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InvalidCastException in WebApiToSwaggerGenerator.GetSupportedHttpMethodsFromAttributes

See original GitHub issue

Using nswag run nswag.json /runtime:NetCore20, I get an InvalidCastException thrown from the WebApiToSwaggerGenerator class:

System.InvalidCastException: Unable to cast object of type 
'SelectArrayIterator`2[System.String,System.String]' to type
'System.Collections.ICollection'.
   at CallSite.Target(Closure , CallSite , Object )
   at System.Dynamic.UpdateDelegates.UpdateAndExecute1[T0,TRet](CallSite site, T0 arg0)
   at NSwag.SwaggerGeneration.WebApi.WebApiToSwaggerGenerator.<GetSupportedHttpMethodsFromAttributes>d__24.MoveNext() in C:\projects\nswag\src\NSwag.SwaggerGeneration.WebApi\WebApiToSwaggerGenerator.cs:line 482
   at System.Collections.Generic.LargeArrayBuilder`1.AddRange(IEnumerable`1 items)
   at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
   at NSwag.SwaggerGeneration.WebApi.WebApiToSwaggerGenerator.<GetSupportedHttpMethods>d__23.MoveNext() in C:\projects\nswag\src\NSwag.SwaggerGeneration.WebApi\WebApiToSwaggerGenerator.cs:line 429
   at System.Collections.Generic.List`1.AddEnumerable(IEnumerable`1 enumerable)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at NSwag.SwaggerGeneration.WebApi.WebApiToSwaggerGenerator.<GenerateForControllerAsync>d__11.MoveNext() in C:\projects\nswag\src\NSwag.SwaggerGeneration.WebApi\WebApiToSwaggerGenerator.cs:line 143
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.SwaggerGeneration.WebApi.WebApiToSwaggerGenerator.<GenerateForControllersAsync>d__9.MoveNext() in C:\projects\nswag\src\NSwag.SwaggerGeneration.WebApi\WebApiToSwaggerGenerator.cs:line 95
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.Commands.SwaggerGeneration.WebApiToSwaggerCommand.<RunIsolatedAsync>d__91.MoveNext() in C:\projects\nswag\src\NSwag.Commands\Commands\SwaggerGeneration\WebApiToSwaggerCommand.cs:line 198
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.Commands.IsolatedCommandBase`1.IsolatedCommandAssemblyLoader`1.Run(String commandType, String commandData, String[] assemblyPaths, String[] referencePaths) in C:\projects\nswag\src\NSwag.Commands\Commands\IsolatedCommandBase.cs:
line 60
   at NSwag.Commands.IsolatedCommandBase`1.<>c__DisplayClass13_0.<RunIsolatedAsync>b__0() in C:\projects\nswag\src\NSwag.Commands\Commands\IsolatedCommandBase.cs:line 50
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.Commands.IsolatedCommandBase`1.<RunIsolatedAsync>d__13.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.Commands.IsolatedSwaggerOutputCommandBase.<RunAsync>d__4.MoveNext() in C:\projects\nswag\src\NSwag.Commands\Commands\IsolatedSwaggerOutputCommandBase.cs:line 24
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.Commands.NSwagDocumentBase.<GenerateSwaggerDocumentAsync>d__40.MoveNext() in C:\projects\nswag\src\NSwag.Commands\NSwagDocumentBase.cs:line 268
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.Commands.NSwagDocument.<ExecuteAsync>d__4.MoveNext() in C:\projects\nswag\src\NSwag.Commands\NSwagDocument.cs:line 76
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.Commands.Document.ExecuteDocumentCommand.<ExecuteDocumentAsync>d__9.MoveNext() in C:\projects\nswag\src\NSwag.Commands\Commands\Document\ExecuteDocumentCommand.cs:line 77
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NSwag.Commands.Document.ExecuteDocumentCommand.<RunAsync>d__8.MoveNext() in C:\projects\nswag\src\NSwag.Commands\Commands\Document\ExecuteDocumentCommand.cs:line 31
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NConsole.CommandLineProcessor.<ProcessSingleAsync>d__12.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NConsole.CommandLineProcessor.<ProcessAsync>d__11.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at NConsole.CommandLineProcessor.Process(String[] args, Object input)
   at NSwag.Commands.NSwagCommandProcessor.Process(String[] args) in C:\projects\nswag\src\NSwag.Commands\NSwagCommandProcessor.cs:line 54

Looking at the code on master, I think the problem might be here, where the code tries to cast a property of the dynamic object to ICollection:

dynamic acceptVerbsAttribute = method.GetCustomAttributes()
    .SingleOrDefault(a => a.GetType().Name == "AcceptVerbsAttribute");

if (acceptVerbsAttribute != null)
{
    foreach (var verb in ((ICollection)acceptVerbsAttribute.HttpMethods).OfType<object>().Select(v => v.ToString().ToLowerInvariant()))

The AcceptVerbsAttribute.HttpMethods property is of type IEnumerable<string>, so the cast to ICollection is optimistic in this case. Considering the method only enumerates the result, a fix may be to change the cast to IEnumerable instead.