Question

With my Application, i want to make sure if user enter no value in a textbox, and click on the
save button to send data in the sqlserver db.The database side validation prevent this violation and set ErrorMessage which my application will catch and Display a meaninful Message
to the User. For each required field i set it to NOT NULL. But when i test,i can still enter
enter empty textbox values it gets inserted with out value.
what am i missing?

string connectionstring = "Data Source=abcdef;Initial Catalog=HMS;Persist Security Info=True;User ID=sysad;Password=abcdef";
        SqlConnection connection = new SqlConnection(connectionstring);

        string SelectStatement = "SELECT * FROM tablename where RegistrationNo = @RegistrationNo";
        SqlCommand insertcommand = new SqlCommand(SelectStatement, connection);
        insertcommand.Parameters.AddWithValue("@RegistrationNo", textBox10.Text);
        SqlDataReader reader;
        try
        {
            connection.Open();
            reader = insertcommand.ExecuteReader();

            while (reader.Read())
            {
                textBox11.Text = reader["RegistrationNo"].ToString();
                textBox1.Text = reader["Appearance"].ToString();
                textBox2.Text = reader["VolumePH"].ToString();
                textBox3.Text = reader["Mobility"].ToString();
                textBox4.Text = reader["Viability"].ToString();
                textBox5.Text = reader["Head"].ToString();
                textBox6.Text = reader["MiddlePiece"].ToString();
                textBox7.Text = reader["Tail"].ToString();
                textBox8.Text = reader["SpermCount"].ToString();
                dateTimePicker1.Text = reader["Date"].ToString();
                textBox9.Text = reader["Comment"].ToString();



            }//end while
            reader.Close();
        }
        catch (Exception ex)
        {
            throw ex;

        }//end catch

Answer 1

What am i missing?

I think you are missing a distinction between null and an empty string.

Databases distinguish between null and empty. If your ToString succeeds, then you have a non-null string there, and so DB is happy to accept it as a valid value.

In general, using DB for user-side validation is somewhat wasteful: if you know that the field must not be empty, you should check for it in the UI; DB validation should serve as the last resort that preserves the integrity of your data model.