cefsharp/CefSharp · error · ArgumentException

An IFrame instance is required.

Error message

An IFrame instance is required.

What it means

Thrown by ScriptedMethods.ActiveElementAcceptsTextInput when the frame argument is null. The extension method needs a live IFrame to execute JavaScript that inspects document.activeElement. Note it uses ArgumentException (not ArgumentNullException) and the older string param name form.

Source

Thrown at CefSharp.Example/ScriptedMethods.cs:26

namespace CefSharp.Example
{
    /// <summary>
    /// Methods whose functionaity is mostly implemented by evaluating or
    /// executing scripts in the browser.
    /// </summary>
    public static class ScriptedMethods
    {
        /// <summary>
        /// Determine if the active element in a frame accepts text input.
        /// </summary>
        /// <param name="frame">Test the active element in this frame.</param>
        /// <returns>True if the active element accepts text input.</returns>
        public static async Task<bool> ActiveElementAcceptsTextInput(this IFrame frame)
        {
            if (frame == null)
            {
                throw new ArgumentException("An IFrame instance is required.", "frame");
            }

            // Scripts should be minified for production builds. The script
            // could also be read from a file...
            const string script =
                  @"(function ()
                    {
                        var isText = false;
                        var activeElement = document.activeElement;
                        if (activeElement) {
                            if (activeElement.tagName.toLowerCase() === 'textarea') {
                                isText = true;
                            } else {
                                if (activeElement.tagName.toLowerCase() === 'input') {
                                    if (activeElement.hasAttribute('type')) {
                                        var inputType = activeElement.getAttribute('type').toLowerCase();
                                        if (inputType === 'text' || inputType === 'email' || inputType === 'password' || inputType === 'tel' || inputType === 'number' || inputType === 'range' || inputType === 'search' || inputType === 'url') {
                                            isText = true;

View on GitHub (pinned to 16bc6e0711)

Solutions

1. Ensure the browser is initialized before accessing MainFrame.
2. Null-check the frame before calling the extension.
3. Re-fetch the frame at call time rather than caching.

Example fix

// before
var accepts = await frame.ActiveElementAcceptsTextInput();

// after
if (frame == null || !frame.IsValid) return false;
var accepts = await frame.ActiveElementAcceptsTextInput();

if (frame == null || !frame.IsValid) return false;
return await frame.ActiveElementAcceptsTextInput();

public static bool IsFrameUsable(IFrame f) => f != null && f.IsValid;

try { return await frame.ActiveElementAcceptsTextInput(); }
catch (ArgumentException ex) when (ex.ParamName == "frame") { return false; }

When it happens

Trigger: Calling frame.ActiveElementAcceptsTextInput() when frame is null — typically because MainFrame was accessed before browser initialization or after disposal.

Common situations: Calling the extension on a frame reference captured too early. Race with browser disposal in a WinForms/WPF closing handler.

Related errors

• frame
• otherRequestContext
• requestContextHandler
• settings
• other

AI-assisted analysis of cefsharp/CefSharp@16bc6e0711 (2026-08-13). Data as JSON: /api/errors/70fbfc4ac458859d. Report an issue: GitHub.