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ASP.NET Core 1.1 –Error in Response Caching Middleware

After putting a first version of our ASP.NET Core application into acceptance, we got a strange bug back.

Sometimes after a cold start, users mentioned that the dropdown lists inside the application were empty. The application is using the Response Caching middleware to prevent having to fetch those lists over and over again from the database.

When looking at the server logs, we found the following error message:

exception": "System.ArgumentException: An item with the same key has already been added. Key: \r\n at System.ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException(Object key)\r\n at System.Collections.Generic.Dictionary2.Insert(TKey key, TValue value, Boolean add)\r\n at System.Collections.Generic.Dictionary2.Add(TKey key, TValue value)\r\n at Microsoft.AspNetCore.Server.Kestrel.Internal.Http.FrameResponseHeaders.AddValueFast(String key, StringValues value)\r\n at Microsoft.AspNetCore.Server.Kestrel.Internal.Http.FrameHeaders.System.Collections.Generic.IDictionary<System.String,Microsoft.Extensions.Primitives.StringValues>.Add(String key, StringValues value)\r\n at Microsoft.AspNetCore.Server.Kestrel.Internal.Http.FrameHeaders.System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<System.String,Microsoft.Extensions.Primitives.StringValues>>.Add(KeyValuePair2 item)\r\n at Microsoft.AspNetCore.ResponseCaching.ResponseCachingMiddleware.d__10.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter1.GetResult()\r\n at Microsoft.AspNetCore.ResponseCaching.ResponseCachingMiddleware.d__11.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---

Seems there is a bug in the Response Caching Middleware, the response caching middleware is causing an exception when it tries to add the same header. This is a bug that should be fixed in  the ASP.NET Core 1.1.1 patch.

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