Creating an (Lambda) Expression using recursion

Question

I have the following dictionary:

Dictionary<string, PropertyInfo> filterProperties;

The content of this dictionary can be like this:

- "Language": [QueryData.Lang],
- "Id": [Querydata.UserId]

Each string key maps to a property of a my QueryData type.

Now let's suppose I have the following QueryData instance:

QueryData: { Lang= "en", UserId = "mcicero" }

Using the previous dictionary as example, I want to build the following expression:

e => e.Language == "en" && e.Id == "mcicero";

As you can see the dictionary keys are used for accesing the properties of e, and the dictionary values (QueryData properties) are used for specifying constants in the binary equal expressions.

e is of type Entity, and is guaranteed to have this properties.

The resulting expression should be of type Expression<Func<Entity, bool>>

How can I build this expression using recursion?

I say recursion because it sounds like a natural solution, but an iterative one would be preferred.

I tried the iterative alternative and ended up with an ugly and not so understandable code.

However, I am having trouble creating a recursion method for this problem.

you probably want to use dynamic linq. here's some info weblogs.asp.net/scottgu/… — DLeh
– DLeh, Commented Jul 10, 2015 at 13:04
Ignoring all the dictionary and QueryData stuff, you just want to know how to create an Expression like e => e.Language = "en" at runtime from a PropertyInfo and a string. — Stefan Steinegger
– Stefan Steinegger, Commented Jul 10, 2015 at 13:10
Can you explain it better, what you mean with build this expression ? — Francisco Mercedes
– Francisco Mercedes, Commented Jul 10, 2015 at 13:11
@StefanSteinegger Actually, I know how to create such expression, but I do not know how to join each mapped expression with an AND (&&) expression. The dictionary size can be variable — Matias Cicero
– Matias Cicero, Commented Jul 10, 2015 at 13:12

Dark Falcon · Accepted Answer · 2015-07-10 13:21:40Z

3

Leaving the creation of the individual expression to you, simply combine them together in an iterative loop:

// For an IEnumerable<Expression> in "expressions"
Expression left = null;
foreach(var right in expressions)
{
  if(left == null)
    left = right;
  else
    left = Expression.And(left, right);
}
// Combined expression is in "left"
// Don't forget it will be null if there were no expressions provided...

Or in one line with LINQ:

var expr = expressions.Aggregate(Expression.And);

edited Jul 10, 2015 at 13:21

answered Jul 10, 2015 at 13:15

Dark Falcon

44.2k6 gold badges91 silver badges104 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Matias Cicero Over a year ago

Thank you for that one line LINQ statement! Beautiful!

MBoros Over a year ago

This &ing will result in a tree that is as deep as possible, which will reduce performance when there is a lot of nodes. You should try to build a tree as flat as possible, for example by putting them in a queue, taking out the first 2 elements, &ing them and putting at the end, until there is only 1 node left.

Matias Cicero · Accepted Answer · 2015-07-10 13:33:08Z

2

For the interested ones, in one single line:

Expression expr = filterProperties.Select(i => Expression.Equal(
                                                  Expression.Property(entityParameter, i.Key),
                                                  Expression.Constant(i.Value.GetValue(queryObj))
                                         )).Aggregate(Expression.And);

answered Jul 10, 2015 at 13:33

Matias Cicero

26.5k21 gold badges91 silver badges160 bronze badges

Collectives™ on Stack Overflow

Creating an (Lambda) Expression using recursion

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related