Compatibility of .NET Framework 4.5

Compatibility of .NET Framework 4.5

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Fundamentals were a big part of our focus while building .NET 4.5. We divided fundamentals into seven areas called “tenets”. One of these tenets is compatibility. Today’s post is by Manish Agnihotri, a program manager who is driving compatibility across the .NET Framework. -- Brandon

Editor's update: we've added more discussion about the compatibility of .NET Framework 4.5 in a recent post on October 17, 2012.

.NET Framework 4.5 is an in-place update that replaces .NET Framework 4 (rather than a side-by-side installation). Our goal is for .NET 4.5 to be fully backward compatible with applications built for .NET 4 (.NET 3.5 and .NET 4.5 will be side-by-side). We’ll talk about the compatibility story for .NET 3.5 in a later post. One of the first things you’ll notice about .NET 4.5 is the version number (4.0.30319) is the same as .NET 4; this is the practice used by other in-place updates.

Our primary concern is guaranteeing applications you use do not break after you install .NET 4.5. We accomplish this by running hundreds of application in our compatibility lab to find issues as soon as they’re introduced. While designing new features or changing existing code, we keep compatibility in mind. And a small group of us, the Developer Division Compatibility Council (DDCC), monitor changes made by developers. We review potential breaking changes, and help teams understand and assess the compatibility impact of new features and bug fixes. For .NET 4.5, members of DDCC reviewed every proposed breaking change, every new feature, and a majority of the bug fixes for the release.

We’ve put a lot of effort into maintaining a consistently high bar for compatibility across the product, yet we know some issues may get past us. Many applications will exercise the .NET Framework in ways that we did not expect or we lack test coverage for. Still we care about knowing every issue, even those that may seem like corner cases. Once you install .NET 4.5 Developer Preview on a machine that previously had .NET 4, any compatibility issues can be sent to the Connect feedback site.

Types of compatibility issues

There are three kinds of version compatibility testing we do: (1) binary compatibility, (2) source compatibility, and (3) serialization compatibility. You may also find these approaches useful in your testing, and should an issue arise this may help you narrow down the root cause. Having a wide range of scenarios within each kind of tests is also critical to ensuring good compatibility coverage.

Binary compatibility uses binaries built targeting .NET 4 and then are run on .NET 4.5. Essentially, we’re testing that the behavior of newer .NET libraries is equivalent to previous versions. This can range from making sure the return value of a function is the same or that the same exceptions are raised. The hardest issues are multi-threading behaviors – sometimes performance improvements can be a breaking change.

Source compatibility takes an application that builds and runs successfully against .NET 4 then does the same with .NET 4.5. An interesting case is a machine where Visual Studio 2010 runs on a machine with .NET 4.5 installed – since reference assemblies are used for targeting versions of the framework, building an app in VS2010 and then running it is actually binary compatibility testing. To do source compatibility testing, we build against the .NET 4.5 targeting pack (which is part of the Visual Studio 11 Developer Preview).

Serialization compatibility deals with data and has several permutations. First, we let an app running on .NET 4 serialize data (e.g. save to disk) and then an app running on .NET 4.5 de-serializes (e.g. read from disk). Second, we do this in reverse by serializing from .NET 4.5 apps and de-serialize in .NET 4 apps. Third, we take a web service running on .NET 4.5 reading serialized data created by a .NET 4 based client. Fourth and last, we do the inverse with a .NET 4 web service and a .NET 4.5 client. A serialization problem will often show up as corrupted data or some kind of serialization exception.

Feedback welcome

Compatibility is an important part of migrating apps forward, though in rare cases it is necessary for a framework library to make breaking changes. We try to avoid this, using it as a last resort. MSDN documents the known breaking changes in .NET 4.5. We also have a migration guide with advice for testing your app.

If you discover any compatibility issues while working with previews and betas, we want to hear about them via Connect. Hopefully, all our work to make this a highly compatible release makes it easy for you to upgrade to .NET 4.5.

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  • Please add 5 and 7 and type the answer here:
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  • @Omar: maybe you should not consider yourself a developer if you think that .NET - or anything else for that matters - is *compiling* a TBODY tag

  • Hi, We have a serialization issue which only happens in ,NET 4.5 - same code works fine in .NET 4. we're trying to serialize an inherited type with a few properties, both base and inherited class are marked with SerializableAttribute. We get an exception on the client side of Web service saying that there was a MethodAccessException in the server , the server itself does not throw any exceptions , it seems to be a problem in the client serialization process. We've tried implementing ISerialiazble interface, it did not help. any help would be most appreciated. Thanks,


  • I too, like many others posting here, have severe compatibility issues that crop up in all our apps when installing .net 4.5.  The in-place install of .net 4.5 was without a doubt the worst decision made by microsoft since I started using .net 7 years ago.

    I did as others were requested and sent an email to the net45 compatibility team 5 days ago, and have yet to receive any response.

  • Hi Rick, I am responding to your comment.

