How to assign ( set ) Null value to DateTime Variable in C# .Net

How to assign Null value to datetime variable using nullable

I am getting a value from the textbox (I select a date)

but, may be sometimes user not select any value to the datetime textbox

in that cases i compare with null

i do like , but it gives me error

 DateTime? ScheduleDate = txtScheduleDate.Text;

                 

                    if (ScheduleDate.HasValue)
                    {
                        ScheduleDate = Convert.ToDateTime(txtScheduleDate.Text);
                    }