BitArray.CopyTo(Array, int) seems broken

[MichaelL79]

BitArray.CopyTo(Array, int) seems broken

When using BitArray.CopyTo() in .NET Core 3 it seems the bits are always copied to the last position of the provided array.

General

My setup is a .NetStandard 2 class library with a method that contains a BitArray.CopyTo(Array, int). If I use this library from within a WPF application using .Net 4.7.2 everything works as expected. When I use the exact same library from within a .NET Core 3 application (Console or WPF) always the last byte of the array is overwritten and the index parameter is ignored. Example:

bool[] directionBools = { true, true, true, true };
bool[] levelBools = { false, false, false, true };
byte[] byteHolder = new byte[2];
BitArray directionBits = new BitArray(directionBools);
BitArray levelBits = new BitArray(levelBools);

directionBits.CopyTo(byteHolder, 0);
levelBits.CopyTo(byteHolder, 1);

In .NET 4.7.2 and earlier this would result in an array 0F-08. In .NET Core 3 it results in an array 00-08 as the second CopyTo overwrites the 0F that was wrongly copied to index 1 instead of index 0. If you increase the byteHolder length to eg. 5. It would result in 0F-08-00-00-00 in 4.7.2 and 00-00-00-00-08 in Core 3.

Best regards
Michael

[carlossanlop]

Hello @MichaelL79 , thanks for reporting this.
Since this is a potential issue in our libraries, I will transfer it to the dotnet/corefx repo for higher visibility.

[Clockwork-Muse]

Behavior confirmed....

But we have a test to protect against this, that passes!? (Verified under debug of test)..... what?

I know there also used to be a copy distributed along with coreclr, although that's long gone.

[jkotas]

Regression introduced by @ahsonkhan 's change dotnet/corefx#33367

[ericstj]

@ahsonkhan can you provide an update here? Will a fix make it in for Preview9?

[ahsonkhan]

Fixed in dotnet/corefx#40442 for release/3.0 (for preview 9 and above) and in dotnet/corefx#40441 for master.