RicoSuter/NSwag · error · InvalidOperationException

Multiple response tags with code

Error message

Multiple response tags with code '{responseCode}' found in XML documentation for method '{methodInfo.Name}'.

What it means

GetResponseXmlDocsElement uses SingleOrDefault to find the <response code='...'> XML docs node for a given response code; if the method's XML documentation contains more than one response tag with the same code, SingleOrDefault throws InvalidOperationException, which is rethrown with a descriptive message naming the code and method. XML docs require exactly one response element per code.

Solutions

Remove or merge duplicate <response> tags with the same code in the XML doc comments
Give each response tag a unique code attribute (200, 404, etc.)
Rebuild the project so the XML documentation file is regenerated

Example fix

// before
/// <response code="200">Success</response>
/// <response code="200">Ok</response>
// after
/// <response code="200">Success</response>
/// <response code="404">Not found</response>

Defensive patterns

Strategy: validation

Validate before calling

var codes = methodDoc.Descendants("response").Select(r => (string)r.Attribute("code"));
if (codes.GroupBy(c => c).Any(g => g.Count() > 1)) throw new InvalidOperationException("Duplicate response codes in XML docs");

Try / catch

try { GenerateWithXmlDocs(); } catch (InvalidOperationException ex) when (ex.Message.Contains("Multiple response tags")) { /* fix XML doc comments */ }

Prevention

Audit controller XML comments for duplicated <response code="..."> tags
Ensure each HTTP status code appears exactly once per action
Rebuild after editing XML docs so the generated XML file is fresh

When it happens

Trigger: Documenting a controller action with two /// <response code="200">...</response> tags that share the same code, then running NSwag generation with XML docs enabled.

Common situations: Copy-pasted XML doc comments where a developer duplicated a response block and forgot to change the code attribute, e.g. two '200' response tags in ASP.NET Core controllers with GenerateXmlDocumentation enabled.

Related errors

AI-assisted analysis of RicoSuter/NSwag@63daf8fcc3 (2026-09-14). Data as JSON: /api/errors/2a9ccfa97f65e68d. Report an issue: GitHub.

Appendix: source

Thrown at src/NSwag.Generation/Processors/OperationResponseProcessorBase.cs:100

                    }
                }
            }
        }

        /// <summary>Gets the XML documentation element for the given response code or null.</summary>
        /// <param name="methodInfo">The method info.</param>
        /// <param name="responseCode">The response code.</param>
        /// <returns>The XML element or null.</returns>
        protected XElement GetResponseXmlDocsElement(MethodInfo methodInfo, string responseCode)
        {
            var operationXmlDocsNodes = GetResponseXmlDocsNodes(methodInfo);
            try
            {
                return operationXmlDocsNodes?.SingleOrDefault(n => n.Name == "response" && n.Attributes().Any(a => a.Name == "code" && a.Value == responseCode));
            }
            catch (InvalidOperationException ex)
            {
                throw new InvalidOperationException($"Multiple response tags with code '{responseCode}' found in XML documentation for method '{methodInfo.Name}'.", ex);
            }
        }

        private IEnumerable<XElement> GetResponseXmlDocsNodes(MethodInfo methodInfo)
        {
            var operationXmlDocs = methodInfo?.GetXmlDocsElement(_settings.SchemaSettings.GetXmlDocsOptions());
            return operationXmlDocs?.Nodes()?.OfType<XElement>();
        }

        private List<OperationResponseDescription> GetOperationResponseDescriptions(IEnumerable<Attribute> responseTypeAttributes, string successResponseDescription)
        {
            List<OperationResponseDescription> operationResponseDescriptions = [];
            foreach (var attribute in responseTypeAttributes)
            {
                dynamic responseTypeAttribute = attribute;
                var attributeType = attribute.GetType();

                var isProducesAttributeWithNoType = // ignore ProducesAttribute if it has no type, https://github.com/RicoSuter/NSwag/issues/1201

View on GitHub (pinned to 63daf8fcc3)