Question

我在.NET中使用Expressions的经验很少，这就是为什么我宁愿问你们。我该怎么做 - 看下面的评论：

using P = Myclass;
..
System.Linq.Expressions.Expression<Func<P, bool>> myExpression = null;
..
myExpression1 = x => foo1 == true && foo2 == false;
myExpression2 = x => ... ;
..
BinaryExpression resultExpression = System.Linq.Expressions.Expression.OrElse(myExpression1, myExpression2);
..
IQueryable<P> l = l.Where(?resultExpression?); // how to transform BinaryExpression to the suitable type?

谢谢

Answer 1

你不能用这种方式“或”羔羊。你真的想把lambda体“和”在一起。这是一种方法：

public static Expression<Func<T, bool>> OrTheseFiltersTogether<T>( 
  this IEnumerable<Expression<Func<T, bool>>> filters) 
{ 
    Expression<Func<T, bool>> firstFilter = filters.FirstOrDefault(); 
    if (firstFilter == null) 
    { 
        Expression<Func<T, bool>> alwaysTrue = x => true; 
        return alwaysTrue; 
    } 

    var body = firstFilter.Body; 
    var param = firstFilter.Parameters.ToArray(); 
    foreach (var nextFilter in filters.Skip(1)) 
    { 
        var nextBody = Expression.Invoke(nextFilter, param); 
        body = Expression.OrElse(body, nextBody); 
    } 
    Expression<Func<T, bool>> result = Expression.Lambda<Func<T, bool>>(body, param); 
    return result; 
}

然后：

Expression<Func<P, bool>> myFilter1 = x => foo1 == true && foo2 == false;  
Expression<Func<P, bool>> myFilter2 = x => ... ;  
..  
List<Expression<Func<P, bool>>> filters = new List<Expression<Func<P, bool>>>();
filters.Add(myfilter1);
filters.Add(myfilter2);
..  
Expression<Func<P, bool>> resultFilter = filters.OrTheseFiltersTogether();
IQueryable<P> query = query.Where(resultFilter);

Answer 2

你可能想看看Predicatebuilder：

http://www.albahari.com/nutshell/predicatebuilder.aspx

Predicatebuilder允许您以非常简洁易懂的方式运行一些非常强大的表达式（AND / OR / NOT等）。对于简单的表达式，我当然只是从头开始滚动它们但适用于复杂的东西......

我非常喜欢它：）

关于SO本身的一些链接可能会有所帮助：

LINQ to SQL PredicateBuilder

Generated SQL with PredicateBuilder, LINQPad and operator ANY

Answer 3

.Where方法将lambda表达式作为参数，因此您需要将BinaryExpression构建为完整的LambdaExpression。

var resultExpression = Expression.OrElse(myExp1, myExp2);   
   // now the exp is like: p > 100 || p < 10
ParameterExpression parameterExp = Expression.Parameter(typeof(P),"p");
   // construct a parameter with its type P, looks like: p =>
LambdaExpression lambdaExp = Expression.Lambda(resultExpression, parameterExp);
   // merge the two expressions:  p => p > 100 || p < 10
myList.Where(lambdaExp.Compile());

Answer 4

它是表达级别上两个Func<P, bool>的组合。

这种不那么花哨的方式应该是：

Func<P, bool> myExpression1 = x => foo1 == true && foo2 == false;
Func<P, bool> myExpression2 = x => ... ;

IQueryable<P> l = l.Where((p) => myExpression1(p) || myExpression2(p));

IQueryable <t> .Where（）适合表达式在哪里？</t>

4 个答案: