Checking to see if a DateTime variable has had a value assigned

The only way of having a variable which hasn't been assigned a value in C# is for it to be a local variable - in which case at compile-time you can tell that it isn't definitely assigned by trying to read from it :)

I suspect you really want Nullable<DateTime> (or DateTime? with the C# syntactic sugar) - make it null to start with and then assign a normal DateTime value (which will be converted appropriately). Then you can just compare with null (or use the HasValue property) to see whether a "real" value has been set.

do you mean like so:

DateTime datetime = new DateTime();

if (datetime == DateTime.MinValue)
{
    //unassigned
}

or you could use Nullable

DateTime? datetime = null;

 if (!datetime.HasValue)
 {
     //unassigned
 }