cefsharp/CefSharp · error · Exception

IBrowserHost is null

Error message

IBrowserHost is null

What it means

Thrown by the example ChromiumWebBrowserWithScreenshotSupport.TakeScreenshot when this.GetBrowserHost() returns null after the earlier guards passed. The capture path needs IBrowserHost (to set WindowlessFrameRate and call WasResized), so a null host means the native host object is not available despite the browser reporting initialized. This is a defensive check for a narrow race or a disposing host.

Source

Thrown at CefSharp.Wpf.Example/Controls/ChromiumWebBrowserWithScreenshotSupport.cs:68

            {
                throw new Exception("Screenshot already in progress, you must wait for the previous screenshot to complete");
            }

            if (IsBrowserInitialized == false)
            {
                throw new Exception("Browser has not yet finished initializing or is being disposed");
            }

            if (IsLoading)
            {
                throw new Exception("Unable to take screenshot while browser is loading");
            }

            var browserHost = this.GetBrowserHost();

            if (browserHost == null)
            {
                throw new Exception("IBrowserHost is null");
            }

            screenshotTaskCompletionSource = new TaskCompletionSource<InteropBitmap>(TaskCreationOptions.RunContinuationsAsynchronously);

            if (cancellationToken.HasValue)
            {
                var token = cancellationToken.Value;
                cancellationTokenRegistration = token.Register(() =>
                {
                    screenshotTaskCompletionSource.TrySetCanceled();

                    cancellationTokenRegistration?.Dispose();

                }, useSynchronizationContext: false);
            }

            if (frameRate.HasValue)
            {

View on GitHub (pinned to 16bc6e0711)

Solutions

1. Guard with `if (browser.GetBrowserHost() != null)` and defer/retry the capture if null.
2. Cancel captures when disposal begins so the host-null state is never hit.
3. Add a short retry/backoff for the rare initialization race rather than throwing.
4. If the host is persistently null, verify the browser is not in a disposing state before retrying.

Example fix

// before
var shot = browser.TakeScreenshot(size); // throws if host null race

// after
if (browser.GetBrowserHost() != null)
    _ = browser.TakeScreenshot(size);
else
    // defer and retry shortly

if (browser.GetBrowserHost() != null)
{
    _ = browser.TakeScreenshot(size);
}

static bool IsHostReady(ChromiumWebBrowserWithScreenshotSupport b) => b.GetBrowserHost() != null;

When it happens

Trigger: The browser reports IsBrowserInitialized true and not loading, but the host object is momentarily null (initialization race) or being torn down; calling during the dispose sequence after BrowserCore is cleared.

Common situations: A race between the initialized flag and host availability; calling capture exactly as the control is disposed; edge cases in off-screen hosting where the host lags initialization.

Related errors

• Browser has not yet finished initializing or is being dispos
• Screenshot already in progress, you must wait for the previo
• Unable to take screenshot while browser is loading
• Browser has already been created. BrowserSettings must be se
• Browser has already been created. RequestContext must be set

AI-assisted analysis of cefsharp/CefSharp@16bc6e0711 (2026-08-13). Data as JSON: /api/errors/ff86be9c22cc985d. Report an issue: GitHub.