… We are forced to stay on the lowest API level of the framework (.NET Standard) …

This to me highlights the fact that, not only are you potentially restricting yourselves too much, you may also be heading for a nasty fall with your approach.

.NET Standard is not, and never will be “the lowest API level of the framework“. The most restrictive set of APIs for .NET is achieved by creating a portable class library that targets Windows Phone and Silverlight.

Depending on which version of .NET Standard you are targeting, you can end up with a very rich set of APIs that are compatible with .NET Framework, .NET Core, Mono, and Xamarin. And there are many third-party libraries that are .NET Standard compatible that will therefore work on all these platforms.

Then there is .NET Standard 2.1, likely to be released in the Autumn of 2019. It will be supported by .NET Core, Mono and Xamarin. It will not be supported by any version of the .NET Framework, at least for the foreseeable future, and quite likely always. So in the near future, far from being “the lowest API level of the framework“, .NET Standard will supersede the framework and have APIs that aren’t supported by the latter.

So be very careful with “The reasoning behind this is that a new platform could one day arrive that only supports that very low API level” as it’s quite likely that new platforms will in fact support a higher level API than the old framework does.

Then there’s the issue of third-party libraries. JSON.NET for example is compatible with .NET Standard. Any library compatible with .NET Standard is guaranteed – API-wise – to work with all .NET implementations that are compatible with that version of .NET Standard. So you achieve no additional compatibility by not using it and creating your JSON library. You simply create more work for yourselves and incur unnecessary costs for your company.

So yes, you definitely are taking this too far in my view.

We are forced to stay on the lowest API level of the framework (.net standard). The reasoning behind this is that a new platform could one day arrive that only supports that very low API level.

The reasoning here is rather backwards. Older, lower API levels are more likely to become obsolete and unsupported than newer ones. While I agree that staying a comfortable way behind the “cutting edge” is sensible to ensure a reasonable level of compatibility in the scenario you mention, never moving forward is beyond extreme.

We have implemented our own components for (de)serializing JSON, and are in the process of doing the same for JWT. This is available in a higher level of the framework API.
We have implemented a wrapper around the HTTP framework of the standard library because we don’t want to take a dependency on the HTTP impelemntation of the standard library.
All of the code for mapping to/from XML is written “by hand”, again for the same reason.

This is madness. Even if you don’t want to use standard library functions for whatever reason, open source libraries exist with commercially compatible licenses that do all of the above. They’ve already been written, extensively tested from a functionality, security and API design point of view, and used extensively in many other projects.

If the worst happens and that project goes away, or stops being maintained, then you’ve got the code to build the library anyway, and you assign someone to maintain it. And you’re likely still in a much better position than if you’d rolled your own, since in reality you’ll have more tested, cleaner, more maintainable code to look after.

In the much more likely scenario that the project is maintained, and bugs or exploits are found in those libraries, you’ll know about them so can do something about it – such as upgrading to a newer version free of charge, or patching your version with the fix if you’ve taken a copy.

On the whole these things are good for your customers. Even a popular open source library might be impossible for them to use for some reason.

For example, they may have signed a contract with their customers promising not to use open source products.

However, as you point out, these features are not without cost.

• Time to market
• Size of package
• Performance

I would raise these downsides and talk with customers to find out if they really need the uber levels of compatibility you are offering.

If all the customers already use Json.NET for example, then using it in your product rather than your own deserialisation code, reduces its size and improves it.

If you introduce a second version of your product, one which uses third-party libraries as well as a compatible one you could judge the uptake on both. Will customers use the third parties to get the latest features a bit earlier, or stick with the ‘compatible’ version?

Short answer is that you should start introducing third-party dependencies. During your next stand-up meeting, tell everyone that the next week at work will be the most fun they have had in years — they’ll replace the JSON and XML components with open source, standard libraries solutions. Tell everyone that they have three days to replace the JSON component. Celebrate after it’s done. Have a party. This is worth celebrating.

Basically it all comes down to effort vs. risk.

By adding an additional dependency or update your framework or use higher level API, you lower your effort but you take up risk. So I would suggest doing a SWOT analysis.

• Strengths: Less effort, because you don’t have to code it yourself.
• Weaknesses: It’s not as custom designed for your special needs as a handcrafted solution.
• Opportunities: Time to market is smaller. You might profit from external developments.
• Threats: You might upset customers with additional dependencies.

As you can see the additional effort to develop a handcrafted solution is an investment into lowering your threats. Now you can make a strategic decision.