private static ReadOnlySpan<byte> s_hexEncodeMap => new byte[] { (byte)'0', (byte)'1', (byte)'2', (byte)'3', (byte)'4', (byte)'5', (byte)'6', (byte)'7', (byte)'8', (byte)'9', (byte)'A', (byte)'B', (byte)'C', (byte)'D', (byte)'E', (byte)'F' };

/// <summary>
/// A faster version of String.Concat(<paramref name="str"/>, <paramref name="separator"/>, <paramref name="number"/>.ToString("X8"))
/// </summary>
/// <param name="str"></param>
/// <param name="separator"></param>
/// <param name="number"></param>
/// <returns></returns>
public static string ConcatAsHexSuffix(string str, char separator, uint number)
{
    var length = 1 + 8;
    if (str != null)
    {
        length += str.Length;
    }

    return string.Create(length, (str, separator, number), (buffer, tuple) =>
    {
        var (tupleStr, tupleSeparator, tupleNumber) = tuple;
        var hexEncodeMap = s_hexEncodeMap;

        var i = 0;
        if (tupleStr != null)
        {
            tupleStr.AsSpan().CopyTo(buffer);
            i = tupleStr.Length;
        }

        var number = (int)tupleNumber;
        buffer[i + 8] = (char)hexEncodeMap[number & 0xF];
        buffer[i + 7] = (char)hexEncodeMap[(number >> 4) & 0xF];
        buffer[i + 6] = (char)hexEncodeMap[(number >> 8) & 0xF];
        buffer[i + 5] = (char)hexEncodeMap[(number >> 12) & 0xF];
        buffer[i + 4] = (char)hexEncodeMap[(number >> 16) & 0xF];
        buffer[i + 3] = (char)hexEncodeMap[(number >> 20) & 0xF];
        buffer[i + 2] = (char)hexEncodeMap[(number >> 24) & 0xF];
        buffer[i + 1] = (char)hexEncodeMap[(number >> 28) & 0xF];
        buffer[i] = tupleSeparator;
    });
}

.method private hidebysig specialname static 
	valuetype [System.Private.CoreLib]System.ReadOnlySpan`1<uint8> get_s_hexEncodeMap () cil managed 
{
	// Method begins at RVA 0x2050
	// Code size 13 (0xd)
	.maxstack 8

	IL_0000: ldsflda valuetype '<PrivateImplementationDetails>'/'__StaticArrayInitTypeSize=16' '<PrivateImplementationDetails>'::CE27CB141098FEB00714E758646BE3E99C185B71
	IL_0005: ldc.i4.s 16
	IL_0007: newobj instance void valuetype [System.Private.CoreLib]System.ReadOnlySpan`1<uint8>::.ctor(void*, int32)
	IL_000c: ret
}

static ReadOnlySpan<byte> span => new byte[] { 1,2,3,4 };

byte Case1(int i)  => span[i & 2];