Could not load type System Runtime CompilerServices ExtensionAttribute from assembly mscorlib

Question

When starting up my web site for the first time  I m getting this error  Could not load type  System Runtime CompilerServices ExtensionAttribute  from assembly  mscorlib  Version 4 0 0 0  Culture neutral  PublicKeyToken b77a5c561934e089   What am I doing wrong   I am using  NET 4 and am starting the site from Visual Studio   The only thing I ve changed recently is add Simple Injector  via Nuget  into my project   Here s the stack trace   TypeLoadException  Could not load type  System Runtime CompilerServices ExtensionAttribute  from assembly  mscorlib  Version 4 0 0 0  Culture neutral  PublicKeyToken b77a5c561934e089       System ModuleHandle ResolveType RuntimeModule module  Int32 typeToken  IntPtr  typeInstArgs  Int32 typeInstCount  IntPtr  methodInstArgs  Int32 methodInstCount  ObjectHandleOnStack type   0    System ModuleHandle ResolveTypeHandleInternal RuntimeModule module  Int32 typeToken  RuntimeTypeHandle   typeInstantiationContext  RuntimeTypeHandle   methodInstantiationContext   180    System Reflection RuntimeModule ResolveType Int32 metadataToken  Type   genericTypeArguments  Type   genericMethodArguments   192    System Reflection CustomAttribute FilterCustomAttributeRecord CustomAttributeRecord caRecord  MetadataImport scope  Assembly amp  lastAptcaOkAssembly  RuntimeModule decoratedModule  MetadataToken decoratedToken  RuntimeType attributeFilterType  Boolean mustBeInheritable  Object   attributes  IList derivedAttributes  RuntimeType amp  attributeType  IRuntimeMethodInfo amp  ctor  Boolean amp  ctorHasParameters  Boolean amp  isVarArg   115    System Reflection CustomAttribute GetCustomAttributes RuntimeModule decoratedModule  Int32 decoratedMetadataToken  Int32 pcaCount  RuntimeType attributeFilterType  Boolean mustBeInheritable  IList derivedAttributes  Boolean isDecoratedTargetSecurityTransparent   426    System Reflection CustomAttribute GetCustomAttributes RuntimeAssembly assembly  RuntimeType caType   103    System Reflection RuntimeAssembly GetCustomAttributes Type attributeType  Boolean inherit   64    WebActivator AssemblyExtensions GetActivationAttributes Assembly assembly   132    WebActivator ActivationManager RunActivationMethods    216    WebActivator ActivationManager RunPreStartMethods    43    WebActivator ActivationManager Run    69   InvalidOperationException  The pre-application start initialization method Run on type WebActivator ActivationManager threw an exception with the following error message  Could not load type  System Runtime CompilerServices ExtensionAttribute  from assembly  mscorlib  Version 4 0 0 0  Culture neutral  PublicKeyToken b77a5c561934e089        System Web Compilation BuildManager InvokePreStartInitMethods ICollection 1 methods   423    System Web Compilation BuildManager CallPreStartInitMethods    306    System Web Hosting HostingEnvironment Initialize ApplicationManager appManager  IApplicationHost appHost  IConfigMapPathFactory configMapPathFactory  HostingEnvironmentParameters hostingParameters  PolicyLevel policyLevel  Exception appDomainCreationException   677   HttpException  0x80004005   The pre-application start initialization method Run on type WebActivator ActivationManager threw an exception with the following error message  Could not load type  System Runtime CompilerServices ExtensionAttribute  from assembly  mscorlib  Version 4 0 0 0  Culture neutral  PublicKeyToken b77a5c561934e089        System Web HttpRuntime FirstRequestInit HttpContext context   9090876    System Web HttpRuntime EnsureFirstRequestInit HttpContext context   97    System Web HttpRuntime ProcessRequestInternal HttpWorkerRequest wr   258   The first line of all views get highlighted and when you hover over them you get this error  The pre-application start initialisation method Run on type WebActivator ActivationManager threw an exception with the following error message Could not load type  System Runtime CompilerServices ExtensionAttribute  from assembly  mscorlib  Version 4 0 0 0  Culture neutral  PublicKeyToken b77a5c561934e089

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In my case  it was Blend SDK missed out on TeamCity machine  This caused the error due incorrect way of assembly resolving then

User · Answer

I did encounter the same problem while trying to read data from a Firebird Database  After many hours of searching  I found out that the problem was caused by an error I made in the query  Fixing it made it work perfectly  It had nothing to do with the version of the Framework

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In my case after downgrading from  NET 4 5 to  NET 4 0 project was working fine on a local machine  but was failing on server after publishing   Turns out that destination had some old assemblies  which were still referencing  NET 4 5   Fixed it by enabling publishing option  Delete all existing files prior to publish

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In my case I had an issue around using Microsoft ReportViewer WebForms  I removed validate true from add verb line in web config and it started working    lt system web gt       lt httpHandlers gt         lt add verb     path  Reserved ReportViewerWebControl axd  type  Microsoft Reporting WebForms HttpHandler  Microsoft ReportViewer WebForms  Version 10 0 0 0  Culture neutral  PublicKeyToken b03f5f7f11d50a3a    gt

