cefsharp/CefSharp · error · ArgumentNullException

Please provide a valid mimeType

Error message

Please provide a valid mimeType

What it means

Thrown by the legacy ResourceHandler constructor when the mimeType argument is null or empty. Although it is a value-required error, the code uses ArgumentNullException with 'mimeType' as the paramName. The parameter defaults to CefSharp.ResourceHandler.DefaultMimeType, so this only fires when the caller explicitly passes null or empty.

Source

Thrown at CefSharp/Lagacy/ResourceHandler.cs:83

        /// <summary>
        /// If the ErrorCode is set then the response will be ignored and
        /// the errorCode returned.
        /// </summary>
        public CefErrorCode? ErrorCode { get; set; }

        /// <summary>
        /// Initializes a new instance of the <see cref="ResourceHandler"/> class.
        /// </summary>
        /// <param name="mimeType">Optional mimeType defaults to <see cref="CefSharp.ResourceHandler.DefaultMimeType"/></param>
        /// <param name="stream">Optional Stream - must be set at some point to provide a valid response</param>
        /// <param name="autoDisposeStream">When true the Stream will be disposed when this instance is Diposed, you will
        /// be unable to use this ResourceHandler after the Stream has been disposed</param>
        /// <param name="charset">response charset</param>
        public ResourceHandler(string mimeType = CefSharp.ResourceHandler.DefaultMimeType, Stream stream = null, bool autoDisposeStream = false, string charset = null)
        {
            if (string.IsNullOrEmpty(mimeType))
            {
                throw new ArgumentNullException("mimeType", "Please provide a valid mimeType");
            }

            StatusCode = 200;
            StatusText = "OK";
            MimeType = mimeType;
            Headers = new NameValueCollection();
            Stream = stream;
            AutoDisposeStream = autoDisposeStream;
            Charset = charset;
        }

        /// <summary>
        /// Begin processing the request. If you have the data in memory you can execute the callback
        /// immediately and return true. For Async processing you would typically spawn a Task to perform processing,
        /// then return true. When the processing is complete execute callback.Continue(); In your processing Task, simply set
        /// the StatusCode, StatusText, MimeType, ResponseLength and Stream
        /// </summary>
        /// <param name="request">The request object.</param>

View on GitHub (pinned to 16bc6e0711)

Solutions

1. Default the argument by not passing it, or pass CefSharp.ResourceHandler.DefaultMimeType explicitly.
2. Coalesce: mimeType ?? CefSharp.ResourceHandler.DefaultMimeType.
3. Resolve the MIME from the file extension (e.g. MimeMapping) before constructing.
4. Validate the MIME is non-empty at the call site.

Example fix

// before
var h = new ResourceHandler(mimeType: resolvedMimeType); // null -> throws

// after
var mt = string.IsNullOrEmpty(resolvedMimeType) ? CefSharp.ResourceHandler.DefaultMimeType : resolvedMimeType;
var h = new ResourceHandler(mimeType: mt);

var mime = string.IsNullOrEmpty(resolved) ? CefSharp.ResourceHandler.DefaultMimeType : resolved;
var handler = new ResourceHandler(mime);

try { var h = new ResourceHandler(mime); }
catch (ArgumentNullException ex) when (ex.ParamName == "mimeType")
{ var h = new ResourceHandler(CefSharp.ResourceHandler.DefaultMimeType); }

When it happens

Trigger: Constructing new ResourceHandler(mimeType: null) or new ResourceHandler("") (e.g. forwarding a request MIME type that was unset); code that derives the MIME from content and passes through a null.

Common situations: Passing a computed/looked-up MIME that returned null; clearing a header value before constructing; legacy code migrating from an overload that allowed null.

Related errors

• Please provide a valid mimeType
• Please provide a valid array
• Unable to load byte[] with length 0.
• Please provide a valid mimeType
• Please provide a valid filePath

AI-assisted analysis of cefsharp/CefSharp@16bc6e0711 (2026-08-13). Data as JSON: /api/errors/078848fe0f24d4e7. Report an issue: GitHub.