3rdpartylibs/fastreports.wpf/FastReport.BaseExt/Code/Expressions/CSharpILParser.cs

#if REFLECTION_EMIT_COMPILER
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.CSharp;
using System.Reflection.Emit;
using System.Reflection;
using System.Diagnostics;
using System.Linq.Expressions;
using System;
using System.Collections.Generic;
using System.Security.Cryptography;
using FastReport.Utils;
using System.Linq;
using System.Runtime.CompilerServices;
#if DEBUG
using ILGenerator = FastReport.Code.Expressions.ILGeneratorDebugger;
#endif

namespace FastReport.Code.Expressions
{
    /* TODO:
     * new [] {}
     * static Type searching
     * extension methods
     * 'dynamic' support
     */

    internal sealed class CSharpILParser : ILParser
    {
        ILGenerator gen;

        private readonly Report _report;
        private Type[] argTypes;
        private static readonly Type _returnType = typeof(object);


        public CSharpILParser(Report report)
        {
            _report = report;
        }

        protected override MethodInfo CompileExpression(string expression, Type[] argTypes)
        {
            ExpressionSyntax parse = SyntaxFactory.ParseExpression(expression);
            return StartDynamic(parse, argTypes);
        }


        private MethodInfo StartDynamic(ExpressionSyntax expressionSyntax, Type[] argTypes)
        {
            const string myMthdName = "CalcExpression";

            this.argTypes = argTypes;
            DynamicMethod dynamicMethod = new DynamicMethod(myMthdName,
                _returnType,
                argTypes,
                typeof(ILGenerator).Module,
                true
                //typeof(string).Module
                );

#if DEBUG
            gen = new ILGeneratorDebugger(dynamicMethod.GetILGenerator());
#else
            gen = dynamicMethod.GetILGenerator();
#endif

            StartParse(expressionSyntax);

            return dynamicMethod;
        }

        private void StartParse(ExpressionSyntax expressionSyntax)
        {
            Type returnType = ParseToIL(expressionSyntax);
            CastTo(returnType, _returnType);
            gen.Emit(OpCodes.Ret);
        }


        public Type ParseToIL(ExpressionSyntax expression, Type leftType = null)
        {
            if (expression is BinaryExpressionSyntax binary)
                return BinaryOp(binary);
            else if (expression is LiteralExpressionSyntax literal)
                return PutValueOnStack(literal.Token.Value);
            else if (expression is InvocationExpressionSyntax invocation)
                return ParseToIL(invocation.Expression, leftType);
            else if (expression is MemberAccessExpressionSyntax memberAccess)
                return MemberAccessOp(memberAccess);
            else if (expression is IdentifierNameSyntax identifierName)
                return SearchIdentifier(identifierName, leftType);
            else if (expression is PredefinedTypeSyntax predefinedType)
                return ParsePredefinedType(predefinedType.Keyword.Value.ToString());
            else if (expression is PostfixUnaryExpressionSyntax postfixUnary)
                return UnaryPostOp(postfixUnary);
            else if (expression is PrefixUnaryExpressionSyntax prefixUnary)
                return UnaryPreOp(prefixUnary);
            else if (expression is ConditionalExpressionSyntax conditional)         // a ? b : c
                return ConditionalOp(conditional);
            else if (expression is AssignmentExpressionSyntax assignment)           // =
                return AssignmentOp(assignment);
            else if (expression is ParenthesizedExpressionSyntax parenthesized)     // (a)
                return ParseToIL(parenthesized.Expression, leftType);
            else if (expression is CastExpressionSyntax cast)                       // ()a
                return ExplicitCastOp(cast);
            else if (expression is ConditionalAccessExpressionSyntax conditionalAccess)  // ?.
                return ConditionalAccessOp(conditionalAccess);
            else if (expression is MemberBindingExpressionSyntax memberBinding)
                return MemberAccessOp(memberBinding, leftType);
            else if (expression is ElementAccessExpressionSyntax elementAccess)     // a[0]
                return ElementAccessOp(elementAccess);
            else if (expression is ArrayCreationExpressionSyntax arrayCreation)         // new int[] { }
            {
                // TODO:
                var arrayType = ParseToIL(arrayCreation.Type);
            }
            else if (expression is ImplicitArrayCreationExpressionSyntax implicitArrayCreation) // new[] { }
            {

