调用任何内部表达式并通过代理

Question

我有一个场景，我需要使用linq（与nhibernate）来打一个动态查询。最后的查询应该是这样的：

long[] values = { ... }; 

var result = Queryable<Entity>.Where(x => x.Documents.Any(d => values.Contains(d.Id))) 
           .ToList(); 

通用Entity和财产Documents可以改变，它会通过某个用户的配置来定义。收集类型Documents是ICollection<T>，其中T是Document类型。我正在尝试创建Expression树来动态定义这些语句，但我遇到了一些问题。看看我试过的代码和评论。

我创建这个函数返回我想要的Any方法内使用delagate：

public static Func<T, bool> GetFunc<T>(long[] values) 
     where T : Entity 
    { 
     return x => values.Contains(x.Id); 
    } 

，我使用表达式类来作出这样的表达（见代码和注释）：

// define my parameter of expression 
var parameter = Expression.Parameter(typeof(T), "x"); 

// I get an array of IDs (long) as argument and transform it on an Expression 
var valuesExpression = Expression.Constant(values); 

// define the access to my collection property. propertyFilter is propertyinfo for the `Documents` of the sample above. 
// I get an expression to represent: x.Documents 
var collectionPropertyExpression = Expression.Property(parameter, propertyFilter); 

// get the T generic type of the ICollection<T> from propertyFilter. I get the `Documents` of sample above. 
var entityFilterType = propertyFilter.PropertyType.GetGenericArguments()[0]; 

// get the definition of `Any` extension method from `Enumerable` class to make the expression 
var anyMethod = typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public) 
                 .First(x => x.Name == "Any" && x.GetParameters().Length == 2) 
                 .MakeGenericMethod(entityFilterType); 

// get a methodBase for GetFunc to get the delagete to use inside the Any 
// using the `Document` generic type 
var collectionBody = typeof(LookUpHelper).GetMethod("GetFunc", BindingFlags.Public | BindingFlags.Static) 
                  .MakeGenericMethod(entityFilterType); 

// call the any passing the collection I need and convert it to a Delegate 
// I get something like: x => values.Contains(x.Id) ... where x if the `Document` 
var func = (Delegate)collectionBody.Invoke(null, new object[] { values }); 

// get the func as an expression .. maybe the problem is here 
var funcExpression = Expression.Constant(func); 

// call the any passing the collection and my delagate as arguments 
var f = Expression.Call(anyMethod, collectionPropertyExpression, funcExpression); 

// I already have an expression and concatenate it using `AndAlso` operator. 
body = Expression.AndAlso(body, f); 

// finally, I built up to lambda expression and apply it on my queryable 
var filterExpression = Expression.Lambda<Func<T, bool>>(body, parameter); 

var result = Queryable.Where(filterExpression).ToList(); 

它执行直到通过ToList方法执行查询。我收到以下错误：

Could not parse expression 'x.Documents.Any(value(System.Func`2[Project.Document,System.Boolean]))': The object of type 'System.Linq.Expressions.ConstantExpression' cannot be converted to type 'System.Linq.Expressions.LambdaExpression'. If you tried to pass a delegate instead of a LambdaExpression, this is not supported because delegates are not parsable expressions.

我不知道我在做什么错。有人可以帮助我吗？

谢谢。

Answer 1

您正在通过一个Func，其中Expression<Func>预计。前者是代表，后者是表达。

public static Expression<Func<T, bool>> GetFunc<T>(long[] values) 
     where T : Entity 
{ 
    return x => values.Contains(x.Id); 
} 

现在你放弃了，因为你已经有了表达式，所以需要使用表达式帮助器类手动构建表达式。