如何从表达式树动态构造一个谓词方法？ [英] How do I dynamically construct a predicate method from an expression tree?

问题描述

这里是场景：
Silverlight 4.0，DataGrid，PagedCollectionView itemssource。
目的是将一个过滤器应用于PCV。过滤器需要是 Predicate< object>（方法） - 其中Method对象实现了一些逻辑，并返回true / false作为包含。
我有什么需要可选地包括3个不同的标准在过滤器逻辑和显式代码很快变得丑陋。我们不想这么做吗？

因此，我发现有一种方法可以使用PredicateBuilder构建一个表达式树，并将其传递给Linq。其中，一个la ：

  IQueryable< Product> SearchProducts（params string [] keywords）
 {
 var predicate = PredicateBuilder.False< Product>（）; 
 
 foreach（关键字中的字符串关键字）
 {
 string temp = keyword; 
 predicate = predicate.Or（p => p.Description.Contains（temp））; 
} 
返回dataContext.Products.Where（谓词）; 
 
 $ / code $ / pre 
 
这不是我想要做的事情/ p> 
 
 
使用3个可选标准，我想写下如下所示： 
 
 
  Ratings.Filter = BuildFilterPredicate（）; // Ratings = PagedCollectionView 
 
 
私有谓词< object> BuildFilterPredicate（）
 {
 bool FilterOnOrder =！String.IsNullOrEmpty（sOrderNumberFilter）; 
 var predicate = PredicateBuilder.False< object>（）; 
 if（ViewMineOnly）
 {
 predicate = predicate.And（Rating r => sUserNameFilter == r.Assigned_To）; （ViewStarOnly）
 {
 predicate = predicate.And（Rating r => r.Star.HasValue&& r.Star.Value> 0; 
} 
 ）; （FilterOnOrder）
 {
 predicate = predicate.And（Rating r => r.ShipmentInvoice.StartsWith（sOrderNumberFilter））; 
 
 if 
} 
返回谓词; 
 
 
 
 $ b当然这不会编译，因为PredicateBuilder会创建一个表达式< Func< T，bool>> 不是一个实际的谓词方法。但是我发现有一些方法可以将表达式树转换为方法，所以在我看来应该有一种方法来完成我所做的事情，而不需要使用一堆嵌套的if / then / else语句。
 
 
 
所以问题是 - 是否有动态构建谓词方法的方法？ 
 
 
 TIA 
解决方案
要为PagedCollectionView执行此操作，您需要有一个谓词。所以它看起来像：
  private Predicate< object> ConvertExpressionToPredicate（Expression< Func< object，bool>> exp）
 {
 Func< object，bool> func = exp.Compile（）; 
谓词< object> predicate = new Predicate< object>（func）; 
 //谓词< object> predicate = t => FUNC（T）; //也可以工作
 //谓词< object>谓词= func.Invoke; //也可以工作
返回谓词; 
 
 
 
 
 
 
构建表达式：
 
 
  private Expression< Func< object，bool>> BuildFilterExpression（）
 {
 ... snip ... 
 var predicate = PredicateBuilder.True< object>（）; $（）
 if（ViewMineOnly）
 {
 predicate = predicate.And（r =>（（Rating）r）.Assigned_To.Trim（）ToUpper（）== sUserNameFilter ||（评分）r）.Assigned_To.Trim（）。ToUpper（）==UNCLAIMED）; （ViewStarOnly）
 {
 predicate = predicate.And（r =>（（Rating）r）.Star.HasValue&&（（Rating） r）.Star.Value> 0）;如果（FilterOnOrder）
 {
 predicate = predicate.And（r =>（（Rating）r）.ShipmentInvoice.Trim（）。ToUpper（）。StartsWith （sOrderNumberFilter））; （ViewDueOnly）
 {
 predicate = predicate.And（r =>（（Rating）r）.SettlementDueDate< = ThisThursday）; 
} 
} 
返回谓词; 
} 
  
然后设置过滤器： 
 
 
  Ratings.Filter = ConvertExpressionToPredicate（BuildFilterExpression（））; 
  
 
Here's the scenario:
Silverlight 4.0, DataGrid, PagedCollectionView itemssource.
The objective is to apply a Filter to the PCV.  The filter needs to be a Predicate<object>(Method) - where Method implements some logic against the object and returns true/false for inclusion.
What I have is a need to optionally include 3 different criteria in the filter logic and explicit code quickly gets ugly.  We don't want that, do we?


So I see that there is a way to build an expression tree using PredicateBuilder and pass that into Linq.Where, a la:
IQueryable<Product> SearchProducts (params string[] keywords)
{
  var predicate = PredicateBuilder.False<Product>();

  foreach (string keyword in keywords)
  {
    string temp = keyword;
    predicate = predicate.Or (p => p.Description.Contains (temp));
  }
  return dataContext.Products.Where (predicate);
}
[this is not what I'm trying to do by the way]

With 3 optional criteria, I want to write something like:
 Ratings.Filter = BuildFilterPredicate();  // Ratings = the PagedCollectionView


private Predicate<object> BuildFilterPredicate()
{
  bool FilterOnOrder = !String.IsNullOrEmpty(sOrderNumberFilter);
  var predicate = PredicateBuilder.False<object>();
  if (ViewMineOnly)
  {
    predicate = predicate.And(Rating r => sUserNameFilter == r.Assigned_To);
  }
  if (ViewStarOnly)
  {
    predicate = predicate.And(Rating r => r.Star.HasValue && r.Star.Value > 0);
  }
  if (FilterOnOrder)
  {
    predicate = predicate.And(Rating r => r.ShipmentInvoice.StartsWith(sOrderNumberFilter));
  }
  return predicate;
}
Of course this won't compile because PredicateBuilder creates an Expression<Func<T, bool>> not an actual predicate method.  But I see that there are ways to convert an expression tree into a method so it seemed to me there ought to be a way to accomplish what I'm after without resorting to a bunch of nested if/then/else statements.

So the question is - is there a way to build a predicate method dynamically?

TIA
解决方案
to do this for a PagedCollectionView, you need to have a Predicate.  So it looks like:
private Predicate<object> ConvertExpressionToPredicate(Expression<Func<object, bool>> exp)
{
  Func<object, bool> func = exp.Compile();
  Predicate<object> predicate = new Predicate<object>(func);
  //Predicate<object> predicate = t => func(t);     // also works
  //Predicate<object> predicate = func.Invoke;      // also works
  return predicate;
}
and build the expression:
private Expression<Func<object, bool>> BuildFilterExpression()
{
  ...snip...
  var predicate = PredicateBuilder.True<object>();
  if (ViewMineOnly)
  {
    predicate = predicate.And(r => ((Rating)r).Assigned_To.Trim().ToUpper() == sUserNameFilter || ((Rating)r).Assigned_To.Trim().ToUpper() == "UNCLAIMED");
  }
  if (ViewStarOnly)
  {
    predicate = predicate.And(r => ((Rating)r).Star.HasValue && ((Rating)r).Star.Value > 0);
  }
  if (FilterOnOrder)
  {
    predicate = predicate.And(r => ((Rating)r).ShipmentInvoice.Trim().ToUpper().StartsWith(sOrderNumberFilter));
  }
  if (ViewDueOnly)
  {
    predicate = predicate.And(r => ((Rating)r).SettlementDueDate <= ThisThursday);
  }
  return predicate;
}
and then set the filter:
Ratings.Filter = ConvertExpressionToPredicate(BuildFilterExpression());


                        
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