I have a project written in VS2012/.Net 4.5 that is essentially a bog-standard, database-first Entity Framework 5 entity model and context, and associated business domain classes. That all works fine when I'm referencing the built assembly from a VS2012/.Net 4.5 application (I've used it successfully with an MVC website and with integration test projects). However when I try to use the exact same assembly from a VS2010/.Net 4 application on the same machine, Visual Studio will add a reference to the assembly with no problem, the classes will also show up under Intellisense (I can add a using statement, for example), but as soon as I try to build the project I get the following error:
Warning 3 The primary reference "MyDotNet45Assembly" could not be resolved because it was built against the ".NETFramework,Version=v4.5" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.0".
My understanding (e.g. from Scott Hanselman here) was that .Net 4.5 was an in-place upgrade to .Net 4.0, so I can't understand why I can't call an assembly created for the newer version. What am I missing or misunderstanding? I can't believe I'm the only person who has hit this issue, but I'm genuinely struggling to find anything on the web that explains what's going on.
I've looked at a number other questions on StackOverflow, none of which seem to quite cover what I'm after, e.g:
Using EF4.0 in a .Net 3.5 app
Using a .Net 4.0 assembly in a .Net 4.5 project
Related
My understanding is that, starting with ASP.NET Core 3.0, .NET Framework is an unsupported target framework, and thus you can only run on the .NET Core runtime.
If this is the case, what NuGet packages can be imported into an ASP.NET Core 3 app?
I assume that you could reference any package that targets netstandard, but what about packages that only target the full framework (i.e., a legacy package that only targets net45)?
What happens if the package you import references an assembly that's not part of .NET Core—i.e., System.Drawing?
TL;DR: You can still reference (packages which depend upon) .NET Framework assemblies from .NET Core 3 and even .NET 5, but you will receive a runtime error if you call into any code which relies upon APIs or libraries not (yet) supported by .NET Core. You can discover these using Microsoft's .NET Portability Analyzer
Background
First off, you're correct that ASP.NET Core 3.x applications can no longer target the .NET Framework, as announced by Microsoft in 2018. That capability previously allowed ASP.NET Core applications to call into .NET Framework libraries, and thus offered an intermediate solution for web applications migrating to .NET Core.
Note: Since the .NET Framework only runs on Windows machines, writing ASP.NET Core web applications which targeted the .NET Framework implicitly restricted those applications to running on Windows.
Behavior
Even when targeting .NET Core or now .NET 5, however, you're still able to reference .NET Framework packages and assemblies, assuming you're on a Windows machine and have the corresponding .NET Framework installed. The inner workings of this are a bit involved, but the short of it is that .NET Core and .NET 5 will evaluate .NET Framework assembles as though they are .NET Standard assemblies. If the API call is also implemented in the .NET Core runtime, it will work fine—but if the API call is exclusively part of .NET Framework, you'll receive an exception.
Surprise! It's really important to emphasize that this is a runtime exception. You will still be able to reference the .NET Framework assembly, write calls to problematic members, and compile your code without any warnings. But as soon as you call into code dependent on a .NET Framework-specific assembly, you'll receive the runtime exception.
Example
With .NET 3.0, a significant portions of .NET Framework libraries have been ported over to .NET Core. In fact, this includes most of the System.Drawing libraries you referenced as an example—though there are good reasons you may not want to use them. If you dig a bit deeper, however, there are plenty of libraries which remain unsupported. One obvious example is the WebConfigurationManager, which could be used to access configuration settings from web.config files.
.NET Framework Code
So, as an example, let's say you have the following function in a .NET Framework class library, which returns an array of keys from your web.config's <AppSetting>s element:
public static class Configuration
{
public static string[] GetAppSettings() => System.Web.Configuration.WebConfigurationManager.AppSettings.AllKeys;
}
ASP.NET Core Code
And then, in an ASP.NET Core controller, you expose an endpoint to retrieve this data:
public class MyController: Controller
{
public IActionResult ApplicationKeys() => Content(String.Join(", ", Configuration.GetAppSettings()));
}
Exception
In an ASP.NET Core 2.x application targeting the .NET Framework, this will work just fine. In an ASP.NET Core 3.x or ASP.NET Core 5 application, however, you'll receive the following runtime error when you call the /My/ApplicationKeys/ route:
System.TypeLoadException: 'Could not load type 'System.Web.Configuration.WebConfigurationManager' from assembly 'System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.'
Avoiding Surprises
If you're anything like me, this will make you incredibly nervous. You'd much rather receive design-time errors—or, at least, compile-time warnings—as soon as you attempt to call into a library relying upon unsupported code. Fortunately, Microsoft offers a .NET Portability Analyzer, which is also available as a Visual Studio Extension, for exactly this purpose.
As of .NET 5, there's also a compatibility analyzer built into the SDK which will identify calls that are not supported by the .NET 5 runtime on particular platforms. This requires that target libraries explicitly annotate their types with the [SupportedOSPlatform()] attribute, so you won't get any warnings for legacy .NET Framework types. But this will help identify similar types of compatibility issues for libraries targeting a variety of platforms.
Example
If you run the Portability Analyzer on the above sample code, for example, it will output an Excel spreadsheet identifying that T:System.Web.Configuration.WebConfigurationManager is Not Supported in e.g. .NET Core,Version=v3.1 or .NET Standard + Platform Extensions,Version=v2.0.
Note: Microsoft used to offer an API Analyzer as a NuGet package, which promised to provide design-time analysis in Visual Studio. Unfortunately, the code hasn't been updated in two years, and the latest release is 0.2.12-alpha. In my evaluation, it was not effective at identifying issues.
