    //Init codes here
}

回答5:

.NET 4.0 does have optional parameters. (google is also your friend, here.)

EDIT (because of Anthony Pegram correct, comment)...

~~And yes, that is how you would do it.~~

But DateTime.Now (static property, on that class) is not know until run-time. As such, you can't use that as an optional value.

.NET 3.5 doesn't ... so then you would have to do what JS Bangs said...

public SomeClassInit(Guid docId) 
{ 
    return SomeClassInit(docId, DateTime.Now);
}

public SomeClassInit(Guid docId, DateTime addedOn = DateTime.Now???) 
{ 
    //Init codes here 
}

or even the null checking/null value parameter from munificent's answer.

Cheers Anthony.

回答6:

C# doesn't have optional parameters in this sense. If you want to make addedOn optional, you should write an overload that doesn't require that parameter, and passes DateTime.Now to the two-argument version.

来源：https://stackoverflow.com/questions/3031110/set-default-value-for-datetime-in-optional-parameter

标签

datetime

default

optional-parameters