NumericConverterExample.cs

public static class NumericConverter
    {
        public enum PadMode
        {
            Zeros,
            Ones,
            Sign
        }
        private const int positiveExt = 0x00;
        private const int negativeExt = 0xFF;
        private const byte signMask = 0x80;
     
        public static int ToInt32(ReadOnlySpan<byte> src, PadMode padMode, ByteOrder srcOrder)
        {
            int msb = srcOrder == ByteOrder.BigEndian ? 0 : src.Length - 1;
            int pad = 0;
            switch (padMode)
            {
                case PadMode.Ones: pad = negativeExt; break;
                case PadMode.Zeros: pad = positiveExt; break;
                case PadMode.Sign: pad = (src[msb] & signMask) != 0 ? negativeExt : positiveExt; break;
            }

            switch (src)
            {
                case var x when x.Length == 1:
                    return pad << 24 | pad << 16 | pad << 8 | src[0];
                case var x when x.Length == 2 && srcOrder == ByteOrder.BigEndian:
                    return pad << 24 | pad << 16 | src[0] << 8 | src[1];
                case var x when x.Length == 2 && srcOrder == ByteOrder.LittleEndian:
                    return pad << 24 | pad << 16 | src[1] << 8 | src[0];
                case var x when x.Length == 3 && srcOrder == ByteOrder.BigEndian:
                    return pad << 24 | src[0] << 16 | src[1] << 8 | src[2];
                case var x when x.Length == 3 && srcOrder == ByteOrder.LittleEndian:
                    return pad << 24 | src[2] << 16 | src[1] << 8 | src[0];
                case var x when x.Length == 4 && srcOrder == ByteOrder.BigEndian:
                    return src[0] << 24 | src[1] << 16 | src[2] << 8 | src[3];
                case var x when x.Length == 4 && srcOrder == ByteOrder.LittleEndian:
                    return src[3] << 24 | src[2] << 16 | src[1] << 8 | src[0];
                default: throw new NotSupportedException();
            }
        }
    }