An item with the same key has already been added when creating new WPF project

Question

I just created a new WPF project in Visual Studio 2015 Pro, and after the Creating Project dialog disappears, I do not get the designer. This ArgumentException just shows there

Answer 1

I got the same exception and reduced both xaml and cs to a minimum without any help. I deleted folders bin and obj, restarted VS, rebooted my computer, but still no luck. With VS 2012 the designer was not loading with a different error message. This post helped me fix the issue

FIX: Delete or rename file "C:\Program" then click on "Click here to reload the designer".

In my case Program was a text file without extension from writing the output of an ipconfig MS-DOS command. There is no need to close the xaml designer, recompile, clear the cache, or restart Visual Studio.

Somehow VS tries to read it when loading the designer (checked with Process Monitor from Sysinternals). For information the stack in Visual Studio 2012 was the same as this bug report.

System.Runtime.Remoting.RemotingException
[] Designer process terminated unexpectedly!
   at Microsoft.Expression.DesignHost.Isolation.Primitives.ProcessDomainFactory.ProcessIsolationDomain.Microsoft.Expression.DesignHost.Isolation.IIsolationDomain.CreateInstance(String assemblyName, String assemblyCodeBase, String typeName)
   at Microsoft.Expression.DesignHost.Isolation.Primitives.IsolationBoundary.Initialize()
   at Microsoft.Expression.DesignHost.Isolation.Primitives.IsolationBoundary.CreateInstance[T](Type type)
   at Microsoft.Expression.DesignHost.Isolation.IsolatedExportProvider.Initialize()
   at Microsoft.VisualStudio.ExpressionHost.Services.VSIsolationService.CreateExportProvider(IIsolationTarget isolationTarget, ICatalogFactory catalogFactory, IExportFilter filter)
   at Microsoft.Expression.DesignHost.Isolation.IsolationService.CreateLease(IIsolationTarget isolationTarget)
   at Microsoft.Expression.DesignHost.IsolatedDesignerService.CreateLease(IIsolationTarget isolationTarget, CancellationToken cancelToken, DesignerServiceEntry& entry)
   at Microsoft.Expression.DesignHost.IsolatedDesignerService.IsolatedDesignerView.CreateDesignerViewInfo(CancellationToken cancelToken)
   at Microsoft.Expression.DesignHost.Isolation.IsolatedTaskScheduler.InvokeWithCulture[T](CultureInfo culture, Func`2 func, CancellationToken cancelToken)
   at Microsoft.Expression.DesignHost.Isolation.IsolatedTaskScheduler.<>c__DisplayClassa`1.<StartTask>b__6()
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()