Set variable based on CheckBox checked property in C# aSP.Net

HI

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?

Best Regards