7

我正在尝试构建一个过滤器表达式来过滤数据库中的数据。

我编写了以下扩展来根据所选过滤器参数动态构建表达式：

public static Expression<Func<T, bool>> And<T>(
        this Expression<Func<T, bool>> leftExpression, 
        Expression<Func<T, bool>> rightExpression)
{
    var invocationExpression = Expression.Invoke(rightExpression, leftExpression.Parameters.Cast<Expression>());
    var andExpression = Expression.Lambda<Func<T, bool>>(
        Expression.AndAlso(leftExpression.Body, invocationExpression), 
        leftExpression.Parameters);

    return andExpression;
}

我以这种方式使用它：

Expression<Func<MyObject, bool>> expression = x => true;

if(MyFilter.SomeParam) {
    expression = expression.And(x=>x.MyProperty == MyFilter.SomeParam);
}

它适用于 NHibernate，但是当我将此代码与 Entity Framework 5 一起使用时，它会失败并显示以下异常消息：

LINQ to Entities 不支持 LINQ 表达式节点类型“Invoke”。

IEnumerable.Where(Func<T1, T2> filterClause)有一种解决方法是从数据库中获取整个集合，然后通过Expression<Func<T1, T2>>.

有什么简单的方法可以让这段代码与实体框架一起工作吗？

4

3 回答 3

6

从Domain Oriented N-Layered .NET 4.0 Sample App尝试这个实现（还有规范模式的实现）：

public static class ExpressionBuilder
{
    public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
    {
        var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
        var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
        return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
    }

    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.And);
    }

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.Or);
    }

}

public class ParameterRebinder : ExpressionVisitor
{
    private readonly Dictionary<ParameterExpression, ParameterExpression> map;

    public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
    {
        this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
    }

    public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
    {
        return new ParameterRebinder(map).Visit(exp);
    }

    protected override Expression VisitParameter(ParameterExpression p)
    {
        ParameterExpression replacement;
        if (map.TryGetValue(p, out replacement))
        {
            p = replacement;
        }

        return base.VisitParameter(p);
    }

}
于 2013-03-08T15:10:57.750 回答
2

请在下面的博客文章中找到解决方案：

一个通用的 PredicateBuilder

这篇博文的作者提供了另一种“Add”方法的实现来结合2个表达式。

/// <summary>
/// Enables the efficient, dynamic composition of query predicates.
/// </summary>
public static class PredicateBuilder
{
    /// <summary>
    /// Creates a predicate that evaluates to true.
    /// </summary>
    public static Expression<Func<T, bool>> True<T>() { return param => true; }

    /// <summary>
    /// Creates a predicate that evaluates to false.
    /// </summary>
    public static Expression<Func<T, bool>> False<T>() { return param => false; }

    /// <summary>
    /// Creates a predicate expression from the specified lambda expression.
    /// </summary>
    public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate) { return predicate; }

    /// <summary>
    /// Combines the first predicate with the second using the logical "and".
    /// </summary>
    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.AndAlso);
    }

    /// <summary>
    /// Combines the first predicate with the second using the logical "or".
    /// </summary>
    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.OrElse);
    }

    /// <summary>
    /// Negates the predicate.
    /// </summary>
    public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression)
    {
        var negated = Expression.Not(expression.Body);
        return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters);
    }

    /// <summary>
    /// Combines the first expression with the second using the specified merge function.
    /// </summary>
    static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
    {
        // zip parameters (map from parameters of second to parameters of first)
        var map = first.Parameters
            .Select((f, i) => new { f, s = second.Parameters[i] })
            .ToDictionary(p => p.s, p => p.f);

        // replace parameters in the second lambda expression with the parameters in the first
        var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

        // create a merged lambda expression with parameters from the first expression
        return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
    }

    class ParameterRebinder : ExpressionVisitor
    {
        readonly Dictionary<ParameterExpression, ParameterExpression> map;

        ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
        {
            this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
        }

        public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
        {
            return new ParameterRebinder(map).Visit(exp);
        }

        protected override Expression VisitParameter(ParameterExpression p)
        {
            ParameterExpression replacement;

            if (map.TryGetValue(p, out replacement))
            {
                p = replacement;
            }

            return base.VisitParameter(p);
        }
    }
}
于 2013-03-08T14:58:33.583 回答
1

如果你总是想“和”你的谓词，你不需要弄乱表达式树，只需为每个谓词添加额外的 where 子句。请参阅http://www.thinqlinq.com/Default/Dynamically-extending-LINQ-queryies-without-building-expression-trees.aspx。

于 2013-03-08T14:57:18.840 回答