cefsharp/CefSharp · error · Exception

RequestContext can only be of type {0} or null

Error message

RequestContext can only be of type {0} or null

What it means

The RequestContext property only accepts instances whose unwrapped type is assignable to Core.ObjectFactory.RequestContextType — the internal CefSharp implementation. A custom IRequestContext implementation or a mock that does not derive from the correct concrete type is rejected.

Source

Thrown at CefSharp.Wpf/HwndHost/ChromiumWebBrowser.cs:256

            }
        }
        /// <summary>
        /// Gets or sets the request context.
        /// </summary>
        /// <value>The request context.</value>
        public IRequestContext RequestContext
        {
            get { return requestContext; }
            set
            {
                if (browserCreated)
                {
                    throw new Exception("Browser has already been created. RequestContext must be " +
                                        "set before the underlying CEF browser is created.");
                }
                if (value != null && !Core.ObjectFactory.RequestContextType.IsAssignableFrom(value.UnWrap().GetType()))
                {
                    throw new Exception(string.Format("RequestContext can only be of type {0} or null", Core.ObjectFactory.RequestContextType));
                }
                requestContext = value;
            }
        }
        /// <summary>
        /// Implement <see cref="IDialogHandler" /> and assign to handle dialog events.
        /// </summary>
        /// <value>The dialog handler.</value>
        public IDialogHandler DialogHandler { get; set; }
        /// <summary>
        /// Implement <see cref="IJsDialogHandler" /> and assign to handle events related to JavaScript Dialogs.
        /// </summary>
        /// <value>The js dialog handler.</value>
        public IJsDialogHandler JsDialogHandler { get; set; }
        /// <summary>
        /// Implement <see cref="IKeyboardHandler" /> and assign to handle events related to key press.
        /// </summary>
        /// <value>The keyboard handler.</value>

View on GitHub (pinned to 16bc6e0711)

Solutions

1. Use RequestContext or RequestContextAdapter from CefSharp.Core — instantiate via new RequestContext(settings) or Cef.GetGlobalRequestContext().
2. If wrapping is needed, inherit from RequestContext and override virtual members rather than implementing IRequestContext from scratch.
3. In tests, use a real RequestContext with a temp cache path instead of a mock.

Example fix

// before — custom implementation rejected
public class MyRequestContext : IRequestContext { ... }
browser.RequestContext = new MyRequestContext();

// after — use the Core type
browser.RequestContext = new RequestContext(new RequestContextSettings { CachePath = cachePath });

// Verify the unwrapped type is the Core RequestContext before assigning
static bool IsValidRequestContext(IRequestContext context)
{
    if (context == null) return true; // null is allowed
    var unwrapped = context.UnWrap();
    return CefSharp.Core.ObjectFactory.RequestContextType
        .IsAssignableFrom(unwrapped.GetType());
}

try
{
    browser.RequestContext = context;
}
catch (Exception ex) when (ex.Message.Contains("RequestContext can only be of type"))
{
    throw new InvalidOperationException(
        "Use RequestContext from CefSharp.Core, not a custom IRequestContext", ex);
}

When it happens

Trigger: Passing a hand-rolled IRequestContext, a mocking-framework proxy, or any wrapper whose UnWrap() returns something that is not the Core RequestContext type.

Common situations: Unit testing with a mocked IRequestContext. Building a decorator/wrapper around IRequestContext without inheriting from RequestContext.

Related errors

• Browser has already been created. RequestContext must be set
• otherRequestContext
• requestContextHandler
• settings
• A call to WithSharedSettings has already been made, it is no

AI-assisted analysis of cefsharp/CefSharp@16bc6e0711 (2026-08-13). Data as JSON: /api/errors/bd7560dda64f6eaf. Report an issue: GitHub.