表达式树和AND条件

c# condition expression-trees filter

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动态构建表达式,具体取决于所选的过滤器参数:

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;
}

我正在以这种方式使用它:

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

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

它适用于NHibernate,但当我在Entity Framework 5中使用此代码时,它会失败,并显示以下异常消息:

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

有一种解决方法从数据库中提取整个集合,然后通过IEnumerable.Where(Func<T1, T2> filterClause)应用过滤条件,但是当Expression<Func<T1, T2>>它不需要所有数据来获取一条记录Expression<Func<T1, T2>>表达式直接翻译成SQL语句。

是否有任何简单的方法可以使此代码与Entity Framework一起使用?

一般承认的答案

面向域的N层.NET 4.0示例应用程序中尝试此实现(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);
    }

}

热门答案

请在下面的博客文章中找到解决方案:

一个通用的PredicateBuilder

这篇博客文章的作者提供了另一种实现“添加”方法来组合2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);
        }
    }
}


许可下: CC-BY-SA with attribution
不隶属于 Stack Overflow
这个KB合法吗? 是的,了解原因
许可下: CC-BY-SA with attribution
不隶属于 Stack Overflow
这个KB合法吗? 是的,了解原因