Make System.Drawing.Common only supported on Windows

INDEX.md

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 | 2021 | [.NET 6.0 Target Frameworks](accepted/2021/net6.0-tfms/net6.0-tfms.md) | [Immo Landwerth](https://github.com/terrajobst) |
 | 2021 | [Compile-time source generation for strongly-typed logging messages](accepted/2021/logging-generator.md) | [Maryam Ariyan](https://github.com/maryamariyan), [Martin Taillefer](https://github.com/geeknoid) |
 | 2021 | [Inbox Source Generators](accepted/2021/InboxSourceGenerators.md) | [Eric StJohn](https://github.com/ericstj) |
+| 2021 | [Make `System.Drawing.Common` only supported on Windows](accepted/2021/system-drawing-win-only/system-drawing-win-only.md) | [Santiago Fernandez Madero](https://github.com/safern) |
 | 2021 | [Objective-C interoperability](accepted/2021/objectivec-interop.md) | [Aaron Robinson](https://github.com/AaronRobinsonMSFT) |
 | 2021 | [Preview Features](accepted/2021/preview-features/preview-features.md) | [Immo Landwerth](https://github.com/terrajobst) |
 | 2021 | [TFM for .NET nanoFramework](accepted/2021/nano-framework-tfm/nano-framework-tfm.md) | [Immo Landwerth](https://github.com/terrajobst), [Laurent Ellerbach](https://github.com/Ellerbach), [José Simões](https://github.com/josesimoes) |

