如何将一个撰写LINQ表达式来调用排序依据一组实体？ [英] How to a compose a Linq Expression to call OrderBy on a set of entities?

问题描述

有人可以解释的语法建设，将排序依据的实体用户指定属性的表达式？

Can someone explain the syntax for building an Expression that will OrderBy a user-specified property on an entity?

这MSDN文章走一段很长的方式来帮助，但它用一个简单的字符串列表交易，我的数据集包含我自己的自定义对象。

This MSDN article goes a long way to helping, but it deals with a simple list of strings, my data set contains my own custom objects.

的 http://msdn.microsoft.com/en-us/library/bb882637.aspx

推荐答案

代码首先，解释以后

IQueryable<T> data = this.Database.ObjectsOfType<T>();

var eachItem = Expression.Parameter(typeof(T), "item");
var propertyToOrderByExpression = Expression.Property(eachItem, propertyName);

var runMe = Expression.Call(
    typeof(Queryable),
    "OrderBy",
    new Type[] { data.ElementType, typeof(IComparable) },
    data.Expression,
    Expression.Lambda<Func<T,IComparable>>(propertyToOrderByExpression, new ParameterExpression[] { eachItem }));

所以，首先我们得到的数据，可查询的对象保持。这有一种根Expression属性的，我们需要的。

So, first we get hold of the data as a Queryable object. This has a kind of 'root' Expression property, and we need that.

eachItem的是代表一个lambda参数占位符表达式中，符号去，如果你愿意。

The eachItem thing is an expression that represents the argument placeholder in a Lambda, the symbol in the goes to, if you will.

然后，我们作出这样做的读操作由propertyName的用户指定的属性名称的表达式。

Then we make an expression that does the read operation the property name specified by the user in propertyName.

我们终于建立由它来调用上可查询的数据排序依据的方法表达。我们说，（在辩论的顺序排列）：

We finally build an expression which does the call to the OrderBy method on the Queryable data. We're saying (in order of argument):

Expression.Call(
 [what's the type on which we want to call a method?],
 [what's the name of the method we're calling?],
 [if this method is generic, what are the types it deals with?],
 {
  [expression representing the data],
  [expression for the lambda using the reader exp + each item exp]
 })

最后两个是在{}因为它的实际上是一个参数数组。我用IComparable的，因为财产可以是任何类型，但显然需要堪比订购。

The last two are in { } since its actually a param array. I used IComparable since the property could be any type but obviously needs to be comparable to be ordered.

路

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