In this article I will explain with an example, how to implement Captcha in Windows Forms (WinForms) Application 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.
 
 

Form Design

The Form consists of following controls:
Label – For labelling TextBoxes.
TextBox – For capturing PersonId, Name, City.
 

Captcha implementation

Following controls are required for Captcha implementation.
PictureBox – For displaying Captcha.
TextBox – For capturing Captcha answer.
ErrorProvider – For displaying error icon.
Here, ErrorProvider control is used for displaying error for different cases.

Cases

        · When TextBox is empty.
        · When Captcha answer is invalid.
Note: For more details on how to use ErrorProvider control, please refer my article Using ErrorProvider control in Windows Forms with C# and VB.Net.
 
Button – For validating Captcha.
Implement Captcha in Windows Forms using C# and VB.Net
 
 

Adding reference of ASPSnippets.Captcha DLL in project

1. Inside the Solution Explorer, right click on Reference option of your Project and then click on Add Reference.
Implement Captcha in Windows Forms using C# and VB.Net
 
2. Then, inside the Reference Manager window, click on Browse button.
Implement Captcha in Windows Forms using C# and VB.Net
 
3. Locate the ASPSnippets.Captcha and select it.
Implement Captcha in Windows Forms using C# and VB.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 Windows Forms using C# and VB.Net
 
Finally, the ASPSnippets.Captcha DLL is now referenced.
Implement Captcha in Windows Forms using C# and VB.Net
 
 

Namespaces

You will need to import the following namespaces.
C#
using System.IO;
using System.Drawing;
using ASPSnippets.Captcha;
 
VB.Net
Imports System.IO
Imports System.Drawing
Imports ASPSnippets.Captcha
 
 

Implementing Arithmetic Captcha using C# and VB.Net

In the code-behind, a property of class Captcha is created.

Form_Load

Inside the Form_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.
 
Then, using ImageData property of Captcha class the BASE64 string is fetched and splatted with comma (,) and converted to BYTE array using FromBase64String method of Convert class.
Finally, the BYTE array is converted to Image using MemoryStream class which is then displayed using the PictureBox control.
C#
private Captcha Captcha { get; set; }
 
private void Form1_Load(object sender, EventArgs e)
{
    this.Captcha = new Captcha(130, 40, 20f, "#FFFFFF", "#61028D", Mode.AlphaNumeric);
    string base64String = this.Captcha.ImageData.Split(',')[1];
 
    //Convert Base64 Encoded string to Byte Array.
    byte[] imageBytes = Convert.FromBase64String(base64String);
 
    //Convert Byte Array to Image and display in PictureBox.
    imgCaptcha.Image = Image.FromStream(new MemoryStream(imageBytes));
}
 
VB.Net
Private Property Captcha As Captcha
 
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Me.Captcha = New Captcha(130, 40, 20.0F, "#FFFFFF", "#61028D", Mode.AlphaNumeric)
    Dim base64String As String = Me.Captcha.ImageData.Split(","c)(1)
 
    'Convert Base64 Encoded string to Byte Array.
    Dim imageBytes As Byte() = Convert.FromBase64String(base64String)
 
    'Convert Byte Array to Image and display in PictureBox.
    imgCaptcha.Image = Image.FromStream(New MemoryStream(imageBytes))
End Sub
 
 

Validating Captcha using C# and VB.Net

When Submit button is clicked, the ErrorProvider is set to empty and a check is performed whether the Captcha answer TextBox is empty or not.
If found empty or NULL, then Captcha required message is displayed using ErrorProvider control.
If found text in the TextBox, then Captcha is validated using IsValid method of Captcha class.
If IsValid method return FALSE, then Error Message is displayed using ErrorProvider.
C#
private void OnSubmit(object sender, EventArgs e)
{
    epError.SetError(txtCaptchaAnswer, string.Empty);
    if (string.IsNullOrEmpty(txtCaptchaAnswer.Text.Trim()))
    {
        epError.SetError(txtCaptchaAnswer, "Captcha required");
    }
    else
    {
        if (!this.Captcha.IsValid(txtCaptchaAnswer.Text.Trim()))
        {
            epError.SetError(txtCaptchaAnswer, "Captcha invalid");
        }
        else
        {
            //Perform operations here.
            int personId = int.Parse(txtPersonId.Text.Trim());
            string name = txtName.Text.Trim();
            string gender = cbGender.Text;
            string city = txtCity.Text.Trim();
        }
    }
}
 
VB.Net
Private Sub OnSubmit(sender As Object, e As EventArgs) Handles btnSubmit.Click
    epError.SetError(txtCaptchaAnswer, String.Empty)
    If String.IsNullOrEmpty(txtCaptchaAnswer.Text.Trim()) Then
        epError.SetError(txtCaptchaAnswer, "Captcha required")
    Else
        If Not Me.Captcha.IsValid(txtCaptchaAnswer.Text.Trim()) Then
            epError.SetError(txtCaptchaAnswer, "Captcha invalid")
        Else
            'Perform operations here.
            Dim personId As Integer = Integer.Parse(txtPersonId.Text.Trim())
            Dim name As String = txtName.Text.Trim()
            Dim gender As String = cbGender.Text
            Dim city As String = txtCity.Text.Trim()
        End If
    End If
End Sub
 
 

Screenshot

Implement Captcha in Windows Forms using C# and VB.Net
 
 

Downloads

Download Code