Hello 3583Bytes,
cf. Arena debug file:

2019-01-02 14:09:34,165 Arena 3.5.1
2019-01-02 14:09:34,165Font Arena Chess Figurine MISSING

2019-01-02 14:09:34,258**----------New game---2019-01-02 14:09:34,258 Mi -------------
2019-01-02 14:09:34,414**----------New game---2019-01-02 14:09:34,414 Mi -------------
2019-01-02 14:09:34,477screen: 1280x1024
2019-01-02 14:09:34,477Monitors: 1
2019-01-02 14:09:34,477Monitor0: 1280x1024
2019-01-02 14:09:34,477FormMonitor: 0
2019-01-02 14:09:34,477**DesktopRect: 0/0 .. 1280/1024
2019-01-02 14:10:36,0231------------------------Starting engine 1 ChessCore-------------------------
2019-01-02 14:10:36,0381Configured Engine 1 Type: WB2
2019-01-02 14:10:36,0381Engine 1 dir: C:\Users\NRL\Downloads\ChessCore-master\ChessCore-master\Releases\ChessCore1.0.1\ChessCore
2019-01-02 14:10:36,0381Engine 1 commandline: C:\Users\NRL\Downloads\ChessCore-master\ChessCore-master\Releases\ChessCore1.0.1\ChessCore\ChessCore.exe
2019-01-02 14:10:36,1471Engine 1 ProcessID: 2152
2019-01-02 14:10:36,1471Engine 1 Prio:32 ThreadPrio:0
2019-01-02 14:10:36,147<--1:Error:
2019-01-02 14:10:36,147<--1: An assembly specified in the application dependencies manifest (ChessCore.deps.json) was not found:
2019-01-02 14:10:36,147-->1:xboard
2019-01-02 14:10:36,147-->1:protover 2
2019-01-02 14:10:36,194<--1: package: 'NUnit', version: '3.11.0'
2019-01-02 14:10:36,210<--1: path: 'lib/netstandard2.0/nunit.framework.dll'
2019-01-02 14:10:37,006-->1:new
2019-01-02 14:10:37,006-->1:random
2019-01-02 14:10:37,006-->1:level 0 5 0
2019-01-02 14:10:37,006-->1:post
2019-01-02 14:10:37,006-->1:hard
2019-01-02 14:10:37,006-->1:easy
2019-01-02 14:11:36,5461
2019-01-02 14:11:36,5461Engine crashed, restarting...
2019-01-02 14:11:36,546-->1:?
2019-01-02 14:11:36,546-->1:force
2019-01-02 14:11:36,546-->1:quit
2019-01-02 14:11:36,9211------------------------Starting engine 1 ChessCore-------------------------
2019-01-02 14:11:36,9211Configured Engine 1 Type: WB2
2019-01-02 14:11:36,9211Engine 1 dir: C:\Users\NRL\Downloads\ChessCore-master\ChessCore-master\Releases\ChessCore1.0.1\ChessCore
2019-01-02 14:11:36,9211Engine 1 commandline: C:\Users\NRL\Downloads\ChessCore-master\ChessCore-master\Releases\ChessCore1.0.1\ChessCore\ChessCore.exe
2019-01-02 14:11:37,0301Engine 1 ProcessID: 3248
2019-01-02 14:11:37,0301Engine 1 Prio:32 ThreadPrio:0
2019-01-02 14:11:37,030<--1:Error:
2019-01-02 14:11:37,030<--1: An assembly specified in the application dependencies manifest (ChessCore.deps.json) was not found:
2019-01-02 14:11:37,030<--1: package: 'NUnit', version: '3.11.0'
2019-01-02 14:11:37,030-->1:xboard
2019-01-02 14:11:37,030-->1:protover 2
2019-01-02 14:11:37,077<--1: path: 'lib/netstandard2.0/nunit.framework.dll'
2019-01-02 14:11:37,859-->1:new
2019-01-02 14:11:37,859-->1:random
2019-01-02 14:11:37,859-->1:level 0 5 0
2019-01-02 14:11:37,859-->1:post
2019-01-02 14:11:37,859-->1:hard
2019-01-02 14:11:37,859-->1:easy
2019-01-02 14:11:38,7331Start calc, move no: 0
2019-01-02 14:11:38,733-->1:level 0 5 0
2019-01-02 14:11:38,733-->1:new
2019-01-02 14:11:38,733-->1:random
2019-01-02 14:11:38,733-->1:level 0 5 0
2019-01-02 14:11:38,733-->1:post
2019-01-02 14:11:38,733-->1:hard
2019-01-02 14:11:38,733-->1:easy
2019-01-02 14:11:38,733-->1:time 30000
2019-01-02 14:11:38,733-->1:otim 30000
2019-01-02 14:11:38,733-->1:white
2019-01-02 14:11:38,733-->1:go
2019-01-02 14:16:40,277-->1:?

