C# PredicateBuilder Entities: The parameter 'f

问题:

I needed to build a dynamic filter and I wanted to keep using entities. Because of this reason I wanted to use the PredicateBuilder from albahari.

I created the following code:

var invoerDatums = PredicateBuilder.True<OnderzoeksVragen>();
var inner = PredicateBuilder.False<OnderzoeksVragen>();

foreach (var filter in set.RapportInvoerFilter.ToList())
{
    if(filter.IsDate)
    {
        var date = DateTime.Parse(filter.Waarde);
        invoerDatums = invoerDatums.Or(o => o.Van >= date && o.Tot <= date);
    }
    else
    {
        string temp = filter.Waarde;
        inner = inner.Or(o => o.OnderzoekType == temp);
    }
}

invoerDatums = invoerDatums.And(inner);
var onderzoeksVragen = entities.OnderzoeksVragen
                               .AsExpandable()
                               .Where(invoerDatums)
                               .ToList();

When I ran the code there was only 1 filter which wasn't a date filter. So only the inner predicate was filled. When the predicate was executed I got the following error.

The parameter 'f' was not bound in the specified LINQ to Entities query expression.

While searching for an answer I found the following page. But this is already implemented in the LINQKit.

Does anyone else experienced this error and know how to solve it?

回答1:

I ran across the same error, the issue seemed to be when I had predicates made with PredicateBuilder that were in turn made up of other predicates made with PredicateBuilder

e.g. (A OR B) AND (X OR Y) where one builder creates A OR B, one creates X OR Y and a third ANDs them together.

With just one level of predicates AsExpandable worked fine, when more than one level was introduced I got the same error.

I wasn't able to find any help but through some trial and error I was able to get things to work. Every time I called a predicate I followed it with the Expand extension method.

Here is a bit of the code, cut down for simplicity:

public static IQueryable<Submission> AddOptionFilter(
    this IQueryable<Submission> query, 
    IEnumerable<IGrouping<int, int>> options)
{
    var predicate = options.Aggregate(
        PredicateBuilder.False<Submission>(),
        (accumulator, optionIds) => accumulator.Or(ConstructOptionMatchPredicate(optionIds).Expand()));
        query = query.Where(predicate.Expand());            
    return query;
}

Query is an IQueryable which has already had AsExpandable called, ConstructOptionNotMatchPredicate returns an Expression.

Once we got past the error we were certainly able to build up complicated filters at runtime against the entity framework.

Edit:

Since people are still commenting on and up voting this I assume it is still useful so I am sharing another fix. Basically I have stopped using LinqKit and it's predicate builder in favour of this Universal Predicate Builder that has the same API but doesn't need Expand calls, well worth checking out.

回答2:

I got this error and Mant101's explanation got me the answer, but you might be looking for a simpler example that causes the problem:

// This predicate is the 1st predicate builder
var predicate = PredicateBuilder.True<Widget>();

// and I am adding more predicates to it (all no problem here)
predicate = predicate.And(c => c.ColumnA == 1);
predicate = predicate.And(c => c.ColumnB > 32);
predicate = predicate.And(c => c.ColumnC == 73);

// Now I want to add another "AND" predicate which actually comprises 
// of a whole list of sub-"OR" predicates
if(keywords.Length > 0)
{
    // NOTICE: Here I am starting off a brand new 2nd predicate builder....
    // (I'm not "AND"ing it to the existing one (yet))
    var subpredicate = PredicateBuilder.False<Widget>();

    foreach(string s in keywords)
    {
        string t = s;  // s is part of enumerable so need to make a copy of it
        subpredicate = subpredicate.Or(c => c.Name.Contains(t));
    }

    // This is the "gotcha" bit... ANDing the independent
    // sub-predicate to the 1st one....

    // If done like this, you will FAIL!
//  predicate = predicate.And(subpredicate); // FAIL at runtime!

    // To correct it, you must do this...
    predicate = predicate.And(subpredicate.Expand());  // OK at runtime!
}

Hope this helps! :-)