确定日期字段是否包含NULL值

Question

I've got this little block of code in a code-behind form:

using (SqlConnection connection1 = new SqlConnection(str))
{
    using (SqlCommand cmd = new SqlCommand("usp_select_claim_id_MERGE", connection1))
    {
        cmd.CommandType = CommandType.StoredProcedure;

        connection1.Open();
        {
            using (SqlDataReader DT = cmd.ExecuteReader())
            {

            //Format the Orig Open Date to strip off the time and just leave the date
            if ((DT["Orig_Open_Date"]) == null)
              {
                OrigOpenDt = Convert.ToDateTime("1/1/1900");
                OrigOpenDt2 = "";
              }
              else
              {
                OrigOpenDt = Convert.ToDateTime(DT["Orig_Open_Date"]);
                OrigOpenDt2 = OrigOpenDt.ToString("MM/dd/yyyy");
              }

When it hits the IF statement, it's telling me it's false, so the code jumps to the "else" statement. But when I look at the SQL Server table at the record in question, it looks like this (I'm actually just looking at the first record in the image below):

Looks to me like Orig_Open_Date (the column all the way to the right) has NULLs in it.

If I grab a record where there is actually a date in Orig_Open_Date, it works fine. If there's no date, it still tries to run the "else" statement, so my "if" statement must be wrong.

Admittedly, I'm a C# n00b. What am I doing wrong here?

Answer 1

尝试DBNull.Value

if(value == DBNull.Value)

Answer 2

您也可以在SQL语句中执行此操作：

Select ISNULL(Orig_Open_Date,'1/1/1900') as Orig_Open_Date

Answer 3

The reason why you recieved a empty value is because of the null value that the reader retrun from the DB. I often encounter this problem and now i always check for DBNull.value.

Exemple :

db.GetCommand().CommandText = "SELECT * FROM EMPLOYE";
IDataReader reader = db.GetCommand().ExecuteReader();

if(reader.Read())
{
if(reader["HierdDate"] == DBNull.Value)
// Do that
else
//Do this
}

It's always a good thing to check the DBNull value that a DataReader can return.