How to inject JavaScript in WebBrowser control?

To inject JavaScript in WebBrowser control, you need to create a Windows Forms application and use the WebBrowser control's document manipulation capabilities. This allows you to dynamically add JavaScript code to web pages loaded in your application.

Prerequisites

• Create a Windows Forms application in Visual Studio
• Drag a WebBrowser control to the form
• Set the Url property of the WebBrowser control
• Right-click the project, choose Add reference... ? COM ? Type Libraries
• Select "Microsoft HTML Object Library"

Method 1: Injecting Script via DOM Manipulation

This approach creates a script element and appends it to the document head:

private void InjectJavaScript(object sender, WebBrowserDocumentCompletedEventArgs e)
{
    // Get the head element
    HtmlElement headElement = webBrowser.Document.GetElementsByTagName("head")[0];

    // Create script element
    HtmlElement scriptElement = webBrowser.Document.CreateElement("script");
    IHTMLScriptElement scriptObj = (IHTMLScriptElement)scriptElement.DomElement;

    // Set JavaScript code
    scriptObj.text = @"
        function sayHello() {
            alert('Welcome to our application!');
        }
        
        function changeBackgroundColor() {
            document.body.style.backgroundColor = 'lightblue';
        }
    ";
    
    // Append to head
    headElement.AppendChild(scriptElement);
    
    // Execute the injected function
    webBrowser.Document.InvokeScript("sayHello");
}

Method 2: Direct Script Execution

You can also execute JavaScript directly without creating DOM elements:

private void ExecuteJavaScript()
{
    string jsCode = @"
        var newDiv = document.createElement('div');
        newDiv.innerHTML = 'This content was added by injected JavaScript!';
        newDiv.style.color = 'red';
        newDiv.style.fontSize = '18px';
        document.body.appendChild(newDiv);
    ";
    
    webBrowser.Document.InvokeScript("eval", new object[] { jsCode });
}

Complete Example

Here's a complete working example showing how to inject and execute JavaScript:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        webBrowser1.DocumentCompleted += WebBrowser_DocumentCompleted;
        webBrowser1.Navigate("https://www.example.com");
    }

    private void WebBrowser_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
    {
        // Ensure document is fully loaded
        if (e.Url.AbsolutePath != webBrowser1.Url.AbsolutePath)
            return;

        InjectCustomScript();
    }

    private void InjectCustomScript()
    {
        try
        {
            HtmlElement head = webBrowser1.Document.GetElementsByTagName("head")[0];
            HtmlElement scriptEl = webBrowser1.Document.CreateElement("script");
            IHTMLScriptElement element = (IHTMLScriptElement)scriptEl.DomElement;
            
            element.text = @"
                function customAlert(message) {
                    alert('Custom Function: ' + message);
                }
                
                function highlightLinks() {
                    var links = document.getElementsByTagName('a');
                    for(var i = 0; i < links.length; i++) {
                        links[i].style.backgroundColor = 'yellow';
                    }
                }
            ";
            
            head.AppendChild(scriptEl);
            
            // Execute the injected functions
            webBrowser1.Document.InvokeScript("customAlert", new object[] { "JavaScript injected successfully!" });
            webBrowser1.Document.InvokeScript("highlightLinks");
        }
        catch (Exception ex)
        {
            MessageBox.Show("Error injecting JavaScript: " + ex.Message);
        }
    }
}

Key Points

• Use the DocumentCompleted event to ensure the page is fully loaded before injecting scripts
• Always wrap JavaScript injection in try-catch blocks for error handling
• The IHTMLScriptElement interface requires adding a reference to Microsoft HTML Object Library
• Use InvokeScript() to execute JavaScript functions after injection
• Check the URL in DocumentCompleted to avoid multiple injections during navigation

Conclusion

JavaScript injection in WebBrowser control enables dynamic web page manipulation in Windows Forms applications. Use DOM manipulation for persistent scripts or direct execution for immediate actions, always ensuring proper error handling.