            }
            throw new NotImplementedException();
        }

        private Type SearchIdentifier(IdentifierNameSyntax identifierName, Type leftType)
        {
            var identifier = identifierName.Identifier.ValueText;

            // leftType can be null => it's static class etc

            if (identifierName.Parent is InvocationExpressionSyntax invocation)
            {
                // method:
                Type[] argumentTypes = ExecuteArguments(invocation.ArgumentList);

                FunctionInfo info = RegisteredObjects.Functions.Find(identifier);

                if (info == null)
                    throw new NotSupportedException($"Method '{identifier}' wasn't found");

                return MethodCall(leftType, info.Function);
            }
            else
            {
                // class, parameter, property, field etc

                if (identifier == "Report")  // hard
                {
                    return LoadReport();
                }
                else if (identifier == "Value")
                {
                    gen.Emit(OpCodes.Ldarg_1);

                    return argTypes[1];
                }
                else if (identifier == "Engine")
                {
                    var reportType = LoadReport();
                    return MemberAccessOp(identifier, identifierName, reportType);
                }

                var type = ParseKnownType(identifier);

                if (type != null)
                    return type;

                // report objects
                Base obj = _report.FindObject(identifier);
                if (obj != null)
                {
                    // IT'S DYNAMIC STATE! We should't store it!

                    var reportType = LoadReport();
                    var argType = PutValueOnStack(identifier);

                    var findObjectMethod = reportType.GetMethod(nameof(_report.FindObject), new[] {argType});

                    var findObjectReturnType = MethodCall(reportType, findObjectMethod);

                    // cast to necessary type:
                    var reportObjectType = obj.GetType();
                    CastTo(findObjectReturnType, reportObjectType);

                    return reportObjectType;
                }

                if (type == null)
                    type = Type.GetType(identifier, false, false);

                if (type != null)
                    return type;
            }

            throw new NotSupportedException();
        }

        private Type LoadReport()
        {
            gen.Emit(OpCodes.Ldarg_0);

            return argTypes[0];
        }

        private static Type ParsePredefinedType(string typeName)
        {
            switch (typeName)
            {
                case "object": return typeof(object);
                case "string": return typeof(string);
                case "bool": return typeof(bool);
                case "byte": return typeof(byte);
                case "sbyte": return typeof(sbyte);
                case "char": return typeof(char);
                case "short": return typeof(short);
                case "ushort": return typeof(ushort);
                case "int": return typeof(int);
                case "uint": return typeof(uint);
                case "long": return typeof(long);
                case "ulong": return typeof(ulong);
                case "float": return typeof(float);
                case "double": return typeof(double);
                case "decimal": return typeof(decimal);
            }
            throw new NotSupportedException();
        }

        private static Type ParseKnownType(string typeName)
        {
            switch (typeName)
            {
                case "Object": return typeof(object);
                case "String": return typeof(string);
                case "Boolean": return typeof(Boolean);
                case "Byte": return typeof(Byte);
                case "SByte": return typeof(SByte);
                case "Char": return typeof(Char);
                case "Int16": return typeof(Int16);
                case "UInt16": return typeof(UInt16);
                case "Int32": return typeof(Int32);
                case "UInt32": return typeof(UInt32);
                case "Int64": return typeof(Int64);
                case "UInt64": return typeof(UInt64);
                case "Single": return typeof(Single);
                case "Double": return typeof(Double);
                case "Decimal": return typeof(Decimal);
                case "DateTime": return typeof(DateTime);
                case "DateTimeOffset": return typeof(DateTimeOffset);
            }
            return null;
        }


        private Type MemberAccessOp(MemberAccessExpressionSyntax memberAccess)
        {
            Type type = ParseToIL(memberAccess.Expression);

            var name = memberAccess.Name.Identifier.ValueText;
            return MemberAccessOp(name, memberAccess, type);
        }

        private Type MemberAccessOp(MemberBindingExpressionSyntax memberAccess, Type leftType)
        {
            var name = memberAccess.Name.Identifier.ValueText;
            return MemberAccessOp(name, memberAccess, leftType);
        }

        private Type MemberAccessOp(string name, ExpressionSyntax expression, Type leftType)
        {
            if (expression.Parent is InvocationExpressionSyntax invocation)
            {
                // method
                Type[] argumentTypes = ExecuteArguments(invocation.ArgumentList);

                var methodInfo = CustomMethodSearcher.Search(leftType, name, argumentTypes);

                return MethodCall(leftType, methodInfo);
            }
            else
            {
                // property or field
                var property = leftType.GetProperty(name);

