restsharp/RestSharp · error · ArgumentException

Failed to put file as content because flag AlwaysMultipartFo

Error message

Failed to put file as content because flag AlwaysMultipartFormData is enabled

What it means

RestRequest.ValidateParameters throws when AlwaysSingleFileAsContent is enabled (sending one file as the raw request body) but AlwaysMultipartFormData is also enabled. The two modes are incompatible: single-file-as-content puts the file bytes directly in the body, while multipart wraps everything in a multipart/form-data envelope.

Source

Thrown at src/RestSharp/Request/RestRequestExtensions.File.cs:79

        /// <param name="options">File parameter header options</param>
        /// <returns></returns>
        public RestRequest AddFile(
            string                name,
            Func<Stream>          getFile,
            string                fileName,
            ContentType?          contentType = null,
            FileParameterOptions? options     = null
        )
            => request.AddFile(FileParameter.Create(name, getFile, fileName, contentType, options));

        internal void ValidateParameters() {
            if (!request.AlwaysSingleFileAsContent) return;

            var postParametersExists = request.Parameters.GetContentParameters(request.Method).Any();
            var bodyParametersExists = request.Parameters.Any(p => p.Type == ParameterType.RequestBody);

            if (request.AlwaysMultipartFormData)
                throw new ArgumentException("Failed to put file as content because flag AlwaysMultipartFormData is enabled");

            if (postParametersExists)
                throw new ArgumentException("Failed to put file as content because POST parameters were added");

            if (bodyParametersExists)
                throw new ArgumentException("Failed to put file as content because body parameters were added");
        }
    }
}

View on GitHub (pinned to 6a50821692)

Solutions

1. Decide on one mode: for a single file as raw body keep AlwaysSingleFileAsContent = true and set AlwaysMultipartFormData = false.
2. For multipart uploads, leave AlwaysMultipartFormData = true and do not set AlwaysSingleFileAsContent.
3. Review all request flags in one place before adding files.

Example fix

// before
request.AlwaysSingleFileAsContent = true;
request.AlwaysMultipartFormData   = true; // throws on AddFile/Execute

// after
request.AlwaysSingleFileAsContent = true;
request.AlwaysMultipartFormData   = false;

if (request.AlwaysSingleFileAsContent && request.AlwaysMultipartFormData)
    throw new InvalidOperationException("Cannot enable both AlwaysSingleFileAsContent and AlwaysMultipartFormData");
request.AddFile(name, bytes, fileName);

When it happens

Trigger: Setting both request.AlwaysSingleFileAsContent = true and request.AlwaysMultipartFormData = true, then adding a file (AddFile) which calls ValidateParameters. Also triggered at request execution time via request.ValidateParameters() in the pipeline.

Common situations: Copy-pasting options from a multipart upload example into a single-file-content flow; toggling flags during experimentation without resetting the conflicting one.

Related errors

• Failed to put file as content because body parameters were a
• AdvancedResponseWriter is not null. Only one response writer
• ResponseWriter is not null. Only one response writer can be
• Failed to put file as content because POST parameters were a
• Both BaseUrl and Resource are empty

AI-assisted analysis of restsharp/RestSharp@6a50821692 (2026-08-13). Data as JSON: /api/errors/d263c887e75b933b. Report an issue: GitHub.