The answer to this is a somewhat hotly debated topic... perhaps mainly within the Open Source Hardware community.
Fundamentally - "Is it permissible to refer to a project as open source if the tools required to edit / build the sources are not themselves free for use?"
Examples:
- Projects with build-time dependencies on tools that are not free-to-use (e.g: hardware projects designed using Altium)
- Projects with run-time dependencies and operating environments that are not free-to-use (e.g: software projects designed for use on Windows)
In my mind, if the answer was "No", then an extreme example would be that any software that only runs on a non-free operating such as Windows cannot be referred to as "Open Source" (even... Even if it couldcould be run in an emulated environment such as Wine), it wasn't designed for this usage.
However, my view is that if the sources are open for anyone to inspect and review, then "Yes" it's permissible to refer to the project as "Open Source".
Bear in mind that the "Source" in "Open Source" is referring to the source files - the original component parts of a project which are used with various tools to produce a usable end result.
"Sources" covers:
- Application source code
- Build infrastructure or scripts (e.g: Makefiles)
- Project files (e.g: VCProj)
- Documentation
- etc...
opensource.com defines "Open Source" as:
something people can modify and share because its design is publicly accessible
Note that the C# tools and .NET are indeed "free-to-use", but they are "closed source":
https://visualstudio.microsoft.com/downloads
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