Web Forms :: Set Variable Based On CheckBox Checked Property In C#?

I used below code fo radiobutton to inserting data in database

string price = RadioButton2.Checked ?"1": "null";
SqlCommand _cmd = new SqlCommand("insertproduct2", _cn);
_cmd.CommandType = CommandType.StoredProcedure;
_cn.Open();
_cmd.Parameters.AddWithValue("@Price", price);
_cn.Close();

Now I use same code for checkbox I want if users checked chekbox in table insert "1"

else insert "Null" but when I wrote below code

string Off = checkbox1.Checked ? "1" : "null";

it make error Under checked attribute it didn't recognize Checked attribute here. what should i do?