Checking DateTime for null?

[joe_pool_is]

How do I check for an uninitialized DateTime?

 

If I start my code by simply declaring a DateTime value:

 

DateTime time1;

 

Then the compiler gives me the error that the field was uninitialized.

 

Seting the DateTime value to null is not legal though, it seems:

 

DateTime time1 = null;

 

How am I supposed to be initializing a DateTime value?

[joe_pool_is]

Some of you may be wondering, "Why would you want a null DateTime?"

 

Here's an example of somewhere that I have to initialize the DateTime object with data that is *not* what I want:

/// <summary>
/// Synchronize the Time on the SQL Server with the time on the device
/// </summary>
private void SyncWithSqlTime(DateTime sqlTime) // pass in or query for time to set on device
{
 SYSTEMTIME st = new SYSTEMTIME();
 sqlTime = DateTime.Now; // checking to see if sqlTime has been set doesn't seem possible
 m_lastError = string.Empty;
 try
 {
   m_db.Open(); // Sql Connection object
   using (SqlCommand cmd = new SqlCommand("SELECT GetDate() AS [CurrentDateTime]", m_db))
   {
     SqlDataReader r = cmd.ExecuteReader();
     while (r.Read() == true)
     {
       sqlTime = (DateTime)r["CurrentDateTime"];
     }
   }
 }
 catch (InvalidOperationException e)
 {
   Console.WriteLine(e.StackTrace);
 }
 catch (SqlException e)
 {
   Console.WriteLine(e.StackTrace);
 }
 finally
 {
   m_db.Close();
 }
 st.FromDateTime(sqlTime); // Convert to SYSTEMTIME
 SetLocalTime(ref st); // PInvoke Win32 API to set time
}

[MrPaul]

Nullable types

 

If using .NET 2 or later, you can use nullable types:

 

DateTime? sqlTime = null;

 

Alternatively, I tend to initialize variables to something like DateTime.MinValue and then test for that.

 

Good luck :cool:

[joe_pool_is]

Re: Nullable types

 

Thanks, MrPaul.

 

I'd seen variables declared using a Question Mark (?) before, but I had no idea what they were called, so there was no way to look them up.

 

Now, I know how to set a DateTime to a null value, and I know what the Question Mark means! I got a 2-for-1!