                // if it's not a property => try to find a field
                if (property == null)
                {
                    var field = leftType.GetField(name);

                    gen.Emit(OpCodes.Ldfld, field);
                    return field.FieldType;
                }

                // only get
                var methodInfo = property.GetGetMethod();
                return MethodCall(leftType, methodInfo);
            }
        }

        private Type MethodCall(Type targetType, MethodInfo methodInfo)
        {
            if (methodInfo.IsStatic)
            {
                gen.EmitCall(OpCodes.Call, methodInfo, null);
            }
            else
            {
                if (targetType.IsValueType)
                {
                    // if local:
                    var local = gen.DeclareLocal(targetType);

                    gen.Emit(OpCodes.Stloc, local);
                    gen.Emit(OpCodes.Ldloca_S, local);
                    gen.EmitCall(OpCodes.Call, methodInfo, null);
                }
                else
                    gen.EmitCall(OpCodes.Callvirt, methodInfo, null);
            }
            return methodInfo.ReturnType;
        }

        private Type AssignmentOp(AssignmentExpressionSyntax assignment)
        {
            var type = ParseToIL(assignment.Right);

            ParseToIL(assignment.Left);
            if (false)   // if field
            {
                var local = gen.DeclareLocal(type);

                gen.Emit(OpCodes.Starg_S, local);
            }

            // TODO: assignment
            throw new NotImplementedException();

            //return type;
            return typeof(void);
        }

        private Type[] ExecuteArguments(BaseArgumentListSyntax argumentList)
        {
            var arguments = argumentList.Arguments;

            List<Type> types = new List<Type>();
            foreach (var argument in arguments)
            {
                var resultTypeArg = ParseToIL(argument.Expression);
                types.Add(resultTypeArg);
            }
            return types.ToArray();
        }

        private Type ConditionalOp(ConditionalExpressionSyntax conditional)
        {
            Label ifTrue = gen.DefineLabel();
            Label next = gen.DefineLabel();
            var conditionType = ParseToIL(conditional.Condition);
            //if (conditionType != typeof(bool))
            //    throw new Exception();
            gen.Emit(OpCodes.Brtrue_S, ifTrue);

            var type1 = ParseToIL(conditional.WhenFalse);
            gen.Emit(OpCodes.Br_S, next);

            gen.MarkLabel(ifTrue);
            var type2 = ParseToIL(conditional.WhenTrue);

            gen.MarkLabel(next);

            //if (type1 != type2)
            //    throw new Exception();
            return type1;
        }

        private Type ConditionalAccessOp(ConditionalAccessExpressionSyntax conditionalAccess)
        {
            Label ifTrue = gen.DefineLabel();
            Label next = gen.DefineLabel();

            var type = ParseToIL(conditionalAccess.Expression);
            gen.Emit(OpCodes.Dup);
            gen.Emit(OpCodes.Brtrue_S, ifTrue);

            gen.Emit(OpCodes.Pop);
            gen.Emit(OpCodes.Ldnull);
            gen.Emit(OpCodes.Br_S, next);

            gen.MarkLabel(ifTrue);
            var resultType = ParseToIL(conditionalAccess.WhenNotNull, type);

            gen.MarkLabel(next);
            return resultType;
        }

        private Type UnaryPreOp(PrefixUnaryExpressionSyntax prefixUnary)
        {
            var kind = prefixUnary.Kind();
            var op = prefixUnary.Operand;

