DTLib.Demystifier/Internal/ILReader.cs

using System.Reflection.Emit;

namespace DTLib.Demystifier.Internal;

internal class ILReader
{
    private static readonly OpCode[] singleByteOpCode;
    private static readonly OpCode[] doubleByteOpCode;

    private readonly byte[] _cil;
    private int ptr;

    static ILReader()
    {
        singleByteOpCode = new OpCode[225];
        doubleByteOpCode = new OpCode[31];

        var fields = GetOpCodeFields();

        for (var i = 0; i < fields.Length; i++)
        {
            var code = (OpCode)fields[i].GetValue(null)!;
            if (code.OpCodeType == OpCodeType.Nternal)
                continue;

            if (code.Size == 1)
                singleByteOpCode[code.Value] = code;
            else
                doubleByteOpCode[code.Value & 0xff] = code;
        }
    }


    public ILReader(byte[] cil)
    {
        _cil = cil;
    }

    public OpCode OpCode { get; private set; }
    public int MetadataToken { get; private set; }
    public MemberInfo? Operand { get; private set; }

    public bool Read(MethodBase methodInfo)
    {
        if (ptr < _cil.Length)
        {
            OpCode = ReadOpCode();
            Operand = ReadOperand(OpCode, methodInfo);
            return true;
        }

        return false;
    }

    private OpCode ReadOpCode()
    {
        var instruction = ReadByte();
        if (instruction < 254)
            return singleByteOpCode[instruction];
        return doubleByteOpCode[ReadByte()];
    }

    private MemberInfo? ReadOperand(OpCode code, MethodBase methodInfo)
    {
        MetadataToken = 0;
        int inlineLength;
        switch (code.OperandType)
        {
            case OperandType.InlineMethod:
                MetadataToken = ReadInt();
                Type[]? methodArgs = null;
                if (methodInfo.GetType() != typeof(ConstructorInfo) &&
                    !methodInfo.GetType().IsSubclassOf(typeof(ConstructorInfo)))
                    methodArgs = methodInfo.GetGenericArguments();
                Type[]? typeArgs = null;
                if (methodInfo.DeclaringType is not null) typeArgs = methodInfo.DeclaringType.GetGenericArguments();
                try
                {
                    return methodInfo.Module.ResolveMember(MetadataToken, typeArgs, methodArgs);
                }
                catch
                {
                    // Can return System.ArgumentException : Token xxx is not a valid MemberInfo token in the scope of module xxx.dll
                    return null;
                }

            case OperandType.InlineNone:
                inlineLength = 0;
                break;

            case OperandType.ShortInlineBrTarget:
            case OperandType.ShortInlineVar:
            case OperandType.ShortInlineI:
                inlineLength = 1;
                break;

            case OperandType.InlineVar:
                inlineLength = 2;
                break;

            case OperandType.InlineBrTarget:
            case OperandType.InlineField:
            case OperandType.InlineI:
            case OperandType.InlineString:
            case OperandType.InlineSig:
            case OperandType.InlineSwitch:
            case OperandType.InlineTok:
            case OperandType.InlineType:
            case OperandType.ShortInlineR:
                inlineLength = 4;
                break;

            case OperandType.InlineI8:
            case OperandType.InlineR:
                inlineLength = 8;
                break;

            default:
                return null;
        }

        for (var i = 0; i < inlineLength; i++) ReadByte();

        return null;
    }

    private byte ReadByte()
    {
        return _cil[ptr++];
    }

    private int ReadInt()
    {
        var b1 = ReadByte();
        var b2 = ReadByte();
        var b3 = ReadByte();
        var b4 = ReadByte();
        return b1 | (b2 << 8) | (b3 << 16) | (b4 << 24);
    }

    private static FieldInfo[] GetOpCodeFields()
    {
        return typeof(OpCodes).GetFields(BindingFlags.Public | BindingFlags.Static);
    }
}