Question

以下是该方案： Silverlight 4.0，DataGrid，PagedCollectionView itemssource。目标是将过滤器应用于PCV。过滤器需要是Predicate<object>(Method) - 其中Method对对象实现一些逻辑并返回true / false以包含。我所需要的是在过滤器逻辑中可选地包括3个不同的标准，并且显式代码很快变得难看。我们不希望这样，是吗？

所以我看到有一种方法可以使用PredicateBuilder构建表达式树并将其传递给Linq.Where，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);
}

[这不是我想要做的事情]

有3个可选标准，我想写一些类似的东西：

 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;
}

当然这不会编译，因为PredicateBuilder创建的Expression<Func<T, bool>>不是实际的谓词方法。但我发现有一些方法可以将表达式树转换为一个方法，所以在我看来应该有一种方法可以实现我所追求的，而不需要使用一堆嵌套的if / then / else语句。

所以问题是 - 有没有办法动态构建谓词方法？

TIA

Answer 1

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();
  Predicate<object> predicate = new Predicate<object>(func);
  //Predicate<object> predicate = t => func(t);     // also works
  //Predicate<object> predicate = func.Invoke;      // also works
  return predicate;
}

并构建表达式：

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 || ((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;
}

然后设置过滤器：

Ratings.Filter = ConvertExpressionToPredicate(BuildFilterExpression());

Answer 2

我遇到了同样的问题。我有3个标准。我做的是以下内容：

验证每个条件的一种方法
验证对象的一种方法

代码看起来很干净，很容易维护。

Ratings.Filter = new predicate<objects>(validateObject);

private bool validateObject(object o)
{
  return validateFirstCriteria(o) && 
         validateSecondCriteria(o) && 
         validateThirdCriteria(o);
}

private bool validateFirstObject(object o)
{
  if (ViewMineOnly)
  {
    Rating r = o as Rating;
    if (o != null)
    {
      return  (r.Star.HasValue && r.Star.Value > 0);
    }
  }
  return false;
}
private bool validateSecondObject(object o)
{
  if (ViewStarOnly)
  {
    Rating r = o as Rating;
    if (o != null)
    {
      return sUserNameFilter == r.Assigned_To;
    }
  }
  return false;
}
private bool validateThirdObject(object o)
{
  if (FilterOnOrder)
  {
    Rating r = o as Rating;
    if (o != null)
    {
      return r.ShipmentInvoice.StartsWith(sOrderNumberFilter);
    }
  }
  return false;
}

修改

如果你想坚持表达树。你可以看看这里：http://msdn.microsoft.com/en-us/library/bb882536.aspx

您可以将表达式树转换为lambda表达式，然后编译lambda表达式。之后，您可以将其用作方法。例如：

// The expression tree to execute. BinaryExpression be = Expression.Power(Expression.Constant(2D), Expression.Constant(3D)); // Create a lambda expression. Expression<Func<double>> le = Expression.Lambda<Func<double>>(be); // Compile the lambda expression. Func<double> compiledExpression = le.Compile(); // Execute the lambda expression. double result = compiledExpression(); // Display the result. Console.WriteLine(result); // This code produces the following output: // 8

Answer 3

感谢Benjamin的提示和这篇文章 - ＆gt; How to convert Func<T, bool> to Predicate<T>? 我想到了。
其实质是：

private static Predicate<T> ConvertExpressionToPredicate(Expression<Func<T, bool>> exp)
{
  Func<T, bool> func = exp.Compile();
  Predicate<T> predicate = new Predicate<T>(func);
  //Predicate<T> predicate = t => func(t);     // also works
  //Predicate<T> predicate = func.Invoke;      // also works
  return predicate;
}

这会将表达式树编译为单个函数并返回一个谓词来调用该函数。
在使用中，它看起来像：

private static bool ViewStarOnly;
private static bool LongNameOnly;
static void Main(string[] args)
{
  List<Dabble> data = GetSomeStuff();
  ViewStarOnly = true;
  LongNameOnly = true;
  Expression<Func<Dabble, bool>> exp = BuildFilterExpression();
  List<Dabble> filtered = data.FindAll(ConvertExpressionToPredicate(exp));
  PrintSomeStuff(filtered);
}

private static Predicate<Dabble> ConvertExpressionToPredicate(Expression<Func<Dabble, bool>> exp)
{
  Func<Dabble, bool> func = exp.Compile();
  Predicate<Dabble> predicate = new Predicate<Dabble>(func);
  //Predicate<Dabble> predicate = t => func(t);     // also works
  //Predicate<Dabble> predicate = func.Invoke;      // also works
  return predicate;
}

private static Expression<Func<Dabble, bool>> BuildFilterExpression()
{
  var predicate = PredicateBuilder.True<Dabble>();
  if (ViewStarOnly)
  {
    predicate = predicate.And(r => r.Star.HasValue && r.Star.Value > 0);
  }
  if (LongNameOnly)
  {
    predicate = predicate.And(r => r.Name.Length > 3);
  }
  return predicate;
}

谢谢！

如何从表达式树动态构造谓词方法？

3 个答案: