无法从“ System.DateTime输出？”中进行转换？ 退出System.DateTime

Question

  if(startDate == "")
  {
      DateTime? startDateParser = null;
      DateTime.TryParse(startDate, out startDateParser);
  }

if the string startDate is empty i would like it to output a null however it complains

cannot convert from 'out System.DateTime?' to 'out System.DateTime'

parsing the null into a sql statement which the parameter accepts nulls however gets parsed in as 01/01/0001

command.Parameters.AddWithValue("@StartDate", (r.StartDate == null ? DBNull.Value : (object)r.StartDate));

any help will be greatly appreciated

Answer 1

A System.DateTime cannot be null. If you require a parameter of this type to allow nulls, you can do as your error suggests and use System.DateTime? which allows null values.

SO has many questions in relation to this topic, such as the following: datetime-null-value

Taken from the accepted answer:

For normal DateTimes, if you don't initialize them at all then they will match DateTime.MinValue , because it is a value type rather than a reference type.

You can also use a nullable DateTime, like this:

DateTime? MyNullableDate;

Or the longer form:

Nullable<DateTime> MyNullableDate;

And, finally, there's a built in way to reference the default of any type. This returns null for reference types, but for our DateTime example it will return the same as DateTime.MinValue :

default(DateTime)

Answer 2

Use this code to resolve "cannot convert from 'out System.DateTime?' to 'out System.DateTime'" 'out System.DateTime?' to 'out System.DateTime'" problem.

DateTime tmpDate;
DateTime? dateValue = DateTime.TryParse(startDate, out temp) ? temp : (DateTime?)null;

Can use the date value you parse for the result variable. You can use either null or a minimum value.

object resultVal = dateValue.HasValue ? dateValue.Value : DateTime.MinValue; // or DBNull.Value

command.Parameters.AddWithValue("@StartDate",resultVal  );

Answer 3

Remember, with out you're giving the method your variable to manipulate. A DateTime struct is an 8 byte value type. Variables for holding such values are 8 bytes and are expected to directly contain the value.

A DateTime? is a nullable DateTime - which is also a struct. But a Nullable<T> always contains an instance of the underlying and an extra bool . Which means that it will be at least 9 bytes in size (in practice, almost certainly 12 in most common implementations). A variable for such a type is (say) 12 bytes and contains the underlying value and the bool (and the bool may be "first" in the layout)

Now do you see why, if you're supplying the variable for some other method to manipulate, it has to be exactly the right sort of variable in order for the code inside that method to work with it correctly.