If you're interested, I found and fixed a bug that caused the engine to prematurely declare checkmate against the computer (it's in my fork of ChessCore, specifically near the start of Engine.AiPonderMove()).

It took me a while to track down the problem, but basically the issue is that WhiteAttackBoard[] gets regenerated BEFORE the player's move gets executed, and only moves that were viable prior to that move are considered when AiPonderMove() goes to look for escape routes for a king in check.

Board to illustrate the problem:

r4r2/pppb1p1k/8/3p2Q1/q4P2/2np4/6PP/5RK1/ w - 0 27

then... move the white queen from G5 to H5.

Without my fix, the engine declares checkmate against black & the game is over.

With my fix, the engine correctly moves the computer's black king to G8, and proceeds to win a few moves later.

Since the performance cost of running PieceValidMoves.GenerateValidMoves(ChessBoard) is nontrivial on Android, I added an extra bit of logic so that it ALWAYS executes whenever either king is in check, but otherwise executes every turn ONLY at the two hardest difficulty levels, on the rationale that a premature declaration of checkmate is going to be noticed & confusing even to inexperienced players, but if the computer insidiously overlooks opportunities to save OTHER pieces, it's either a minor concern relative to performance, or a welcome way to make the engine slightly easier to play against.

Where do I type this in to run the code?

First off, thank you for releasing the code. Very educational and useful.
Code should demonstrate the problem. which is caused by attack boards not being regenerated after every ply. Maybe this is your design intention. Not sure.

Consider the following method:
`

    static void TestValidMoves()
    {
        string startFen = "8/8/8/2k5/2P5/2N5/2N3Q1/3K4 w - - 0 1";
        ChessEngine.Engine.Engine e = new ChessEngine.Engine.Engine();
        e.InitiateBoard(startFen);

        // 1. Qg7
        e.MovePiece(54, 14);

        // returns "8/6Q1/8/2k5/2P5/2N5/2N5/3K4 w - - 2 2"
        // (I've modified 50 move logic to increment after every ply, so this may look different to you
        // but should not interfere with GetValidMoves)

        var fen1 = e.FEN;
        
        // c6
        byte square = 26;
        var pieceType = e.GetPieceTypeAt(square);
        var pieceColor = e.GetPieceColorAt(square);

        int c = square % 8;
        int r = square / 8;
        
        //  Returns { 1, 2 } { 3, 2 } and { 2, 4 } (squares 17, 19 and 34 ) square 18 not in list, even though valid.
        var vm = e.GetValidMoves((byte)c, (byte)r);
        
        // 1...Kc6  but engine allows move
        e.MovePiece(26, 18);

        // returns "8/6Q1/2k5/8/2P5/2N5/2N5/3K4 w - - 2 2"
        var fen2 = e.FEN;

        e.InitiateBoard(fen1);
        // now square 18 is included in list
        vm = e.GetValidMoves((byte)c, (byte)r);

    }

`

Hello 3583Bytes,

cf. http://s000.tinyupload.com/index.php?file_id=54336671776373947969 - the former link is no longer available

Regards

Hello3583Bytes,

cf. this package here: http://tonyschess.x10host.com/ChessCore101AllDLLs.rar and the ChessCore results against Fairy-Max on three different graphical interfaces
Debug Arena-ChessGUI-PGN.zip including Arena and ChessGUI debug files

The message "illegal moves" is sometimes visible on these GUIs

Regards,