How do I dynamically construct a predicate method from an expression tree?

expression-trees filter predicatebuilder silverlight

Question

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

Popular Answer

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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Is this KB legal? Yes, learn why