如何选择表达式的评估值 <Func<T> &gt;在实体框架查询中

Question

I'm creating a system to generate variable search queries in EF. To accomplish this I dynamically build up an Expression<Func<T>> like so.

Expression<Func<Entity, float>> matchScore = x => 0;
if (!string.IsNullOrWhiteSpace(NameLTB.Value))
{
    var words = NameLTB.Value.Split(' ').ToArray();
    var perWordValue = 2f / words.Length;
    foreach (var word in words)
        matchScore = ExpressionExt.Compose(matchScore, x => x.Name.Contains(word) ? perWordValue : 0f, Expression.Add);
}
//...

Then I order my query on this EF-compatible Expression<T> like so.

List<Entity> result;
using (var context = new DatabaseContext())
    result = context.Table.OrderByDescending(matchScore).Take(MaxResults).ToList();

The problem here is I don't know what the score was that caused the order to be the way it is. I'd love to be able to do this.

List<Entity> result;
using (var context = new DatabaseContext())
    result = 
        (
            from x in context.Table
            let y = matchScore
            orderby y descending
            select new { Entity = x, MatchScore = y }
        )
        .ToList();

Of course the issue here is y is Expression<T> which isn't valid. I could use .Select(matchScore) but then don't have the original entity to go with it.

Answer 1

You can combine expressions with LINQKit like this:

Expression<Func<Entity, float>> matchScore = x => 0;
Expression<Func<Entity, float>> matchScoreInc;
string s = "a b";
if (!string.IsNullOrWhiteSpace(s))
{
    var words = s.Split(' ').ToArray();
    var perWordValue = 2f / words.Length;
    foreach (var word in words)
    {
        matchScoreInc = x => matchScore.Invoke(x) + (x.Name.Contains(word) ? perWordValue : 0f);
        matchScore = matchScoreInc.Expand();
    }
}

var q =
    from x in table.AsExpandable()
    let y = matchScore.Invoke(x)
    orderby y descending
    select new { Entity = x, MatchScore = y };

Note the use of Invoke , Expand and AsExpandable . Demo: