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HeaderCollection Key Already Added: Cache-Control

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Ben asked on 03 Mar 2020, 09:01 PM

I'm trying to set things up for the first time in a .Net Core ASP 3.1 Application.

My ReportsController appears to be recognized properly (the constructor is called with the IReportServiceConfiguration) and the client attempts to call:

api/reports/resources/js/telerikReportViewer (with a ?_=<number> on the end)

But i'm getting this error:

"Message": "An error has occurred.","ExceptionMessage": "An item with the same key has already been added. Key: Cache-Control"

Full:

{

"Message": "An error has occurred.","ExceptionMessage": "An item with the same key has already been added. Key: Cache-Control","ExceptionType": "System.ArgumentException","StackTrace": " at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)\r\n at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)\r\n at Microsoft.AspNetCore.HttpSys.Internal.HeaderCollection.Add(String key, StringValues value)\r\n at Telerik.Reporting.Services.AspNetCore.ReportsControllerBase.GetResourceCore(String folder, String resourceName, Type type)\r\n at Telerik.Reporting.Services.AspNetCore.ReportsControllerBase.GetResource(String folder, String resourceName)\r\n at lambda_method(Closure , Object , Object[] )\r\n at Microsoft.Extensions.Internal.ObjectMethodExecutor.Execute(Object target, Object[] parameters)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeActionMethodAsync()\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeNextActionFilterAsync()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextExceptionFilterAsync>g__Awaited|25_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)"

}

9 Answers, 1 is accepted

answered on 13 May 2020, 01:58 PM

Hi,

I had the same error reason but not the same error message, only when we tried to display a report.

This is the error:

{

"message":"An error has occurred.",

"exceptionMessage":"An item with the same key has already been added.",

"exceptionType":"System.ArgumentException","stackTrace":" at

Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpHeaders.ThrowDuplicateKeyException()\r\n at

Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpHeaders.System.Collections.Generic.IDictionary<System.String,Microsoft.Extensions.Primitives.StringValues>.Add(String key, StringValues value)\r\n at

Telerik.Reporting.Services.AspNetCore.ReportsControllerBase.GetResourceCore(String folder, String resourceName, Type type)\r\n   at

Telerik.Reporting.Services.AspNetCore.ReportsControllerBase.GetResource(String folder, String resourceName)\r\n at

lambda_method(Closure , Object , Object[] )\r\n at

Microsoft.Extensions.Internal.ObjectMethodExecutor.Execute(Object target, Object[] parameters)\r\n at

Microsoft.AspNetCore.Mvc.Internal.ActionMethodExecutor.SyncActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)\r\n   at

Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()\r\n at

Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeNextActionFilterAsync()\r\n at

Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)\r\n at

Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)\r\n   at

Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()\r\n at

Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextExceptionFilterAsync()"

}

In our code I found a line where we added the "Cache-Control" header on application startup:

context.Response.Headers.Append("Cache-Control", "no-cache");

The error seems to be that the Telerik Reporting also tries to add the same response header. When it exists the error occures. After removing our line everything works fine.

@Ben

It may help only to add the header if it still does not exist!?

if (!context.Response.Headers.ContainsKey("Cache-Control"

)) { add header ... }