Ran into a interesting problem today when implementing swagger UI on one of our WebAPI 2 projects.

Locally it was working fine. But when the site was deployed to dev/test it would display an ambiguous error message

<Error>
<Message>An error has occurred.</Message>
</Error>

After hunting around I found that swashbuckle respects the customErrors mode in the system.web section of the web config.

Setting this Off displayed the real error, in our case a missing dependency

<system.web>
<customErrors mode="Off"/>
</system.web>

<Error>
<Message>An error has occurred.</Message>
<ExceptionMessage>
Could not find file 'C:\Octopus\Applications\Development\Oztix.GreenRoom.WebAPI\2.0.332.0\Oztix.GreenRoom.WebAPI.XML'.
</ExceptionMessage>
<ExceptionType>System.IO.FileNotFoundException</ExceptionType>
<StackTrace>
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath) at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost) at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, String msgPath, Boolean bFromProxy) at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize) at System.Xml.XmlUrlResolver.GetEntity(Uri absoluteUri, String role, Type ofObjectToReturn) at System.Xml.XmlTextReaderImpl.OpenUrlDelegate(Object xmlResolver) at System.Threading.CompressedStack.runTryCode(Object userData) at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData) at System.Threading.CompressedStack.Run(CompressedStack compressedStack, ContextCallback callback, Object state) at System.Xml.XmlTextReaderImpl.OpenUrl() at System.Xml.XmlTextReaderImpl.Read() at System.Xml.XPath.XPathDocument.LoadFromReader(XmlReader reader, XmlSpace space) at System.Xml.XPath.XPathDocument..ctor(String uri, XmlSpace space) at Swashbuckle.Application.SwaggerDocsConfig.<>c__DisplayClass8.<IncludeXmlComments>b__6() at Swashbuckle.Application.SwaggerDocsConfig.<GetSwaggerProvider>b__e(Func`1 factory) at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext() at Swashbuckle.Swagger.SwaggerGenerator.CreateOperation(ApiDescription apiDescription, SchemaRegistry schemaRegistry) at Swashbuckle.Swagger.SwaggerGenerator.CreatePathItem(IEnumerable`1 apiDescriptions, SchemaRegistry schemaRegistry) at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer) at Swashbuckle.Swagger.SwaggerGenerator.GetSwagger(String rootUrl, String apiVersion) at Swashbuckle.Application.SwaggerDocsHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) at System.Net.Http.HttpMessageInvoker.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) at System.Web.Http.Dispatcher.HttpRoutingDispatcher.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) at System.Web.Http.HttpServer.<SendAsync>d__0.MoveNext()
</StackTrace>
</Error>