如何在沒有調用的情況下合併兩個C#Lambda表達式?

c# entity-framework lambda linq-to-entities

我想合併以下表達式:

// example class
class Order
{
    List<OrderLine> Lines       
}
class OrderLine { }

Expression<Func<Order, List<OrderLine>>> selectOrderLines = o => o.Lines;
Expression<Func<List<OrderLine>, Boolean>> validateOrderLines = lines => lines.Count > 0;

// now combine those to
Expression<Func<Order, Boolean>> validateOrder;

我使用selectOrderLines上的調用並將結果提供給validateOrderLines,但是因為我在Entity Framework中使用這些表達式,我必須實際創建一個乾淨的表達式,它應該代表:

Expression<Func<Order, Boolean>> validateOrder = o => o.Lines.Count > 0;

我怎樣才能做到這一點?

一般承認的答案

最優雅的方式是使用表達式訪問者 。特別是,這個MSDN博客條目描述瞭如何使用它來組合謂詞(使用布爾和或或)而不需要調用。

編輯實現布爾組合不是你想要的,我寫了一個ExpressionVisitor的示例用法,它解決了你的特定問題:

public class ParameterToMemberExpressionRebinder : ExpressionVisitor
{
    ParameterExpression _paramExpr;
    MemberExpression _memberExpr;

    ParameterToMemberExpressionRebinder(ParameterExpression paramExpr, MemberExpression memberExpr) 
    {
        _paramExpr = paramExpr;
        _memberExpr = memberExpr;
    }

    protected override Expression Visit(Expression p)
    {
        return base.Visit(p == _paramExpr ? _memberExpr : p);
    }

    public static Expression<Func<T, bool>> CombinePropertySelectorWithPredicate<T, T2>(
        Expression<Func<T, T2>> propertySelector,
        Expression<Func<T2, bool>> propertyPredicate)
    {
        var memberExpression = propertySelector.Body as MemberExpression;

        if (memberExpression == null)
        {
            throw new ArgumentException("propertySelector");
        }

        var expr = Expression.Lambda<Func<T, bool>>(propertyPredicate.Body, propertySelector.Parameters);
        var rebinder = new ParameterToMemberExpressionRebinder(propertyPredicate.Parameters[0], memberExpression);
        expr = (Expression<Func<T, bool>>)rebinder.Visit(expr);

        return expr;
    }

    class OrderLine
    {
    }

    class Order
    {
        public List<OrderLine> Lines;
    }

    static void test()
    {
        Expression<Func<Order, List<OrderLine>>> selectOrderLines = o => o.Lines;
        Expression<Func<List<OrderLine>, Boolean>> validateOrderLines = lines => lines.Count > 0;
        var validateOrder = ParameterToMemberExpressionRebinder.CombinePropertySelectorWithPredicate(selectOrderLines, validateOrderLines);

        // validateOrder: {o => (o.Lines.Count > 0)}
    }
}

熱門答案

此擴展程序有效:

public static class Utility
    {
        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // build parameter 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 parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression 
            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);
        }
    }

樣品使用:

Expression<Func<Product, bool>> filter1 = p => a.ProductId == 1;
Expression<Func<Product, bool>> filter2 = p => a.Text.StartWith("test");
Expression<Func<Product, bool>> filterCombined = filter1.And(filter2);


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不隸屬於 Stack Overflow
這個KB合法嗎? 是的,了解原因
許可下: CC-BY-SA with attribution
不隸屬於 Stack Overflow
這個KB合法嗎? 是的,了解原因