            Type returnType = ParseToIL(op);
            if (returnType == typeof(decimal))
                return BinaryCustomTypeOp(returnType, kind);

            switch (kind)
            {
                case SyntaxKind.BitwiseNotExpression:       // ~
                    gen.Emit(OpCodes.Not);
                    break;

                case SyntaxKind.UnaryMinusExpression:       // -
                    gen.Emit(OpCodes.Neg);
                    break;

                case SyntaxKind.LogicalNotExpression:       // !
                    returnType = CompareWithZero();
                    break;

                case SyntaxKind.PreIncrementExpression:     // ++a
                    gen.Emit(OpCodes.Ldc_I4_1);
                    gen.Emit(OpCodes.Add);
                    break;

                case SyntaxKind.PreDecrementExpression:     // --a
                    gen.Emit(OpCodes.Ldc_I4_1);
                    gen.Emit(OpCodes.Sub);
                    break;
            }
            return returnType;
        }

        private Type UnaryPostOp(PostfixUnaryExpressionSyntax postfixUnary)
        {
            var op = postfixUnary.Operand;
            var kind = postfixUnary.Kind();

            Type returnType = ParseToIL(op);
            if (returnType == typeof(decimal))
                return BinaryCustomTypeOp(returnType, kind);

            switch (kind)
            {
                case SyntaxKind.PostIncrementExpression:    // a++
                    gen.Emit(OpCodes.Ldc_I4_1);
                    gen.Emit(OpCodes.Add);
                    break;

                case SyntaxKind.PostDecrementExpression:    // a--
                    gen.Emit(OpCodes.Ldc_I4_1);
                    gen.Emit(OpCodes.Sub);
                    break;
            }
            return returnType;
        }


        private Type PutValueOnStack(object value)
        {
            if (value == null)
            {
                gen.Emit(OpCodes.Ldnull);
                return typeof(object);
            }

            var type = value.GetType();
            if (type == typeof(int))
            {
                gen.Emit(OpCodes.Ldc_I4, Convert.ToInt32(value));
            }
            else if (type == typeof(bool))
            {
                if (Convert.ToBoolean(value))
                    gen.Emit(OpCodes.Ldc_I4_1);
                else
                    gen.Emit(OpCodes.Ldc_I4_0);
            }
            else if (type == typeof(float))
            {
                gen.Emit(OpCodes.Ldc_R4, Convert.ToSingle(value));
            }
            else if (type == typeof(double))
            {
                gen.Emit(OpCodes.Ldc_R8, Convert.ToDouble(value));
            }
            //else if (type == typeof(char)) // => int32
            //{}
            else if (type == typeof(string))
            {
                gen.Emit(OpCodes.Ldstr, Convert.ToString(value));
            }
            else if (type == typeof(long))
            {
                gen.Emit(OpCodes.Ldc_I8, Convert.ToInt64(value));
            }
            else
            {
                gen.Emit(OpCodes.Ldc_I4, Convert.ToInt32(value));
            }
            return type;
        }

        private Type ElementAccessOp(ElementAccessExpressionSyntax elementAccess)
        {
            var type = ParseToIL(elementAccess.Expression);
            var args = ExecuteArguments(elementAccess.ArgumentList);

            if(type.IsArray)
            {
                var elementType = type.GetElementType();
                if (elementType.IsClass || elementType.IsInterface)
                    gen.Emit(OpCodes.Ldelem_Ref);
                else
                    gen.Emit(OpCodes.Ldelem, elementType);

                return elementType;
            }
            else
            {
                string propertyName = "Item";
                if (type == typeof(string))
                    propertyName = "Chars";

                var property = type.GetProperty(propertyName, args);
                var method = property.GetGetMethod();
                return MethodCall(type, method);
            }
        }

        private Type BinaryOp(BinaryExpressionSyntax binary)
        {
            var kind = binary.Kind();
            switch(kind)
            {
                case SyntaxKind.LogicalOrExpression:
                case SyntaxKind.LogicalAndExpression:
                    return LogicalOp(binary, kind);

                case SyntaxKind.IsExpression:
                case SyntaxKind.AsExpression:
                    return IsAsOp(binary, kind);

                default:
                    return BinaryOpWithCast(binary, kind);
            }
        }

        private Type BinaryOpWithCast(BinaryExpressionSyntax binary, SyntaxKind kind)
        {
            Type leftType = ParseToIL(binary.Left);
            Type rightType = ParseToIL(binary.Right);

            if (leftType == typeof(Variant) || rightType == typeof(Variant))
                return VariantOp(kind, leftType, rightType);

