ImmutableArray tests are failing:

    System.Collections.Immutable.Tests.ImmutableArrayTest.CopyToInvalid(source: []) [FAIL]
      Assert.Equal() Failure
                    � (pos 4)
      Expected: dest
      Actual:   destinationArray
                    � (pos 4)
      Stack Trace:
        /_/src/libraries/Common/tests/CoreFx.Private.TestUtilities/System/AssertExtensions.cs(46,0): at System.AssertExtensions.Throws[T](String netCoreParamName, String netFxParamName, Action action)
        /_/src/libraries/System.Collections.Immutable/tests/ImmutableArrayTest.cs(1635,0): at System.Collections.Immutable.Tests.ImmutableArrayTest.CopyToInvalid(IEnumerable`1 source)
    System.Collections.Immutable.Tests.ImmutableArrayTest.CopyToInvalid(source: [1]) [FAIL]

It should be fine to fix the test failures by relaxing the argument name validation in this test.

@jkotas Do you mean something like:

// Skip checking .NETFramework parameter name is either 'destinationArray' or 'dest'
AssertExtensions.Throws<ArgumentNullException>("destinationArray", netFxParamName: null, () => array.CopyTo(null));
AssertExtensions.Throws<ArgumentNullException>("destinationArray", netFxParamName: null, () => array.CopyTo(null, 0));
AssertExtensions.Throws<ArgumentNullException>("destinationArray", netFxParamName: null, () => array.CopyTo(0, null, 0, 0));
AssertExtensions.Throws<ArgumentNullException>("destinationArray", netFxParamName: null, () => array.CopyTo(-1, null, -1, -1)); // The destination should be validated first.

@Therzok, still working on this?

@stephentoub I'll setup the repo on a Windows machine to see why it's failing, the tests passed locally on OS X. Sorry for the delays.

If you need help, let me know (and if I have push permissions to your branch, I can just fix it for you, too).

I've had a few hoops and hurdles. If you have MSBuild Build Tools installed and they don';t have the required pre-reqs, you could end up opening the wrong developer command prompt to build. I'm trying to get the in-VS tests to run right now, looking at the test explorer, it doesn't seem to run them. Although it seems the top menu run seems to work.

Okay, so, I've managed to get it setup and run it on .netcore. Whenever I try to run the .NET Framework config, I'm getting Illegal characters in path., with no extra explanation from VS.

I fixed up the commit. Thanks for the contribution.

Thanks for that!