    I have responded to you email last Wednesday, can you confirm you got my email? I will send you another email right now, just in case my email landed in your junk mail folder or something.


    .NET Framework Compatibility.


    From Rick,

    I too, like many others posting here, have severe compatibility issues that crop up in all our apps when installing .net 4.5.  The in-place install of .net 4.5 was without a doubt the worst decision made by microsoft since I started using .net 7 years ago.

    I did as others were requested and sent an email to the net45 compatibility team 5 days ago, and have yet to receive any response.

  • I have an error in FrameWork 4.5  (object of type 'system.int32' cannot be converted to type 'system.int16' )

    My application (VS 2010) work fine with FrameWork 4.0, but when I Installed FrameWork 4.5, our application got this error. When I unstalled FrameWork 4.5 et reinstalle FramWork 4.0, our application work fine .

    thanks you

  • Hi Quang,

    We're trying to reproduce in-house. Can you contact us on netfx45compat at Microsoft dot com to discuss (if you have repro project handy that would be great)?

  • Hello,

    After installing VS2011, our application is NOT working anymore on PC with .NET 4.0.3. (even if we are targeting .NET 4.0 in VS)

    Framework 4.5 is NOT fully backward compatible with 4.0 !

    * We can't use .NET 4.5 as our application must be supported under Windows XP.

    * We can't reproduce the crash on our development PCs as VS2011 replaced .NET 4.0 files.

    What can we do ?

    Thanks for your help

  • Hello Sphane,

    I see that you have sent us email on netfx45compat at Microsoft dot com. Please send us project to reproduce the issue and we will investigate. Please make sure that your computer is upto date with all latest Windows Updates.


    Varun Gupta

    .NET Framework Compatibility Team

  • Hello,  i raised a bug #790909 on Microsoft Connect. Please check, 4.5 upgrade is breaking our custom formatter to compress view state.

  • RE: bug #790909 on Microsoft Connect

    Thank you for filing bug report. We are looking into it. Could you contact us on netfx45compat at Microsoft dot com as well?

  • I ran into a problem coz of this inplace update of dlls.

    We have an application targetting .Net 4.0 framework. However one of your build server was updated to .Net 4.5.

    And while building the app, it used 4.5 instead of 4.0. Compile worked fine... but one of the Yield method returning IEnumerable type now throws an exception of type System.MissingMethodException when executed on on a machine running 4.0. How do i solve the problem.

    The build mechanism uses command line msbuild to compile the project

  • @Parag Agrawal [This is response to your comment on build server upgraded to .NET 4.5].

    I have seen this happen when .NET 4.5 is installed on Build Server but .NET 4.0 reference assemblies are not installed on build server. If .NET4.0 reference assemblies are installed on the build machine and TargetFrameworkVersion is set to .NET4.0, your app would only use .NET4.0 surface area (even though it's build on .NET4.5 runtime).

    (You can install .NET4.0 reference assemblies via Windows SDK 7 for .NET 4.0 -

    If you still see this error, please contact us on netfx45compat at Microsoft dot com.


    Varun Gupta

    .NET Framework Compatibility

  • Accroding to microsoft 4.5 is 100% compatible with 4.0 but recently we have discovered Serialization compatbility between client & server. The client machine installed 4.0 framewrok version and at server 4.5, the custom object serialized by 4.5 framework won't be able to de-serialized at client (4.0 ) and throw runtime exception "Cannot get the member 'System.Web.Hosting.IISAPIRuntime2.ProcessRequest'. I would appriciate if you could provide some solution to resolved this Issue.

    Call Stack:




     Cannot get the member 'System.Web.Hosting.IISAPIRuntime2.ProcessRequest'

    Server stack trace:

      at System.Reflection.MemberInfoSerializationHolder.GetRealObject(StreamingContext context)

      at System.Runtime.Serialization.ObjectManager.ResolveObjectReference(ObjectHolder holder)

      at System.Runtime.Serialization.ObjectManager.DoFixups()

      at System.Runtime.Serialization.Formatters.Binary.ObjectReader.Deserialize(HeaderHandler handler, __BinaryParser serParser, Boolean fCheck, Boolean isCrossAppDomain, IMethodCallMessage methodCallMessage)

      at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize(Stream serializationStream, HeaderHandler handler, Boolean fCheck, Boolean isCrossAppDomain, IMethodCallMessage methodCallMessage)

      at System.Runtime.Remoting.Channels.CoreChannel.DeserializeBinaryResponseMessage(Stream inputStream, IMethodCallMessage reqMsg, Boolean bStrictBinding)

      at System.Runtime.Remoting.Channels.BinaryClientFormatterSink.SyncProcessMessage(IMessage msg)

    Exception rethrown at [0]:

      at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)

      at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)

  • Hi Awdhes, Thank you for contacting us. Could you send us an email on netfx45compat at Microsoft com to discuss your report?


    Varun Gupta

    Microsoft .NET Framework Compatibility Team

  • I have already raise this case to Microsoft support with premium account, we tried several option suggested by expert but that didn't work. Issue is still open

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