The primary reference could not be resolved because it was built against a higher version of the .NET framework than the currently targeted framework

Question

I am trying to build a project that references a 3rd party SlingshotClient.dll. The project builds fine on other developers workstations. However, I am getting the error below. One difference that I could imagine is contributing to my issue is that I also have VS 2012 and .NET Frameworks 4.5 installed on my machine. I believe the other developers that can build this successfully, don't have those installed.

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1578,5): warning MSB3274: The primary reference "SlingshotClient, Version=1.0.0.0, Culture=neutral, PublicKeyToken=bb5a8745472e181a, processorArchitecture=MSIL" could not be resolved because it was built against the ".NETFramework,Version=v4.5" framework. This is a higher versio n than the currently targeted framework ".NETFramework,Version=v4.0".

The error seems to be saying that our project targets the framework version 4.0 (which is correct), but the referenced SlingshotClient.dll was built targeting framework version 4.5. When I look at the SlingshotClient.dll in the dissembler, it looks to me that the assembly was built targeting framework version 4.0

Answer 1

Apparently if you have only .NET Frameworks 4.0 installed, you can add as a reference a .NET assembly that targets 4.5 to a project that targets .NET 4.0. This will compile and run with no errors.

As soon as you install .NET Frameworks 4.5, compile will fail. The solution is to target your project for .NET Framework 4.5 or get a version of the referenced assembly that targets .NET 4.0.

.NET 4.5 is a drop in replacement for 4.0. 4.0 doesn't know anything about 4.5, and I assume it is just looking at the first digit of the version number which for 4.0 or 4.5 is the same, so it is allowed to compile. As soon as you install 4.5, your 4.0 projects are compiled using the 4.5 Frameworks and it now knows about 4.5 and complains.

Answer 2

As a workaround, you can add the following to your .csproj file within the <Project> element.

<PropertyGroup>
<ResolveAssemblyReferenceIgnoreTargetFrameworkAttributeVersionMismatch>true</ResolveAssemblyReferenceIgnoreTargetFrameworkAttributeVersionMismatch>
</PropertyGroup>

Be aware that while this does make the compilation errors go away, those errors are there for a reason and ignoring them blatantly can absolutely cause issues when the binary you are referencing tries to perform code that does not work on your lower version of .NET.

Reference