src/Library/NetPro.Dapper/Expressions/Evaluator.cs

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using System;
using System.Collections.Generic;
using System.Linq.Expressions;

namespace NetPro.Dapper.Expressions
{
    /// <summary>
    ///     Reference
    ///     From:http://blogs.msdn.com/b/mattwar/archive/2007/08/01/linq-building-an-iqueryable-provider-part-iii.aspx
    /// </summary>
    internal class Evaluator
    {
        public static Expression PartialEval(Expression exp, Func<Expression, bool> canBeEval)
        {
            return new SubtreeEvaluator(new Nominator(canBeEval).Nominate(exp)).Eval(exp);
        }

        public static Expression PartialEval(Expression exp)
        {
            return PartialEval(exp, CanBeEvaluatedLocally);
        }

        private static bool CanBeEvaluatedLocally(Expression exp)
        {
            return exp.NodeType != ExpressionType.Parameter;
        }

        private class SubtreeEvaluator : ExpressionVisitor
        {
            private readonly HashSet<Expression> _candidates;

            internal SubtreeEvaluator(HashSet<Expression> candidates)
            {
                _candidates = candidates;
            }

            internal Expression Eval(Expression exp)
            {
                return Visit(exp);
            }

            public override Expression Visit(Expression exp)
            {
                if (exp == null)
                {
                    return null;
                }

                if (_candidates.Contains(exp))
                {
                    return Evaluate(exp);
                }

                return base.Visit(exp);
            }

            private Expression Evaluate(Expression exp)
            {
                if (exp.NodeType == ExpressionType.Constant)
                {