windows Application with sql server 2008

Question

i need example program code which includes windows form and database code. 
 
i want to know how to give if condition with database connectivity. i have code like this

 private void button1_Click(object sender, EventArgs e)

        {

             

            {

                SqlConnection con = new SqlConnection(@"server=CSE-PC; integrated security=true; Database=Bas");

                con.Open();

                SqlCommand cmd1 = new SqlCommand("insert into devi Values('" + comboBox1.Text + "',+'" + comboBox2

                    .Text + "','" + comboBox3.Text + "','" + comboBox4.Text + "','" + comboBox5.Text + "','" + comboBox6.Text + "','" + comboBox7.Text + "')", con);

                cmd1.ExecuteNonQuery();          hera, i need to give if condition for null. i.e . if box is empty it does not store. it need to show message as MessageBox.Show("doesnot store null value");

                con.Close();

                MessageBox.Show("Added Successfully");

                comboBox1.Text = "";

                comboBox2.Text = "";

                comboBox3.Text = "";

                comboBox4.Text = "";

                comboBox5.Text = "";

                comboBox6.Text = "";

                comboBox7.Text = "";

            }

            

        }
 how give if condition for this please help me