restsharp/RestSharp · error · ArgumentException

Failed to put file as content because body parameters were…

Error message

Failed to put file as content because body parameters were added

What it means

ValidateParameters throws when AlwaysSingleFileAsContent is enabled and the request already has a ParameterType.RequestBody parameter (a body added via AddJsonBody/AddXmlBody/AddStringBody/AddBody). A raw single-file body and a serialized request body cannot both occupy the HTTP body.

Solutions

1. Do not add a request body when using AlwaysSingleFileAsContent; the file IS the body.
2. If you need both a structured body and a file, use multipart form data (remove AlwaysSingleFileAsContent).
3. Check request.TryGetBodyParameter / Parameters for RequestBody entries before enabling single-file mode.

Example fix

// before
request.AddJsonBody(payload);
request.AlwaysSingleFileAsContent = true;
request.AddFile("file", bytes, "f.bin"); // throws

// after
// drop the JSON body; the file is the body:
request.AlwaysSingleFileAsContent = true;
request.AddFile("file", bytes, "f.bin");

if (request.AlwaysSingleFileAsContent) {
    var hasBody = request.Parameters.Any(p => p.Type == ParameterType.RequestBody);
    if (hasBody)
        throw new InvalidOperationException("Remove the request body before single-file-as-content upload");
}
request.AddFile(name, bytes, fileName);

When it happens

Trigger: Setting AlwaysSingleFileAsContent = true, adding a body (AddBody/AddJsonBody/AddXmlBody/AddStringBody), then adding a file or executing the request.

Common situations: Adding a JSON payload and then switching the request to a file-content upload without removing the body; building a request generically where a body and a file both get attached.

Related errors

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AI-assisted analysis of restsharp/RestSharp@6a50821692 (2026-08-13). Data as JSON: /api/errors/030d18585d7f8a37. Report an issue: GitHub.

Appendix: source

Thrown at src/RestSharp/Request/RestRequestExtensions.File.cs:85

            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");
        }
    }
}

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