In this article, we are going to learn Server Side Validation in asp.net. In real scenario, we use client side validation or server side validation or combination of both i.e. client side as well as server side validation. In server side, validation occurs at server side hence it is more secure than client side validation.

In designer file:

In designer file, create a registration form using TextBox, DropDownlist and Button Control. Designer file will look like as shown below.
Below is complete designer code.

div align="center">
table style="border: 1px solid black;">

td colspan="2">span style="color: red;">*span>indicates required fields
td>


td>First Nametd>
td>
                        TextBox ID="txtFirstName" runat="server">TextBox>
span style="color: red;">*span>
td>


td>Last Nametd>
td>
                        TextBox ID="txtLastName" runat="server">TextBox>
span style="color: red;">*span>
td>


td>Agetd>
td>
                        TextBox ID="txtAge" runat="server">TextBox>
span style="color: red;">*span>
td>


td>Gendertd>
td>
                        DropDownList ID="ddlGender" runat="server">
                            ListItem Text="--select--" Value="--select--">ListItem>
                            ListItem Text="Male" Value="Male">ListItem>
                            ListItem Text="Female" Value="Female">ListItem>
DropDownList>
span style="color: red;">*span>
td>


td>Emailtd>
td>
                        TextBox ID="txtEmail" runat="server">TextBox>
span style="color: red;">*span>
td>


td>
                        Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click">
Button>
td>
td>
                        Label ID="lblError" runat="server" ForeColor="Red">
Label>
td>

table>
div>

Namespace used:

Include the below Namespace.

using System.Text.RegularExpressions;

Complete Code:

protected void Page_Load(object sender, EventArgs e)
        {

        }

protected void btnSubmit_Click(object sender, EventArgs e)
        {
            lblError.Text = string.Empty;

if (txtFirstName.Text.Trim() == string.Empty)
            {
                lblError.Text = "Please enter first name";
                txtFirstName.Focus();
return;
            }
if (txtLastName.Text.Trim() == string.Empty)
            {
                lblError.Text = "Please enter last name";
                txtLastName.Focus();
return;
            }

var age= txtAge.Text.Trim();
var isNumeric = !string.IsNullOrEmpty(age) && age.All(Char.IsDigit);

if (isNumeric==false || Convert.ToInt32(age) Convert.ToInt32(age) > 30)
            {
                lblError.Text = "Please enter valid age between 20 and 30";
                txtAge.Focus();
return;
            }

if (ddlGender.SelectedIndex == 0)
            {
                lblError.Text = "Please select gender";
                ddlGender.Focus();
return;
            }
//regex is used to check valid mail
if (!Regex.IsMatch(txtEmail.Text.Trim(), @"^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$"
            , RegexOptions.IgnoreCase))
            {
                lblError.Text = "Please enter valid Email";
                txtEmail.Focus();
return;
            }
/*

              call your Save method to save data


            */
            lblError.Text = "Record submitted successfully!!";
            lblError.ForeColor = System.Drawing.Color.Green;
            ClearControls();
        }

public void ClearControls()
        {
            txtFirstName.Text = txtLastName.Text = txtAge.Text = txtEmail.Text = string.Empty;
            ddlGender.SelectedIndex = 0;

        }

When page loads, form will look like as shown below.

When you click on submit button without filling form, error message will be shown on Label control.

In the next article, we are going to learn client side validation in asp.net Simple registration form with client side validation in asp.net