            Type returnType = CastToGreaterType(leftType, rightType);

            if (returnType == typeof(decimal) || returnType == typeof(string))
                return BinaryCustomTypeOp(returnType, kind);


            switch (kind)
            {
                case SyntaxKind.AddExpression:          // +
                    gen.Emit(OpCodes.Add);
                    break;

                case SyntaxKind.SubtractExpression:     // -
                    gen.Emit(OpCodes.Sub);
                    break;

                case SyntaxKind.MultiplyExpression:     // *
                    gen.Emit(OpCodes.Mul);
                    break;

                case SyntaxKind.DivideExpression:       // /
                    gen.Emit(OpCodes.Div);
                    break;

                case SyntaxKind.ModuloExpression:       // %
                    gen.Emit(OpCodes.Rem);
                    break;

                case SyntaxKind.EqualsExpression:       // ==
                    gen.Emit(OpCodes.Ceq);
                    returnType = typeof(bool);
                    break;

                case SyntaxKind.NotEqualsExpression:    // !=
                    // == and NOT
                    gen.Emit(OpCodes.Ceq);
                    returnType = CompareWithZero();
                    break;

                case SyntaxKind.LessThanExpression:     // <
                    gen.Emit(OpCodes.Clt);
                    returnType = typeof(bool);
                    break;

                case SyntaxKind.LessThanOrEqualExpression:  // <=
                    // > and NOT
                    gen.Emit(OpCodes.Cgt);
                    returnType = CompareWithZero();
                    break;

                case SyntaxKind.GreaterThanExpression:      // >
                    gen.Emit(OpCodes.Cgt);
                    returnType = typeof(bool);
                    break;

                case SyntaxKind.GreaterThanOrEqualExpression:   // >=
                    // < and NOT
                    gen.Emit(OpCodes.Clt);
                    returnType = CompareWithZero();
                    break;

                case SyntaxKind.BitwiseOrExpression:        // |
                    gen.Emit(OpCodes.Or);
                    break;

                case SyntaxKind.ExclusiveOrExpression:      // ^
                    gen.Emit(OpCodes.Xor);
                    break;

                case SyntaxKind.BitwiseAndExpression:       // &
                    gen.Emit(OpCodes.And);
                    break;
            }

            return returnType;
        }

        private Type BinaryCustomTypeOp(Type type, SyntaxKind kind)
        {
            string methodName = GetOperatorName(kind);
            MethodInfo method;
            if (type == typeof(string) && kind == SyntaxKind.AddExpression)
                method = type.GetMethod(nameof(string.Concat), new Type[] { type, type });
            else
                method = type.GetMethod(methodName);

            return MethodCall(type, method);
        }


        private Type VariantOp(SyntaxKind kind, Type left, Type right)
        {
            Type variantType = typeof(Variant);

            string methodName = GetOperatorName(kind);
            MethodInfo method = variantType.GetMethod(methodName);

            // because 1 argument in Variant must be 'Variant'
            if (left != variantType)
                PreviousCastTo(left, variantType);


            // because 2 argument in Variant operator is object
            var parameters = method.GetParameters();
            if (parameters.Length == 2)
            {
                CastTo(right, parameters[1].ParameterType);
            }

            return MethodCall(variantType, method);
        }


        private static string GetOperatorName(SyntaxKind kind)
        {
            string methodName;
            switch (kind)
            {
                case SyntaxKind.AddExpression:          // +
                    methodName = "op_Addition";
                    break;

                case SyntaxKind.SubtractExpression:     // -
                    methodName = "op_Subtraction";
                    break;

                case SyntaxKind.MultiplyExpression:     // *
                    methodName = "op_Multiply";
                    break;

                case SyntaxKind.DivideExpression:       // /
                    methodName = "op_Division";
                    break;

