How To Validate Windows Form In C# .NET

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.Text.RegularExpressions;
namespace ValidationForm {
    public partial class Form1: Form {
        public Form1() {
            InitializeComponent();
        }
        private void btnOk_Click(object sender, EventArgs e) {
                // find blank TextBoxs and orderby TabIndex with Linq Concept.....................
                var emptytextbox = from Control CurrentControl in Controls
                where CurrentControl is TextBox
                let box = CurrentControl as TextBox
                where String.IsNullOrEmpty(box.Text)
                orderby box.TabIndex
                select box;
                // if there is empty textbox........
                if (emptytextbox.Count() > 0) {
                    // display message box indicating message information.........................
                    MessageBox.Show("Please fill the all TextBox Field", "Missing Information", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    //select first empty TextBox...............
                    emptytextbox.First().Select();
                } else {
                    if (!ValidateInput(txtfname.Text, "^[A-Z][a-zA-Z]*$", "Your First Name is not valid format")) {
                        txtfname.Select();
                    } else if (!ValidateInput(txtlname.Text, "^[A-Z][a-zA-Z]*$", "Your Last Name is not valid format")) {
                        txtlname.Select();
                    } else if (!ValidateInput(txtemailid.Text, @ "^\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*", "Your emailid is not valid format")) {
                        txtemailid.Select();
                    } else if (!ValidateInput(txtaddress.Text, @ "^[0-9]+\s+([a-zA-Z]+|[a-zA-Z]+\s[a-zA-Z]+)$", "Your Address is not valid format")) {
                        txtaddress.Select();
                    } else if (!ValidateInput(txtcityname.Text, @ "^([a-zA-Z]+|[a-zA-Z]+\s[a-zA-Z]+)$", "Your City Name is not valid format")) {
                        txtcityname.Select();
                    } else if (!ValidateInput(txtstatename.Text, @ "^([a-zA-Z]+|[a-zA-Z]+\s[a-zA-Z]+)$", "Your State Name is not valid format")) {
                        txtstatename.Select();
                    }
                    // Phone numbers like 0548-2221005
                    else if (!ValidateInput(txtphoneno.Text, @ "^[0-9]\d{2,4}-\d{6,8}$", "Your Phone No is not valid format")) {
                        txtphoneno.Select();
                    } else if (!ValidateInput(txtzipcode.Text, @ "^\d{6}$", "Please enter valid Zip-Code like-233001")) {
                        txtzipcode.Select();
                    }
                    // Mobile numbers starting from 7,8 and 9 and not from 0,6...
                    else if (!ValidateInput(txtmobileno.Text, @ "^[789]\d{9}$", "Please enter valid Mobile No like- 9451119029")) {
                        txtmobileno.Select();
                    } else {
                        this.Hide();
                        MessageBox.Show("Thank You", "Correct Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
                        Application.Exit();
                    }
                }
            }
            //user regular expressions to validate user input.....................
        private bool ValidateInput(string input, string expression, string message) {
            //store whether the input is valid
            bool valid = Regex.Match(input, expression).Success;
            //if the input doesn't match the regular expression......
            if (!valid) {
                //input was invalid......
                MessageBox.Show(message, "Invalid Input", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            return valid;
        }
    }
}