Help Regarding C# Windows App

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C#2.0

C#

SQL

I am making an attendance management system, it has a table named tblattendance and has four fields:

name nvarchar(100)
timearrive nvarchar(100) allow null
timeleave nvarchar(100) allow null
logincode nvarchar(100)

At time of check in, I check from database whether or not this person already checkedin,if yes he is checkedout else he checked in.

The problem arising is that when I check for timeleave at time of checkout that if person already checked out(i.e timeleave != "") then show a message box else check him out, exception is arising "The value for column "timeleave" in table "tblattendance" is DBNull."

if (a.GetAttendbyDt(uid, day)[0].TimeLeave !=  "")
                          {
                           //show message
                          }
else
//update attendance(check out)

the GetAttendbyDT method returns a datatable by taking userid and current date as parameters.

Can any one solve this problem? How can I check for time leave null or not?