4

我有一个网格视图，我们可以在其中按不同的标准进行过滤。每个条件都是一个表达式。我有一个场景，我可以有超过一千个条件，这些条件会导致我的表达式在调用 Compile 方法时抛出 StackOverflow。

顺便说一句，我仍然是使用表达式树的初学者。

这是我为重现 stackoverflow 所做的示例。

var param = Expression.Parameter(typeof(SomeEntity), "SomeEntity");

        Expression finalExpression = Expression.Default(typeof(bool));

        for (int i = 0; i < 20000; i++) // Create 20000 expressions
        {
            var left = Expression.Property(param, "OrderID");
            var right = Expression.Constant(42.ToString());

            var expression = BinaryExpression.Equal(left, right);

            finalExpression = Expression.OrElse(finalExpression, expression);
        }

        var hello = Expression.Lambda(finalExpression, param);
        hello.Compile();

我的问题是：有没有办法“减少”这个表达式或任何其他防止 stackoverflow 的解决方案？

谢谢

注意：这是表达式在调试器中的样子：

(SomeEntity.OrderID == "42")) 
OrElse (SomeEntity.OrderID == "42")) 
OrElse (SomeEntity.OrderID == "42")) 
OrElse (SomeEntity.OrderID == "42")) 
OrElse (SomeEntity.OrderID == "42")) 
x20000
4

1 回答 1

5

我刚刚成功地测试了这段代码多达 1,000,000 个条件而没有堆栈溢出 - 我怀疑它可以处理你想要的尽可能多的条件。

当Compile在 lambda 表达式上调用时，表达式树会递归地向下遍历以编译它；极深的树（像这样）需要大量的堆栈帧来完成 - 因此StackOverflowException.

我在下面所做的是MaxPredicateConditionCount在编译表达式并将其推送到已经生成的条件集合之前只接受固定数量的条件（由 设置）。如果该预生成表达式的集合达到该最大值，则将它们组合成一个新的表达式，依此类推。通过这种方式，我们可以限制编译表达式所需的递归深度（通过分段执行）。

public class PredicateBuilder<TParameter>
{
    private const int MaxPredicateConditionCount = 500;
    private readonly List<Expression<Func<TParameter, bool>>> _existingPredicates = new List<Expression<Func<TParameter, bool>>>(MaxPredicateConditionCount);
    private readonly ParameterExpression _parameter = Expression.Parameter(typeof(TParameter));
    private Expression<Func<TParameter, bool>> _expression;
    private Expression _workingPredicate;
    private int _workingPredicateConditionCount;
    public bool Built { get; private set; }

    public Expression<Func<TParameter, bool>> LambdaExpression
    {
        get
        {
            if (!Built)
            {
                return null;
            }

            return _expression;
        }
    }

    public void AddCondition<TValue>(string propertyName, TValue value)
    {
        if (Built)
        {
            throw new InvalidOperationException("Predicate has already been built");
        }

        var property = Expression.Property(_parameter, propertyName);
        var constant = Expression.Constant(value, typeof(TValue));
        var equality = Expression.Equal(property, constant);

        if (_workingPredicate == null)
        {
            _workingPredicate = equality;
        }
        else
        {
            if (MaxPredicateConditionCount < ++_workingPredicateConditionCount)
            {
                var compiledWorking = Expression.Lambda<Func<TParameter, bool>>(_workingPredicate, _parameter).Compile();
                _existingPredicates.Add(p => compiledWorking(p));

                if (_existingPredicates.Count + 1 > MaxPredicateConditionCount)
                {
                    var compiled = BuildExistingPredicates().Compile();
                    _existingPredicates.Clear();
                    _existingPredicates.Add(p => compiled(p));
                }

                _workingPredicate = equality;
                _workingPredicateConditionCount = 0;
            }
            else
            {
                _workingPredicate = Expression.OrElse(_workingPredicate, equality);
            }
        }
    }

    private Expression<Func<TParameter, bool>> BuildExistingPredicates()
    {
        Expression compileTemp = Expression.Invoke(_existingPredicates[0], _parameter);

        for (var i = 1; i < _existingPredicates.Count; ++i)
        {
            var nextCall = Expression.Invoke(_existingPredicates[i], _parameter);
            compileTemp = Expression.OrElse(compileTemp, nextCall);
        }

        return Expression.Lambda<Func<TParameter, bool>>(compileTemp, _parameter);
    }

    public void Build()
    {
        Built = true;
        //There were no conditions, assume true
        if (_workingPredicate == null)
        {
            _expression = x => true;
            return;
        }

        _existingPredicates.Add(Expression.Lambda<Func<TParameter, bool>>(_workingPredicate, _parameter));

        _expression = BuildExistingPredicates();

        _existingPredicates.Clear();
        _workingPredicate = null;
        _workingPredicateConditionCount = 0;
    }

    public Func<TParameter, bool> Compile()
    {
        if (!Built)
        {
            Build();
        }

        return _expression.Compile();
    }
}

示例实体

public class SomeEntity
{
    public string OrderID { get; set; }
}

用法

class Program
{
    static void Main()
    {
        var builder = new PredicateBuilder<SomeEntity>();

        for (int i = 0; i < 1000000; i++) // Create 1,000,000 expressions
        {
            builder.AddCondition("OrderID", "42");
            Console.Title = i.ToString();
        }

        builder.Compile();
    }
}
于 2013-01-03T19:07:06.130 回答