How To Pass Null Value To Sqlserver

hi i am using formview

i have a date field textbox which selects date from ajax calender

now my point is when i make this field blank

at that time in my gridview rather then displaying the date field blank

it shows me the value 1 Jan 1900

this is the default date of sqlserver which it returns when we leave the field ''

now when i pass the value null it automatically takes it as ''

in my class file my function is

public object checkdate(TextBox txtdate)
        {
            if (txtdate.Text == "")
            {
                return null;
            }
            else
            {
               return txtdate.Text;
            }
        }

in my cs file i wrote
  object checkemptydate = obj.checkdate(nextvaccinationdate);

string values = patientID + ",'" + vaccination + "',convert(varchar,'" + vaccinationdate + "',106)," + checkemptydate + ",'" + remarks + "'";

note that i even tried this too

string values = patientID + ",'" + vaccination + "',convert(varchar,'" + vaccinationdate + "',106)," + null + ",'" + remarks + "'";

and

string values = patientID + ",'" + vaccination + "',convert(varchar,'" + vaccinationdate + "',106),convert(varchar,'" + checkemptydate + "',106),'" + remarks + "'";

but nothing is working it passes '' not null

how could i solve this problem?

shweta