Set an empty DateTime variable

问题

I would declare an empty String variable like this:

    string myString = string.Empty;

Is there an equivalent for a 'DateTime' variable ?

Update :

The problem is I use this 'DateTime' as a parameter for a 'StoredProcedure' in SQL. E.g:

    DateTime? someDate = null;
    myCommand.Parameters.AddWithValue("@SurgeryDate", someDate);

When I run this code an exception is catched telling me the 'StoredProcedure' expected a '@SurgeryDate' parameter. But i provided it. Any idea why?

回答1:

Since DateTime is a value type you cannot assign null to it, but exactly for these cases (absence of a value) Nullable<T> was introduced - use a nullable DateTime instead:

DateTime? myTime = null;

回答2:

No. You have 2 options:

DateTime date = DateTime.MinValue;

This works when you need to do something every X amount of time (since you will always be over MinValue) but can actually cause subtle errors (such as using some operators w/o first checking if you are not MinValue) if you are not careful.

And you can use Nullable:

DateTime? date = null;

Which is nice and avoids most issues while introducing only 1 or 2.

It really depends on what you are trying to achieve.

回答3:

You can set a DateTime variable to be '1/1/0001 00:00:00' but the variable itself cannot be null. To get this MinTime use:

DateTime variableName = DateTime.MinValue;

回答4:

You may want to use a nullable datetime. Datetime? someDate = null;

You may find instances of people using DateTime.Max or DateTime.Min in such instances, but I highly doubt you want to do that. It leads to bugs with edge cases, code that's harder to read, etc.

回答5:

The method you used (AddWithValue) doesn't convert null values to database nulls. You should use DBNull.Value instead:

myCommand.Parameters.AddWithValue(
    "@SurgeryDate", 
    someDate == null ? DBNull.Value : (object)someDate
);

This will pass the someDate value if it is not null, or DBNull.Value otherwise. In this case correct value will be passed to the database.

回答6:

Either:

DateTime dt = new DateTime();

DateTime dt = default(DateTime);

回答7:

If you set the date to

DateTime dNewDate = new DateTime();

The value is set to {1/1/0001 12:00:00 AM}

回答8:

Option 1: Use a nullable DateTime?

Option 2: Use DateTime.MinValue

Personally, I'd prefer option 1.

回答9:

A string is a sequence of characters. So it makes sense to have an empty string, which is just an empty sequence of characters.

But DateTime is just a single value, so it's doesn't make sense to talk about an “empty” DateTime.

If you want to represent the concept of “no value”, that's represented as null in .Net. And if you want to use that with value types, you need to explicitly make them nullable. That means either using Nullable<DateTime>, or the equivalent DateTime?.

DateTime (just like all value types) also has a default value, that's assigned to uninitialized fields and you can also get it by new DateTime() or default(DateTime). But you probably don't want to use it, since it represents valid date: 1.1.0001 0:00:00.

回答10:

There's no such thing as an empty date per se, do you mean something like:

DateTime? myDateTime = null;

回答11:

The .addwithvalue needs dbnull. You could do something like this:

DateTime? someDate = null;
//...
if (someDate == null)
    myCommand.Parameters.AddWithValue("@SurgeryDate", DBnull.value);

or use a method extension...

  public static class Extensions
    {
        public static SqlParameter AddWithNullValue(this SqlParameterCollection collection, string parameterName, object value)
        {
            if (value == null)
                return collection.AddWithValue(parameterName, DBNull.Value);
            else
                return collection.AddWithValue(parameterName, value);
        }
    }

回答12:

This will work for null able dateTime parameter

. .

SearchUsingDate(DateTime? StartDate, DateTime? EndDate){
     DateTime LastDate;
     if (EndDate != null)
       {
          LastDate = (DateTime)EndDate;
          LastDate = LastDate.AddDays(1);
          EndDate = LastDate;
        }
}

来源：https://stackoverflow.com/questions/9982839/set-an-empty-datetime-variable

标签

sql

datetime