Asp.net 2.0  introduced nullable types that allow you to determine whether a variable has been assigned a value or not and not cause an exception if the variable is null.  A nullable type simply allows a variable to be null in addition to its normal range of values.  A nullable string for example, can be any text representation as well as null.  Likewise a nullable boolean value can be true, false or null.

A nullable type can be represented by the ? operator or by the Nullable <T> syntax.  The following two statements are identical and define a nullable integer:

Nullable<int> myNullableInt;
int? myNullableInt;

Nullable types are most useful when working with database queries that may return null. The following example shows a class with database operations performed against it where some fields may be null.

We’ll start off by defining the class without any nullable operators:

public class Event
{
   public int EventID { get; set; }
   public string EventName { get; set; }
   public DateTime EventDate { get; set; }
}

In this case we’re going to allow the EventDate field to be null in the database.   Without setting a nullable type, you’ll receive a number of errors when retrieving and updating this value.

Firstly, when retrieving from a database:

SqlDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
   myEvent = new Event();
   myEvent.EventID = (int)reader["RegistryID"];
   myEvent.EventName = (string)reader["EventName"];
   myEvent.EventDate = (DateTime)reader["EventDate"];
}

You’ll get the following error:

Specified cast is not valid.

The way to resolve this is to modify the class to make the EventDate property a nullable type:

public DateTime? EventDate { get; set; }

The database code changes the same way:

myEvent.EventDate = reader["EventDate"] as DateTime?;

Finally, there are a few things you need to do differently when displaying, and updating your data.

When displaying the value in a textbox or label, use TryParse to see if this is a valid date – if not then display empty text.  (For more on TryParse, see  James Michael Hare’s excellent post here):

DateTime date;
if (DateTime.TryParse(myEvent.EventDate.ToString(), out date))
   EventDate.Text = date.ToShortDateString();
else
   EventDate.Text = String.Empty;

After editing then use the ?? operator to save a DBNull value if the EventDate is null:

cmd.Parameters.AddWithValue("@EventDate", 
   EventtoUpdate.EventDate ?? (object)DBNull.Value);

The ?? operator simply says use the value of EventtoUpdate.EventDate if it is not null, otherwise save the value as DBNull.Value (the database representation of null)