                case SyntaxKind.ModuloExpression:       // %
                    methodName = "op_Modulus";
                    break;

                case SyntaxKind.EqualsExpression:       // ==
                    methodName = "op_Equality";
                    break;

                case SyntaxKind.NotEqualsExpression:    // !=
                    methodName = "op_Inequality";
                    break;

                case SyntaxKind.LessThanExpression:     // <
                    methodName = "op_LessThan";
                    break;

                case SyntaxKind.LessThanOrEqualExpression:  // <=
                    methodName = "op_LessThanOrEqual";
                    break;

                case SyntaxKind.GreaterThanExpression:      // >
                    methodName = "op_GreaterThan";
                    break;

                case SyntaxKind.GreaterThanOrEqualExpression:   // >=
                    methodName = "op_GreaterThanOrEqual";
                    break;

                case SyntaxKind.UnaryMinusExpression:       // unary -
                    methodName = "op_UnaryNegation";
                    break;

                case SyntaxKind.PreIncrementExpression:     // ++a
                case SyntaxKind.PostIncrementExpression:    // a++
                    methodName = "op_Increment";
                    break;

                case SyntaxKind.PreDecrementExpression:     // --a
                case SyntaxKind.PostDecrementExpression:    // a--
                    methodName = "op_Decrement";
                    break;

                default:
                    throw new NotSupportedException("This operation isn't supported with this type");
            }
            return methodName;
        }

        private Type IsAsOp(BinaryExpressionSyntax binary, SyntaxKind kind)
        {
            var type1 = ParseToIL(binary.Left);
            var type2 = ParseToIL(binary.Right);

            Type returnType = type1;
            switch (kind)
            {
                case SyntaxKind.IsExpression:               // is
                    gen.Emit(OpCodes.Isinst, type2);
                    gen.Emit(OpCodes.Ldnull);
                    gen.Emit(OpCodes.Cgt_Un);
                    returnType = typeof(bool);
                    break;

                case SyntaxKind.AsExpression:               // as
                    // 'as' cannot be applied to non-reference types
                    if (type2.IsValueType)
                        throw new NotSupportedException();
                    gen.Emit(OpCodes.Isinst, type2);
                    returnType = type2;
                    break;
            }

            return returnType;
        }

        private Type CastToGreaterType(Type leftType, Type rightType)
        {
            if (leftType == rightType)
                return leftType;

            Type result;
            result = CompareAndCastTo(leftType, rightType, typeof(Variant));
            if (result != null)
                return result;

            result = CompareAndCastTo(leftType, rightType, typeof(string)); // string is the strongest type
            if (result != null)
                return result;

            result = CompareAndCastTo(leftType, rightType, typeof(decimal));
            if (result != null)
                return result;

            result = CompareAndCastTo(leftType, rightType, typeof(double));
            if (result != null)
                return result;

            result = CompareAndCastTo(leftType, rightType, typeof(float));
            if (result != null)
                return result;

            result = CompareAndCastTo(leftType, rightType, typeof(ulong));
            if (result != null)
                return result;

            result = CompareAndCastTo(leftType, rightType, typeof(long));
            if (result != null)
                return result;


            throw new NotImplementedException();
        }

        private Type CompareAndCastTo(Type leftType, Type rightType, Type comparableType)
        {
            if (rightType == comparableType
                || leftType == comparableType)
            {
                if (leftType == comparableType)
                    return CastTo(rightType, leftType);
                else
                    return PreviousCastTo(leftType, rightType);
            }
            return null;
        }


        private Type PreviousCastTo(Type original, Type toType)
        {
            var local = gen.DeclareLocal(toType);
            gen.Emit(OpCodes.Stloc, local);

