Is it safe to have a .NET6 application which references a .NET Core 3.1 NuGet library?
I tested this case using a simple console application. Everything looks good and there are no errors/warning. However, I haven't found any specific information so prefer to make sure.
This is important, as one of my providers offers only a .NET Core 3.1 NuGet package. Knowing the company it'll take them a year or so to prepare a .NET6 / .NETStandard version.
There is a good chance that your .NET Core 3.1 library will run without any compatibility problems as part of your .NET 6 application.
Quoting the .NET fundamentals article Changes that affect compatibility (emphasis mine):
Throughout its history, .NET has attempted to maintain a high level of compatibility from version to version and across implementations of .NET. …
Along with compatibility across .NET implementations, developers expect a high level of compatibility across versions of a given implementation of .NET. In particular, code written for an earlier version of .NET Core should run seamlessly on .NET 5 or a later version. In fact, many developers expect that the new APIs found in newly released versions of .NET should also be compatible with the pre-release versions in which those APIs were introduced.
However, compatibility is not guaranteed. The article goes on to say:
This article outlines changes that affect compatibility and the way in which the .NET team evaluates each type of change. Understanding how the .NET team approaches possible breaking changes is particularly helpful for developers who open pull requests that modify the behavior of existing .NET APIs.
Furthermore, in an answer to the question Clarification on backwards compatibility of .NET Core, a member of the .NET runtime team says:
We do not guarantee 100% compatibility between major versions. This is true for both ASP.NET Core and the runtime itself. We intentionally make breaking changes where we believe that they are necessary to move the platform forward and the cost of the .NET ecosystem adjusting to them is low enough.
Breaking changes that could potentially affect the compatibility of your .NET Core 3.1 library are those that are documented for .NET 5 and .NET 6.
But, if you are not experiencing any problems with your .NET Core 3.1 library, it would appear that none of the documented compatibility problems apply.
Ultimately, of course, you'll have a higher degree of confidence in the library when your vendor provides one that has been updated for .NET 6.
Finally, with the introduction of .NET 5, there became a lesser need for .NET Standard:
.NET Standard is a formal specification of .NET APIs that are available on multiple .NET implementations. The motivation behind .NET Standard was to establish greater uniformity in the .NET ecosystem. .NET 5 and later versions adopt a different approach to establishing uniformity that eliminates the need for .NET Standard in most scenarios. However, if you want to share code between .NET Framework and any other .NET implementation, such as .NET Core, your library should target .NET Standard 2.0. No new versions of .NET Standard will be released, but .NET 5, .NET 6, and all future versions will continue to support .NET Standard 2.1 and earlier.