accepted/2021/system-drawing-win-only/system-drawing-win-only.md

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+# Make `System.Drawing.Common` only supported on Windows
+
+**Owner** [Santiago Fernandez Madero](https://github.com/safern)
+
+In the .NET Core 2.x days we created the `Microsoft.Windows.Compatibility`
+package which exposes many Windows-only APIs so that more people can migrate to
+.NET Core. As part of this, we also ported `System.Drawing.Common`. This was
+mainly meant for .NET Core users that still target Windows, however since Mono
+already had a cross-platform implementation, we decided to bring that over. The
+cross-platform functionality is provided via the native library `libgdiplus`.
+Sadly, `libgdiplus` is a community supported library and at this point
+effectively unmaintained.
+
+While `libgdiplus` has helped a lot of customers expand to cross-platform it has
+also caused some headaches. `System.Drawing.Common` is very Windows driven as it
+is mostly a wrapper around GDI+ and is heavily coupled to Windows Forms, which
+keeps evolving and diverging from cross-platform graphics. This has caused some
+confusion over why some things don't work well on Unix or don't work at all.
+
+Since the inclusion of the Mono cross-platform implementation we spent lot of
+time redirecting issues to `libgdiplus` that never got fixed, or helping people
+install `libgdiplus` correctly (because it's not part of .NET Core proper). We
+have thought about our options, and to support `System.Drawing.Common` properly
+on Unix we'd need to make `libgdiplus` a first-class citizen and ship it as part
+of the product:
+
+* `libgdiplus` doesn't ship with all operating systems by default, unlike other
+  native libraries we depend on (icu, openssl).
+
+* `libgdiplus` is community driven, so a lot of issues/PRs just go stale and are
+  never looked at.
+
+* There is no servicing story for bugs and security issues.
+
+However, making `libgdiplus` a first-class citizen is costly. It's around 30k
+lines of pretty old C code, virtually untested, and lacks a lot of
+functionality. `libgdiplus` also has a lot of external dependencies for all the
+image processing and text rendering, such as `cairo`, `pango`, and other native
+libraries, which makes shipping even more challenging.
+
+We've also hit some memory leaks or double free'd memory that weren't fixed in
+`libgdiplus` and that we had to work around because they were potential security
+issues. This makes it harder as we are basically on our “own” for those security
+issues. `libgdiplus` is a library we don't own, but that we must spend time
+investigating issues and proposing fixes.
+
+We have noticed (such as from analysis of NuGet packages) that
+`System.Drawing.Common` is used on cross-platform mostly for image manipulation
+like QR code generators and text rendering. We haven't noticed heavy graphics
+usage as our cross-platform graphics support is very incomplete. For example,
+right now some components crash on macOS because the native APIs we use have
+been deprecated and are no longer included with recent macOS releases. We have
+never gotten a report about that crash; we discovered it ourselves while making
+`System.Drawing.Common` trim compatible.
+
+The usages we see of `System.Drawing.Common` in non-Windows environments are
+typically well supported with SkiaSharp and ImageSharp.
+
+Thus, we believe it is not worth investing in making `System.Drawing.Common`
+work on non-Windows platforms and instead propose we only keep evolving it in
+the context of Windows Forms and GDI+.
+
+## Requirements
+
+### Goals
+
+* Consumers of `System.Drawing.Common` will be informed that starting with .NET
+  6, it's only supported on Windows, via the [platform compatibility analyzer]
+* Cross-platform consumers can reference older versions of the package to
+  continue using `System.Drawing.Common` via `libgdiplus`
+
+### Non-Goals
+
+* Closing the gap of `System.Drawing.Common` for non-Windows platforms or
+  bundling `libgdiplus` as part of the .NET 6 installer
+
+## Stakeholders and Reviewers
+
+* Libraries team
+* ML.NET
+* Xamarin team
+
+## Design
+
+In the past we decided to include the following disclaimer in our docs:
+
+> The `System.Drawing.Common` NuGet package works on Windows, Linux, and macOS.
+> However, there are some platform differences. On Linux and macOS, the GDI+
+> functionality is implemented by the `libgdiplus` library. This library is not
+> installed by default in most Linux distributions and doesn't support all the
+> functionality of GDI+ on Windows on macOS. There are also platforms where
+> `libgdiplus` is not available at all. To use types from the
+> `System.Drawing.Common` package on Linux and macOS, you must install
+> `libgdiplus` separately. For more information, see Install .NET on Linux or
+> Install .NET on macOS.
+>
+> If you can't use System.Drawing with your application, recommended
+> alternatives include ImageSharp, SkiaSharp, and Windows Imaging Components.
+
+However, this disclaimer doesn't feel clear enough to steer people towards using
+other graphics libraries.
+
+The proposal is to do the following for .NET 6:
+
+1. **Mark the assembly as only being supported on Windows**. That will cause the
+   platform compatibility analyzer to warn when consumed from a project that
+   isn't targeting net6.0-windows (or marked as Windows-specific).
+
+2. Throw `PlatformNotSupportedException` when trying to load `libgdiplus`
+   pointing to an aka.ms link with the reasons behind it and recommending
+   stable, well-maintained open-source library alternatives
+
+3. Add targets to the package that when it is restored using a non-windows RID,
+   it warns that we no longer support it on non-windows and point to the aka.ms
+   link with more info.
+
+The challenge is that `System.Drawing.Common` is used by 1st party consumers
+such as ML.NET, which uses it for image manipulation and even exposes an API
+that returns a Bitmap. At the same time, https://github.com/dotnet/iot is in the
+process of moving to use ImageSharp and so far they had a great migration
+experience. We need to engage with 1st party customers to see what impact this
+proposal will have on them.
+
+***Note:** This only affect `System.Drawing.Common` and would leave
+`System.Drawing.Primitives` as-is. That assembly contains primitive types that
+don't depend on GDI+, such `Rectangle`, `Point`, and `Size`. Any types in
+`System.Drawing.Common` that don't depend on GDI+, will be moved to
+`System.Drawing.Primitives`.*
+
+# Q & A
+
+## Will this prevent applications to move to .NET 6?
+
+No, `System.Drawing.Common` is a standalone package that does not ship as part
+of the shared framework. For that reason if an application wants to keep using
+`System.Drawing.Common` on `Unix` and move to .NET 6, they can reference the
+`5.0.x` version of the package.
+
+## How will applications migrate to recommended libraries?
+
+We will provide with the right documentation via a breaking change notice and a
+document that will contain guidance and samples.
+
+## What does the usage data of `System.Drawing.Common` reveal?
+
+In order to assess how `System.Drawing.Common` we looked at the API usage from
+libraries published to nuget.org.
+
+We looked at member refs to types that begin with `System.Drawing.` and
+aggregated the hits by type and member name. We also pulled in the TFM of the
+assemblies in question to help categorize the hits. There's a lot of data (there
+are a lot of members * TFMs), so instead of just huge raw tables of nonsense,
+we'll provide a summary of what we observed. 
+
+If we look at .NET Core family of TFMs, the top APIs are:
+
+* `System.Drawing.Bitmap` (constructor)
+* `System.Drawing.Font` (constructor)
+* `System.Drawing.Graphics.DrawImage()`/`DrawString()`/`FillRectangle()`
+* `System.Drawing.Point(F)`
+* `System.Drawing.Rectangle(F)`
+* `System.Drawing.Size(F)`
+
+The hits here are in the 40k range, and quickly fall off as we get to
+`System.Drawing.Brush` at ~15k
+
+These usages are consistent with drawing/augmenting images scenarios.
+Interestingly, `set_SmoothingMode` has fewer than 1k hits, which suggests that
+most people are getting less than stellar results from their current usage, and
+they're likely to be much happier moving to our suggested solutions, which have
+better quality by default.
+
+.NET Standard TFMs have similar usage, but `System.Drawing.Color` shows up at
+the top of the hits, along with operations that indicate color calculations are
+a top scenario, and the bulk of other auxillary types like point, rectangle,
+size, etc. have reduced usage. Overall, .NET Standard usage is slightly less
+than .NET Core, which was a surprise to us, and we're not exactly sure what to
+make of it. It could suggest that .NET Core usage overall is beginning to
+eclipse .NET Standard. Or, it could mean something else entirely, like .NET
+Standard libraries tend to be used in different scenarios that are doing less
+image manipulation, and more raw color calculations. Finding out, is a lot more
+work, and we don't think it helps us here.
+
+Other interesting takeaways:
+
+* These types are used by Xamarin.iOS and MonoAndroid targets, particularly the
+  `Rectangle`/`Size`/`Point` types/APIs, although overall usage is in the 13k
+  range, and falls off quickly. This suggests a usage pattern other than drawing
+  things on images, but it's not clear that this decision has any effect on that
+  part of the ecosystem.
+
+* For comparison, the .NET Framework usage of these types is in the 531k range,
+  which very similar usage patterns to .NET Core. If you combine "no TFM"
+  assemblies, you get an extra almost 200k of hits, so this dwarfs .NET Core
+  usage, but that's no surprise.
+
+[platform compatibility analyzer]: https://docs.microsoft.com/en-us/dotnet/standard/analyzers/platform-compat-analyzer