C# 如何在没有调用的情况下合并两个C Lambda表达式?
我想合并以下表达式:C# 如何在没有调用的情况下合并两个C Lambda表达式?,c#,entity-framework,linq-to-entities,lambda,C#,Entity Framework,Linq To Entities,Lambda,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>, Boolea
// 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;
我在selectOrderLines上使用了一个调用并将结果提供给validateOrderLines,但由于我在Entity Framework中使用了这些表达式,因此我必须创建一个干净的表达式,它应该表示:
Expression<Func<Order, Boolean>> validateOrder = o => o.Lines.Count > 0;
我怎样才能做到这一点呢?最优雅的方法是使用。特别地,本文描述了如何使用它来组合使用boolean和/或不使用Invoke的谓词 在意识到布尔组合不是您想要的之后,我编写了ExpressionVisitor的示例用法,它解决了您的特定问题:
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)}
}
}
此扩展工作:
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);
}
}
样本使用:
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);
我需要做什么才能添加括号?我的意思是,如果我想构建一个像ProductId==1或ProductId==2和a.text.startswitha这样的过滤器,我在哪里可以找到parameterRebinder?如果出于某种原因,您仍然在寻找parameterRebinder,它可以在Ben处找到您仍然在做的任何更改?我需要做一些特别的合并你可能想帮我做这个?没有运气就想把你的改变成我的需要