如何在没有调用的情况下合并两个C＃Lambda表达式？ [英] How to merge two C# Lambda Expressions without an invoke?

问题描述

我想合并以下表达式：

// 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;

我使用selectOrderLine上的调用工作，并将结果提供给validateOrderLines，但因为我在Entity Framework中使用这些表达式，我必须实际创建一个干净的表达式，它应该代表：

I got it to work using a invoke on the selectOrderLines and supplying the result to the validateOrderLines, but because I'm using these expressions in Entity Framework, I have to actually create a clean expression which should represent:

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

我该怎么做？

推荐答案

最优雅的方式是使用表达式访问者。特别是，这个 MSDN博客条目描述了如何使用它来组合谓词（使用布尔和或），而不使用Invoke。

The most elegant way is to use an Expression Visitor. In particular, this MSDN Blog Entry describes how to use it to combine predicates (using boolean And or Or) without Invoke.

EDITED 实现了布尔组合并不是你想要的，我写了一个解决您的特殊问题的ExpressionVisitor的示例用法：

EDITED Having realized boolean combination is not what you wanted, I wrote a sample usage of ExpressionVisitor that solves for your particular problem:

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)}
    }
}

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