如何合并没有调用两个C＃Lambda表达式？(How to merge two C# Lambda

我想合并下面的表达式：

// 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;

我把它用在一个selectOrderLines调用，并将结果提供给validateOrderLines工作，但因为我使用实体框架中这些表情，我必须真正创造一个干净的表达应该代表：

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

我怎样才能做到这一点？

Answer 1:

最优雅的方式是使用表达式游客 。 特别是，该MSDN博客条目描述了如何使用它，而不调用到谓词（使用布尔And或Or）相结合。

EDITED已经认识布尔组合是不是你想要的，我写ExpressionVisitor的样品使用，解决您的特定问题：

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)}
    }
}

Answer 2:

该扩建工程：

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);
        }
    }

使用示例：

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);

文章来源: How to merge two C# Lambda Expressions without an invoke?