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We ran into this problem and tracked it down to the Geocoding net NuGet package that we were using to help with our Google Maps views  Geocoding net version 3 1 0 published 2 4 2014    The Geocoding dll appears to be  Net 4 0 when you examine the package file or view it using Jet Brains    Dot Peek application  however  a colleague of mine says that it was compiled using ilmerge so it is most likely related to the ilmerge problems listed above     It was a long process to track it down   We fetched different changesets from TFS till we narrowed it down to the changeset that added the aforementioned NuGet package  After removing it  we were able to deploy to our  NET 4 server

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Could not load type  System Runtime CompilerServices ExtensionAttribute  from assembly mscorlib   Yes  this technically can go wrong when you execute code on  NET 4 0 instead of  NET 4 5   The attribute was moved from System Core dll to mscorlib dll in  NET 4 5   While that sounds like a rather nasty breaking change in a framework version that is supposed to be 100  compatible  a  TypeForwardedTo  attribute is supposed to make this difference unobservable   As Murphy would have it  every well intended change like this has at least one failure mode that nobody thought of   This appears to go wrong when ILMerge was used to merge several assemblies into one and that tool was used incorrectly   A good feedback article that describes this breakage is here   It links to a blog post that describes the mistake   It is rather a long article  but if I interpret it correctly then the wrong ILMerge command line option causes this problem      targetplatform  v4 c  windows Microsoft NET Framework v4 0 30319    Which is incorrect   When you install 4 5 on the machine that builds the program then the assemblies in that directory are updated from 4 0 to 4 5 and are no longer suitable to target 4 0   Those assemblies really shouldn t be there anymore but were kept for compat reasons   The proper reference assemblies are the 4 0 reference assemblies  stored elsewhere      targetplatform  v4 C  Program Files  x86  Reference Assemblies Microsoft Framework  NETFramework v4 0    So possible workarounds are to fall back to 4 0 on the build machine  install  NET 4 5 on the target machine and the real fix  to rebuild the project from the provided source code  fixing the ILMerge command     Do note that this failure mode isn t exclusive to ILMerge  it is just a very common case   Any other scenario where these 4 5 assemblies are used as reference assemblies in a project that targets 4 0 is liable to fail the same way   Judging from other questions  another common failure mode is in build servers that were setup without using a valid VS license   And overlooking that the multi-targeting packs are a free download   Using the reference assemblies in the c  program files  x86  subdirectory is a rock hard requirement   Starting at  NET 4 0  already important to avoid accidentally taking a dependency on a class or method that was added in the 4 01  4 02 and 4 03 releases   But absolutely essential now that 4 5 is released

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I had this problem  except the type it couldn t load was System Reflection AssemblyMetadataAttribute  The web application was built on a machine with  NET 4 5 installed  runs fine there   with 4 0 as the target framework  but the error presented itself when it was run on a web server with only 4 0 installed  I then tried it on a web server with 4 5 installed and there was no error   So  as others have said  this is all due to the screwy way Microsoft released 4 5  which basically is an upgrade to  and overwrite of  version 4 0  The System Reflection assembly references a type that doesn t exist in 4 0  AssemblyMetadataAttribute  so it will fail if you don t have the new System Reflection dll   You can either install  NET 4 5 on the target web server  or build the application on a machine that does not have 4 5 installed   Far from an ideal resolution

User · Answer

Just adding this answer to help Google save some punter the hours I have spent to get here  I used ILMerge on my  Net 4 0 project  without the  targetplatform option set  assuming it would be detected correctly from my main assembly  I then had complaints from users only on Windows XP aka WinXP  This now makes sense as XP will never have    Net 4 0 installed whereas most newer OSes will  So if your XP users are having issues  see the fixes above

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I just ran into this annoying problem today   We use SmartAssembly to pack obfuscate our  NET assemblies  but suddenly the final product wasn t working on our test systems   I didn t even think I had  NET 4 5  but apparently something installed it about a month ago   I uninstalled 4 5 and reinstalled 4 0  and now everything is working again   Not too impressed with having blown an afternoon on this

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I had this exact same problem with a site  Kentico CMS   starting development in 4 5  finding out the production server only supports 4 0  tried going back down to target framework of 4 0  Compiling the other posts in this thread  specifically changing target framework to  Net 4 and  Net 4 5 still being referenced   I searched through my solution and found that a handful of the NuGet packages were still using libraries with targetFramework  net45    packages config  before    lt  xml version  1 0  encoding  utf-8   gt   lt packages gt     lt package id  AutoMapper  version  3 1 0  targetFramework  net45    gt     lt package id  EntityFramework  version  5 0 0  targetFramework  net45    gt     lt package id  Microsoft AspNet WebApi Client  version  5 0 0  targetFramework  net45    gt     lt package id  Newtonsoft Json  version  4 5 11  targetFramework  net45    gt   lt  packages gt    I changed the projects target framework back to 4 5  removed all NuGet libraries  went back down to 4 0 and re-added the libraries  had to use some previous versions that were not dependent on 4 5    packages config  after    lt  xml version  1 0  encoding  utf-8   gt   lt packages gt     lt package id  AutoMapper  version  3 1 1  targetFramework  net40    gt     lt package id  EntityFramework  version  6 0 2  targetFramework  net40    gt     lt package id  Microsoft AspNet WebApi Client  version  4 0 30506 0  targetFramework  net40    gt     lt package id  Microsoft Net Http  version  2 0 20710 0  targetFramework  net40    gt     lt package id  Newtonsoft Json  version  4 5 11  targetFramework  net40    gt   lt  packages gt

[.net] Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib

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