[Untested] Change solution to C# 7.3

[YoshiRulz]

edit: We bumped to .NET Framework 4.8 in b1ef7bc, so everything noted in this SO answer is now available to us. I've snipped the summaries of the changes that were here, go to MSDN for that: 7.0, 7.1, 7.2, 7.3, 8.0.

[Asnivor]

https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-7

The new tuples features require the ValueTuple types. You must add the NuGet package System.ValueTuple in order to use it on platforms that do not include the types.

You need to add the NuGet package System.Threading.Tasks.Extensions in order to use the ValueTask type

I'm assuming in order to not have to do the above requires .net 4.7 nad VS2017

[YoshiRulz]

Yeah, .NET framework 4.7. The package can't be very big though, we don't have to switch immediately.

[adelikat]

Upgrading .net should be avoided or at least carefully considered. Because the user must have this installed. And not all OSes even support it. We picked .net 4.6.1 for good reason. There are versions of windows 10 that can't go past this (even if you have auto-update on).

[zeromus]

Weird, up to 4.7.2 downloads pages are saying it's even supported on windows 7.

[NarryG]

It's a bit odd. Microsoft doesn't support 4.7+ on 1507 and 1511 (despite supporting them on Win7), but AdDuplex's statistics show that Windows 10 1507 and Windows 10 1511 only make up for 0.6% of all Win10 installs as of this point in time when it comes to how they record (Windows Store Advertising) (https://reports.adduplex.com/#/r/2019-01)

[YoshiRulz]

Tuples are backported in the System.ValueTuple package.

[adelikat]

This PR requires an upgrade to .net 4.7. And that's a decision we do not want to make right now. That's too much of an impact on the user base. I do not approve of this PR.

I don't what the high C# version is that doesn't require a .net change for the user. But if we want to set it to that verison and use some of those language features, that would be fine, and I'd like to see it.

[YoshiRulz]

4.6.1 supports 6.0

after the switch to new CLR (keeping C# 6.0), I'll open another PR for 8.0

edit: added note to desc because some features require .NET Core