创建动态包含子查询的 Linq 表达式 [英] Creating a Linq expression dynamically containing a subquery

问题描述

我最近偶然发现了在运行时动态创建 Linq 表达式的问题.我发现的大多数示例都处理相当简单的任务，即仅将给定数据库实体的一个属性与单个参数进行比较.像这样:

I recently stumpled upon the problem to dynamically create Linq expressions during runtime. Most examples I found deal with the rather simple task of just comparing one property of a given database entity with a single parameter. Like so:

 Session.Query.Where(m => m.Name.Contains("test"))

这也可以通过更通用的方法来实现:

Which could also be achieved with a far more generic approach like this:

var item = Expression.Parameter(typeof (MyClass), "item");
var property = Expression.Property(item, "Name");
var containsMethod = typeof(string).GetMethod("Contains", new[] { typeof(string) });
var searchExpression = Expression.Constant(searchString, typeof(string));
var containsMethodExpression = Expression.Call(property, containsMethod, searchExpression);
var lambda = Expression.Lambda<Func<MyClass, bool>>(containsMethodExpression, item);
query = query.Where(lambda);    

然而，有时任务会更复杂一些，人们希望达到以下目标:

However, sometimes the task is somewhat more complex and one wants to attain something like the following:

Session.Query.Where(m => m.SpecialProperty.Any(f => f.Name.Contains("test")));

其中SpecialProperty"的类型为 List<>，属性Name"的类型为字符串.

Where "SpecialProperty" is of the type List<> and the property "Name" is of the type string.

是否可以动态构建这样的 Linq 表达式，如何实现?这种方法是否有任何性能问题?

Is it possible to build a Linq expression like this dynamically and how could this be achieved? Are there any performance concerns regarding this approach?

推荐答案

找到了一个适用于我的特定用例的解决方案，并且完全符合我的要求.

Found a solution that is working in my particular use case and does exactly what I was looking for.

/* 
building expression tree 
example: Session.Query.Where(m => m.SpecialProperty.Any(f => f.Name.Contains("test")))
*/ 

var innerItem = Expression.Parameter(typeof(MyInnerClass), "f");
var innerProperty = Expression.Property(innerItem, "Name");
var innerMethod = typeof(string).GetMethod("Contains", new[] { typeof(string) });
var innerSearchExpression = Expression.Constant(searchString, typeof(string));
var innerMethodExpression = Expression.Call(innerProperty, innerMethod, new[] { innerSearchExpression });
var innerLambda = Expression.Lambda<Func<MyInnerClass, bool>>(innerMethodExpression, innerItem);

var outerItem = Expression.Parameter(typeof(MyOuterClass), "m");
var outerProperty = Expression.Property(outerItem, info.Name);
/* calling a method extension defined in Enumerable */
var outerMethodExpression = Expression.Call(typeof(Enumerable), "Any", new[] { typeof(MyInnerClass) }, outerProperty, innerLambda);
var outerLambda = Expression.Lambda<Func<MyOuterClass, bool>>(outerMethodExpression, outerItem);
query = query.Where(outerLambda);

需要这种相当乏味的方法，而不是更优雅的单行 LINQ-Expression，以允许参数化类型和方法名称.但是，我当然不介意其他有关可能的性能损失的建议和想法.

This rather dowdy approach is needed instead of the more elegant single line LINQ-Expression to allow for parametrization of types and method names. However, I of course wouldn't mind other suggestions and ideas on possible performance penalties.

此代码片段很可能也有助于解决如何使用非泛型 Lambda 生成子查询.

It is very likely that this code snippet could also assist in solving How to produce a Subquery using non-generic Lambda.

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