In this article I will explain with an example, how to implement Captcha in ASP.Net using C# and VB.Net.
 
 

Download ASPSnippets Captcha DLL

You can download the latest ASPSnippets.Captcha.dll from the following link.
Download DLL file
Note: You will need to add the reference of ASPSnippets.Captcha DLL in your project.
 
 

HTML Markup

The HTML markup consists of following controls:
TextBox – For capturing PersonId, Name and City.
DropDownList – For capturing Gender.
 

Captcha implementation

Following controls are required for Captcha implementation.
TextBox – For capturing Captcha answer.
Image – For displaying Captcha image.
 
CustomValidator – For validating Captcha.

Properties

ErrorMessage – For displaying error message when validation fails.
 

Events

The CustomValidator has been assigned with an OnServerValidate event handler for performing Captcha validation.
 
Button – For submitting the Form.
<table cellpadding="0" cellspacing="0">
    <tr>
        <th colspan="2" align="center">Person Details</th>
    </tr>
    <tr>
        <td>Person Id: </td>
        <td><asp:TextBox ID="txtPersonId" runat="server"></asp:TextBox></td>
    </tr>
    <tr>
        <td>Name: </td>
        <td><asp:TextBox ID="txtName" runat="server"></asp:TextBox></td>
    </tr>
    <tr>
        <td>Gender: </td>
        <td>
            <asp:DropDownList ID="ddlGenders" runat="server" Width="170">
                <asp:ListItem Text="Please Select" Value=""></asp:ListItem>
                <asp:ListItem Text="Male" Value="M"></asp:ListItem>
                <asp:ListItem Text="Female" Value="F"></asp:ListItem>
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>City: </td>
        <td><asp:TextBox ID="txtCity" runat="server"></asp:TextBox></td>
    </tr>
    <tr>
        <td><asp:Image ID="imgCaptcha" runat="server"/></td>
        <td>
            <asp:TextBox ID="txtCaptchaAnswer" runat="server"></asp:TextBox>
            <asp:CustomValidator ID="cvCaptchaValidate" runat="server" OnServerValidate="OnCaptchaValidate"
                ErrorMessage="Captcha invalid" ForeColor="Red"></asp:CustomValidator>
        </td>
    </tr>
    <tr>
        <td></td>
        <td><asp:Button ID="btnSubmit" runat="server" Text="Submit" /></td>
    </tr>
</table>
 
 

Adding reference of ASPSnippets.Captcha DLL in project

1. Inside the Solution Explorer, right click on Project and then click on Add and then click on Reference.
Implement Captcha in ASP.Net
 
2. Then, inside the Reference Manager window, click on Browse button.
Implement Captcha in ASP.Net
 
3. Locate the ASPSnippets.Captcha and select it.
Implement Captcha in ASP.Net
 
4. The ASPSnippets.Captcha DLL is now visible in the Reference Manager window. Select it and click OK button as shown below.
Implement Captcha in ASP.Net
 
Finally, the ASPSnippets.Captcha DLL is now referenced.
Implement Captcha in ASP.Net
 
 
 

Namespaces

You will need to import the following namespace.
C#
using ASPSnippets.Captcha;
 
VB.Net
Imports ASPSnippets.Captcha
 
 

Implementing Arithmetic Captcha in ASP.Net

In the code-behind, a property of class Captcha is created.
Note: The property is a ViewState property so that its value is preserved across PostBacks. For more information please refer Preserving state and values of Custom Properties across PostBack in ASP.Net.
 

Page_Load

Inside the Page_Load event handler, Captcha object is initialized and its width, height, Font size, Font color, background color and Mode are set.
Note: The ASPSnippets.Captcha works in four different Modes i.e. Numeric, Alphabets, AlphaNumeric and Arithmetic. This article will use AlphaNumeric mode.
 
Finally, the Image control is set with the ImageData property.
C#
private Captcha Captcha
{
    get
    {
        return (Captcha)ViewState["Captcha"];
    }
    set
    {
        ViewState["Captcha"] = value;
    }
}
 
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.Captcha = new Captcha(125, 40, 20f, "#FFFFFF", "#61028D", Mode.AlphaNumeric);
        imgCaptcha.ImageUrl = this.Captcha.ImageData;
    }
}
 
VB.Net
Private Property Captcha As Captcha
    Get
        Return CType(ViewState("Captcha"), Captcha)
    End Get
    Set(ByVal value As Captcha)
        ViewState("Captcha") = value
    End Set
End Property
 
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
    If Not Me.IsPostBack Then
        Me.Captcha = New Captcha(125, 40, 20.0F, "#FFFFFF", "#61028D", Mode.AlphaNumeric)
        imgCaptcha.ImageUrl = Me.Captcha.ImageData
    End If
End Sub
 
 

Validating Captcha using C# and VB.Net

When the Submit button is clicked, the OnCaptchaValidate event handler of the CustomValidator is executed.
Inside this event handler, the Captcha is validated using the IsValid method to which the Answer inputted by the User is passed.
If the IsValid method returns FALSE, the Error Message is displayed using the CustomValidator.
C#
protected void OnCaptchaValidate(object sender, ServerValidateEventArgs e)
{
    e.IsValid = this.Captcha.IsValid(txtCaptchaAnswer.Text.Trim());
    if (e.IsValid)
    {
       //Perform operations here.
       int personId = int.Parse(txtPersonId.Text.Trim());
       string name = txtName.Text.Trim();
       string gender = ddlGenders.SelectedItem.Value;
       string city = txtCity.Text.Trim();
    }
}
 
VB.Net
Protected Sub OnCaptchaValidate(ByVal sender As Object, ByVal e As ServerValidateEventArgs)
    e.IsValid = Me.Captcha.IsValid(txtCaptchaAnswer.Text.Trim())
    If e.IsValid Then
        'Perform operations here.
        Dim personId As Integer = Integer.Parse(txtPersonId.Text.Trim())
        Dim name As String = txtName.Text.Trim()
        Dim gender As String = ddlGenders.SelectedItem.Value
        Dim city As String = txtCity.Text.Trim()
    End If
End Sub
 
 

Screenshot

Implement Captcha in ASP.Net
 
 

Demo

View Demo
 
 

Downloads

Download Code