cefsharp/CefSharp · error · ArgumentNullException

Please provide a valid filePath

Error message

Please provide a valid filePath

What it means

Thrown by the FileResourceHandler constructor when filePath is null or empty. The handler's sole purpose is to stream a file, so an absent path leaves nothing to serve; the constructor rejects it with ArgumentNullException before CEF ever tries to open it.

Source

Thrown at CefSharp/Internals/FileResourceHandler.cs:44

        /// Gets or sets the Mime Type.
        /// </summary>
        public string MimeType { get; set; }

        /// <summary>
        /// Initializes a new instance of the <see cref="FileResourceHandler"/> class.
        /// </summary>
        /// <param name="mimeType">mimeType</param>
        /// <param name="filePath">filePath</param>
        public FileResourceHandler(string mimeType, string filePath)
        {
            if (string.IsNullOrEmpty(mimeType))
            {
                throw new ArgumentNullException("mimeType", "Please provide a valid mimeType");
            }

            if (string.IsNullOrEmpty(filePath))
            {
                throw new ArgumentNullException("filePath", "Please provide a valid filePath");
            }

            if (!File.Exists(filePath))
            {
                throw new FileNotFoundException("Unable to create FileResourceHandler", filePath);
            }

            MimeType = mimeType;
            FilePath = filePath;
        }

        bool IResourceHandler.ProcessRequest(IRequest request, ICallback callback)
        {
            //Should never be called
            throw new NotImplementedException("This method should never be called");
        }

        void IResourceHandler.GetResponseHeaders(IResponse response, out long responseLength, out string redirectUrl)

View on GitHub (pinned to 16bc6e0711)

Solutions

1. Ensure filePath is a non-empty, fully qualified path before constructing the handler.
2. Return null from GetResourceHandler when no file path is available instead of forcing construction.
3. Resolve/validate the path upstream (e.g. Server.MapPath, AppDomain.BaseDirectory combine) and check it.

Example fix

// before
return new FileResourceHandler("text/html", resolvedPath);

// after
if (string.IsNullOrEmpty(resolvedPath)) return null;
return new FileResourceHandler("text/html", resolvedPath);

if (string.IsNullOrEmpty(filePath))
{
    return null; // let default handling proceed
}
return new FileResourceHandler(mimeType, filePath);

static bool HasFilePath(string path) => !string.IsNullOrEmpty(path);

When it happens

Trigger: Constructing new FileResourceHandler("text/html", null) or new FileResourceHandler("text/html", "") where the path variable was never assigned, was cleared, or came from a failed lookup.

Common situations: Serving an asset whose path was resolved from a config/lookup that returned null/empty; passing a user-supplied download target that was unset; placeholder field not yet populated before handler construction.

Related errors

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

AI-assisted analysis of cefsharp/CefSharp@16bc6e0711 (2026-08-13). Data as JSON: /api/errors/f416201c80e12567. Report an issue: GitHub.