Samo bih ti preporučio da koristiš parametriziranje kako bi spriječio SQL injection...
Dodaj System.Data namespace prvo...
Code:
string selectString = ("SELECT count(username) FROM tabela WHERE username= @username");
SqlCommand mySqlCommand = new SqlCommand(selectString,mySqlConnection);
mySqlCommand.Parameters.Add("@username", SqlDbType.String).Value = textbox1.Text;
int count = (int)mySqlCommand.ExecuteScalar();