            // cast leftType:
            toType = CastTo(original, toType);

            gen.Emit(OpCodes.Ldloc, local);
            return toType;
        }

        private Type LogicalOp(BinaryExpressionSyntax binary, SyntaxKind kind)
        {
            var type1 = ParseToIL(binary.Left);
            Label answer = gen.DefineLabel();
            OpCode answerOpcode;
            Label next = gen.DefineLabel();

            switch (kind)
            {
                case SyntaxKind.LogicalOrExpression:    // ||
                    gen.Emit(OpCodes.Brtrue_S, answer);

                    answerOpcode = OpCodes.Ldc_I4_1;
                    break;

                case SyntaxKind.LogicalAndExpression:   // &&
                    gen.Emit(OpCodes.Brfalse_S, answer);

                    answerOpcode = OpCodes.Ldc_I4_0;
                    break;

                default: throw new Exception();
            }

            Type type2 = ParseToIL(binary.Right);
            gen.Emit(OpCodes.Br_S, next);

            gen.MarkLabel(answer);
            gen.Emit(answerOpcode);

            gen.MarkLabel(next);

            return typeof(bool);
        }

        private Type ExplicitCastOp(CastExpressionSyntax cast)
        {
            var originType = ParseToIL(cast.Expression);
            var type = ParseToIL(cast.Type);
            return CastTo(originType, type, true);
        }

        private Type CastTo(Type original, Type toType, bool _explicit = false)
        {
            if (original.IsClass)
            {
                if(toType.IsValueType)
                {
                    if (original != typeof(object)) // Unbox only for object
                        throw new NotSupportedException($"You cannot cast type \"{original}\" to type \"{toType}\"");

                    gen.Emit(OpCodes.Unbox_Any, toType);
                }
                else if (toType == typeof(object))
                    return toType;      // do nothing
                else
                    gen.Emit(OpCodes.Castclass, toType);
            }
            else if (original.IsValueType)
            {
                if (toType == typeof(object))
                    gen.Emit(OpCodes.Box, original);
                // decimal has special operators for parsing into different types (exclude object)
                else if (original == typeof(decimal))
                {
                    var methods = original.GetMethods(BindingFlags.Static | BindingFlags.Public);
                    foreach (var method in methods)
                        if (method.ReturnType == toType && method.IsSpecialName)
                            return MethodCall(original, method);
                }
                else if (toType == typeof(decimal))
                {
                    var method = toType.GetMethod("op_Implicit", new Type[] { original });
                    if (method != null)
                        return MethodCall(toType, method);

                    // float and double require explicit casting
                    if (_explicit)
                    {
                        method = toType.GetMethod("op_Explicit", new Type[] { original });
                        if (method != null)
                            return MethodCall(toType, method);
                    }

                    throw new NotSupportedException($"You cannot cast type \"{original}\" to type \"{toType}\"");
                }
                else if (toType == typeof(double))
                    gen.Emit(OpCodes.Conv_R8);
                else if (toType == typeof(float))
                    gen.Emit(OpCodes.Conv_R4);
                else if (toType == typeof(ulong))
                    gen.Emit(OpCodes.Conv_U8);
                else if (toType == typeof(long))
                    gen.Emit(OpCodes.Conv_I8);
                else if (toType.IsPrimitive)
                {
                    // else int, uint etc
                    // do nothing
                    // TODO:
                    // int + uint => long
                }
                else
                {
                    var method = toType.GetMethod("op_Implicit", new Type[] { original });
                    if (method != null)
                        return MethodCall(toType, method);

                    if (_explicit)
                    {
                        method = toType.GetMethod("op_Explicit", new Type[] { original });
                        if (method != null)
                            return MethodCall(toType, method);
                    }

                    if (toType.IsClass || toType.IsInterface)
                        throw new NotSupportedException($"You cannot cast type \"{original}\" to type \"{toType}\"");
                }
            }
            else if (original.IsInterface)
            {
                if (toType == typeof(object))
                    return toType;      // do nothing

                if (toType.IsValueType)
                {
                    if (original.IsAssignableFrom(toType))
                        throw new NotSupportedException($"You cannot cast type \"{original}\" to type \"{toType}\"");

                    gen.Emit(OpCodes.Unbox_Any, toType);
                }
                else
                    gen.Emit(OpCodes.Castclass, toType);
            }
            else
                gen.Emit(OpCodes.Castclass, toType);

            return toType;
        }

        private Type CompareWithZero()
        {
            gen.Emit(OpCodes.Ldc_I4_0);
            gen.Emit(OpCodes.Ceq);
            return typeof(bool);
        }
    }
}
#endif