if condition with sql command

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

using System.Data.SqlClient;

using System.Collections;

 

 

namespace WindowsFormsApplication19

{

          

    public partial class Form1 : Form

    {

        string str;

        public string CUSTOMERTABLE = "devi";

 

        int i;

        string strc00;

       

        public Form1()

        {

            InitializeComponent();

            CUSTOMERTABLE = "devi";

            BasDataSet dataSet = new BasDataSet();

            BasDataSetTableAdapters.deviTableAdapter adapter = new BasDataSetTableAdapters.deviTableAdapter();

            adapter.Fill(dataSet.devi);

 

            comboBox1.DataSource = dataSet.devi;

            comboBox1.DisplayMember = "CustomerId";

        }

 

        private void button1_Click(object sender, EventArgs e)

        {

 

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

            con.Open();

 

  

No 1 :condition:        if ((comboBox1.Text == "") && (comboBox2.Text == "") && (comboBox3.Text == "") && (comboBox4.Text == "") && (comboBox5.Text == ""))

 

            {

                MessageBox.Show("doesn't store null value");

            }

            Else

No 2 :condition: 

                if ((comboBox1.Text == "") || (comboBox2.Text == "") || (comboBox3.Text == "") || (comboBox4.Text == "") || (comboBox5.Text == ""))

 

                {

                    MessageBox.Show("Enter the nessary fields");

                }

If above two condition is failed then I need  to check 3^rd conditions with database. How can I get this please help me I need to check with database of table of columnname "Phonenum"

Else

No 3 :condition: check 3^rd conditions with database 

                {

 

Here my condition is the phonenum is already exist or not.

 

                    if (the phonenum is already exist or not.)

                    {

                        MessageBox.Show("this Customerdetails already  registered");

                    }

 

                    Else

{

 

MessageBox.show(" Enter New Customerdetails ");

 

ComboBox1.Text=="";

ComboBox2.Text=="";

.

.

.

}

No 4 :condition: if the above condition is satisfied then go to 4^th condition.

 

                    {

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();

                        con.Close();

                        MessageBox.Show("Added Successfully");

                   }

                }

        }

And one problem is: I have button as new user

 

If I click new user I need to create Automatic id in my 1^st click starts with C001,C002. After that I am closing my application again I am

   Executing means its need to continue with Next value of id i.e. C003,C004

The below condition is create new id with database of table of CutomerID

 

I have connect with sql connectivity code like in add button but it shows error

 

 

How can I get this please tell me

       

        private void Newuser_Click_1(object sender, EventArgs e)

        {

            if (i > 0)

            {

                int j = i + 1;

                strc00 = "C00" + j.ToString();

            }

    else

            {

                strc00 = "C001";

            }

 

            comboBox1.Items.Add(strc00);

            i++;

            MessageBox.Show("id created");

           

}