Runtime: Environment.ExpandEnvironmentVariables on Linux has Windows behavior

[FeodorFitsner]

Environment.ExpandEnvironmentVariables() on Linux expands variables in %VAR_NAME% format rather than $VAR_NAME.

SDK version: 2.1.4
OS: Ubuntu 16.04

A simple test:

Environment.ExpandEnvironmentVariables("$HOME/test");

Expected value: /home/{user}/test
Actual value: $HOME/test

However running:

Environment.ExpandEnvironmentVariables("%HOME%/test");

gives expanded result /home/{user}/test.

[danmosemsft]

My guess is this was done intentionally -- so existing code passing %VAR_NAME% "just works" when used on Unix. For code written for Unix, it is certainly unexpected.
The API could also respect $ - if it is worth diverging further and becoming potentially confusing.
Another option is an API
```c#
public string Environment.ExpandEnvironmentVariables(string name, EnvironmentVariablesExpandFlavor flavor = EnvironmentVariablesExpandFlavor.PlatformDefault);

[Flags]
public enum EnvironmentVariablesExpandFlavor
{
PlatformDefault = 0x1,
PercentSigns = 0x2,
DollarSigns = 0x4
}
```
with better names...

Maybe @stephentoub recalls context.

[stephentoub]

Maybe @stephentoub recalls context.

Mainly because that's how it's documented:
https://docs.microsoft.com/en-us/dotnet/api/system.environment.expandenvironmentvariables

A string containing the names of zero or more environment variables. Each environment variable is quoted with the percent sign character (%).

I could be convinced that you'd never do that on unix and this is really just surfacing a Windows-ism and so we should use $ on unix instead of %. But we'd want to understand how badly breaking that would be. I'd be worried that code has already done string.Replace('$','%') to make it work the desired way on unix, and then we'd be breaking those folks, for example.

[joshfree]

I think we should add a new overload that supports this.

I'd be worried that code has already done string.Replace('$','%') to make it work the desired way on unix,

ps - string.Replace('$', '%') would add $ suffixes to your expanded env vars 😉

[gfoidl]

For cross-platform development both variants should be allowed, similar to the path-separator (\ vs. /).

_Batch_ allows the $ as valid variable name, but then it would have to be quoted by %, so %$%. This case can be handled.
In _Bash_ the % isn't allowd in variable names, so this case can be handled too.

Further there wouldn't be any breaking changes, since %var% and $var would work.

[stephentoub]

Further there wouldn't be any breaking changes, since %var% and $var would work.

Except $var on Windows may start to be reinterpreted, changing output in some cases.

[aaronfranke]

It may also be worth considering expanding ~. I know that it is used by some programs in file names, so I'd suggest only expanding it if it's the first character in the given path string.