Connected.Framework/Connected.Expressions/Evaluation/SubtreeEvaluator.cs

using Connected.Expressions.Reflection;
using Connected.Interop;
using System.Linq.Expressions;
using System.Reflection;
using ExpressionVisitor = Connected.Expressions.Visitors.ExpressionVisitor;

namespace Connected.Expressions.Evaluation;

internal sealed class SubtreeEvaluator : ExpressionVisitor
{
    private SubtreeEvaluator(ExpressionCompilationContext context, HashSet<Expression> candidates, Func<ConstantExpression, Expression>? onEval)
    {
        Candidates = candidates;
        OnEval = onEval;
        Context = context;
    }

    public ExpressionCompilationContext Context { get; }
    private HashSet<Expression> Candidates { get; set; }
    private Func<ConstantExpression, Expression>? OnEval { get; set; }

    internal static Expression Eval(ExpressionCompilationContext context, HashSet<Expression> candidates, Func<ConstantExpression, Expression>? onEval, Expression exp)
    {
        if (new SubtreeEvaluator(context, candidates, onEval).Visit(exp) is not Expression subtreeExpression)
            throw new NullReferenceException(nameof(subtreeExpression));

        return subtreeExpression;
    }

    protected override Expression? Visit(Expression? exp)
    {
        if (exp is null)
            return null;

        if (Candidates.Contains(exp))
            return Evaluate(exp);

        return base.Visit(exp);
    }

    protected override Expression VisitConditional(ConditionalExpression c)
    {
        if (Candidates.Contains(c.Test))
        {
            var test = Evaluate(c.Test);

            if (test is ConstantExpression && ((ConstantExpression)test).Type == typeof(bool))
            {
                if ((bool)((ConstantExpression)test).Value)
                    return Visit(c.IfTrue);
                else
                    return Visit(c.IfFalse);
            }
        }

        return base.VisitConditional(c);
    }

    private Expression PostEval(ConstantExpression e)
    {
        if (OnEval is not null)
            return OnEval(e);

        return e;
    }

    private Expression Evaluate(Expression e)
    {
        var type = e.Type;

        if (e.NodeType == ExpressionType.Convert)
        {
            var u = (UnaryExpression)e;

            if (Nullables.GetNonNullableType(u.Operand.Type) == Nullables.GetNonNullableType(type))
                e = ((UnaryExpression)e).Operand;
        }

        if (e.NodeType == ExpressionType.Constant)
        {
            if (e.Type == type)
                return e;
            else if (Nullables.GetNonNullableType(e.Type) == Nullables.GetNonNullableType(type))
                return Expression.Constant(((ConstantExpression)e).Value, type);
        }

        if (e is MemberExpression me)
        {
            if (me.Expression is ConstantExpression ce)
            {
                var value = me.Member.GetValue(ce.Value);
                var constant = Expression.Constant(value, type);

                Context.Parameters.Add(me.Member.Name, constant);

                return PostEval(constant);
            }
        }

        if (type.GetTypeInfo().IsValueType)
            e = Expression.Convert(e, typeof(object));

        var lambda = Expression.Lambda<Func<object>>(e);
        var fn = lambda.Compile();

        return PostEval(Expression.Constant(fn(), type));
    }
}