cefsharp/CefSharp · error · ArgumentNullException

Please provide a valid mimeType

Error message

Please provide a valid mimeType

What it means

Thrown by the ByteArrayResourceHandler constructor when the mimeType argument is null or empty. ByteArrayResourceHandler is a placeholder that wraps a byte array for a CefStreamResourceHandler; CEF needs a valid MIME type to generate correct response headers. The guard fails fast rather than handing CEF an undefined content type.

Source

Thrown at CefSharp/Internals/ByteArrayResourceHandler.cs:38

        /// Underlying byte array that represents the data
        /// </summary>
        public byte[] Data { get; private set; }

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

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

            if (data == null)
            {
                throw new ArgumentNullException("data", "Please provide a valid array");
            }

            if (data.Length == 0)
            {
                throw new Exception("Unable to load byte[] with length 0.");
            }

            MimeType = mimeType;
            Data = data;
        }

        bool IResourceHandler.ProcessRequest(IRequest request, ICallback callback)
        {

View on GitHub (pinned to 16bc6e0711)

Solutions

1. Pass an explicit, non-empty MIME type such as "text/html" or "application/json".
2. If the type is dynamic, resolve it via a lookup (e.g. MimeMapping.GetMimeMap) with a fallback default before constructing the handler.
3. Guard the value: if (string.IsNullOrEmpty(mimeType)) mimeType = "application/octet-stream"; before the constructor call.

Example fix

// before
var handler = new ByteArrayResourceHandler(mimeType, bytes);

// after
if (string.IsNullOrEmpty(mimeType)) mimeType = "text/html";
var handler = new ByteArrayResourceHandler(mimeType, bytes);

if (string.IsNullOrEmpty(mimeType))
{
    mimeType = "application/octet-stream"; // or resolve from file extension
}
var handler = new ByteArrayResourceHandler(mimeType, data);

When it happens

Trigger: Constructing new ByteArrayResourceHandler(null, data), new ByteArrayResourceHandler("", data), or passing a mimeType variable that was never assigned. Typical in IResourceRequestHandler.GetResourceHandler implementations that serve in-memory content.

Common situations: Serving dynamically generated HTML/JSON/images where the MIME type is computed at runtime and may be unset; copying a snippet that omitted the first argument; passing a content-type resolved from an extension lookup that returned null.

Related errors

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

AI-assisted analysis of cefsharp/CefSharp@16bc6e0711 (2026-08-13). Data as JSON: /api/errors/5d06c3924a3c8f46. Report an issue: GitHub.