Sample Project
I've put together a sample project on GitHub which demonstrates the above behavior. It includes the following projects:
ASP.NET Core 2.0 Website targeting .NET Framework 4.8
ASP.NET Core 3.1 Website targeting .NET Core 3.1
.NET Framework class library with calls to the legacy WebConfigurationManager
Both ASP.NET Core websites include two endpoints which call into the same .NET Framework 4.8 class library. The first is a "Hello world" example which will execute fine on both projects, since it relies exclusively on common APIs:
http://localhost:5000/Basic/Index
The second will fail on the ASP.NET Core 3.1 project, since it calls into the legacy WebConfigurationManager API:
http://localhost:5000/Basic/Configuration
Disclaimer: This is a quick and dirty repository that I put together to verify my understanding prior to posting this. If there's interest, I'll tidy it up and document it. For now, however, it may prove useful for those of you who need to see this in action.
Acknowledgments
#Chris Pratt offered an excellent answer covering similar material last year. It's worth reading.
I have an application that consists of a client-side application and a WebApi website.
My client-side stuff is targetting .NET 4 so that I don't have to insist that users install .NET 4.5. My website, however, is entirely under my control, so I'm targetting .NET 4.5.
There is one shared assembly, which I use for data access. It uses Entity Framework 5.
When I build the client application, the DLL used is version 4.4.xxx, whereas when I build the web application, the DLL is 5.0.xxx.
Up until now, I've been able to run the client application with no problems, and I've also been able to run the web application, again without problems.
However, I've now re-created my web application project from scratch (*), and suddenly I can't run it. I get a YSOD saying "Could not load file or assembly 'EntityFramework, Version=4.4.0.0 ..." at the point where my data-access assembly is first invoked.
Now, it's perfectly clear what that error means - it can't find the v4.4 DLL as used by the data-access assembly - but I don't understand why that's a problem with my new project when it wasn't a problem with my old project. As far as I can see, the same DLLs are referenced in each project.
(*) I should explain why I'm re-creating my project. I originally created the project in VS 2012 RC, and then later upgraded to the release version. Although this supposedly upgraded my project, I've had a few problems with it, and have also noticed some differences v. a newly-created project. So, to be on the safe side - and hopefully circumvent those other problems - I'm re-creating it from scratch.
So, my question: why is this suddenly a problem, and what can I do to resolve it?
Is the code depending on the assemblies set to require the exact version? If so, you will need a publisher policy for the EntityFramework assembly or bindingRedirect for the referencing application (web project).
We have a large & complex system running under .Net 4.0. We want to start introducing code written for .Net 4.5, but can't (yet) assume that .Net 4.5 is installed on all the developer machines, build machines and test labs. So I'm looking for a way to compile certain projects in MSBuild using assemblies which are available on disk, on machines which have only .Net 4.0 installed.
Per the documentation it seems possible. The 4.5 reference assemblies are available in the msbuild environment, and I've been experimenting with the TargetFrameworkVersion, FrameworkPathOverride and TargetFrameworkMoniker attributes, but so far without success.
Any hints, suggestions or working samples would be most helpful.
To build a .NET Framework 4.5 application, you must also have the .NET Framework 4.5 reference assemblies.
Installation
The .NET Framework 4.5 replaces the common language runtime (CLR), targets, tasks, and tools of the .NET Framework 4 without renaming them. Essentially, it creates a modified and improved .NET Framework 4.
I just installed the ASP.NET MVC 4.0 RC build on top of a Visual Studio 2010 SP1. My project that used to compile and work with the beta version of ASP.NET MVC 4.0, raises an error when I access the site saying that the Newtonsoft.Json assembly version 4.5 can not be found.
As I did not use this third party library in my current project, I configured Fusion Log to try to isolate the assembly that was relying on Newtonsoft.Json assembly.
And the guilty assembly is System.Net.Http.Formatting.dll which references Newtonsoft.Json version 4.5.
The beta build referenced System.Json.dll and not the Newtonsoft assembly.
I can hardly believe that Microsoft is starting to rely on external assemblies even if they publish more and more code in open source.
Does anyone have an explanation of what happened?
They are actually relying in third-party software (just like they ship MVC with JQuery and knockout.js). Scott Guthrie announced that MVC 4 will be shipped with JSON.NET (Newtonsoft). See below:
Json.NET: We plan to use the community developed Json.NET
serialization stack in our default JSON formatter in ASP.NET Web API.
Json.NET provides the flexibility and performance required for a
modern web framework.
http://aspnet.codeplex.com/wikipage?title=ASP.NET%20MVC%204%20RoadMap
You can keep the System.Json.dll btw, I've got them both. If you have any problems with JSON.NET, just reinstall it :)
I've thrown together some code to tinker with the new .Net 4.0/VS 2010 pieces, but I can't seem to find a build of my logging framework of choice (log4net) for 4.0, and I'm getting reference errors with the 2.0 version. Is there a 4.0 version available somewhere? I'm not asking for new features, just a version that's already been rebuilt against the new assemblies. Anyone know where I can find a build of 1.2.10 built for the 4.0 framework?
log4net has a known issue of referencing a System.Web component which is not part of the .NET Framework 4 Client Profile and the VS2010 by default sets project target to be the lightweighted Client Profile.
The solution is to change the target to .NET Framework 4:
Right click project -> properties -> Application.
Change:
.NET Framework 4 Client Profile
to be:
.NET Framework 4
... and that should work, i.e. you can use log4net again...