How do I dynamically create an Expression<Func<MyClass, bool>> predicate?

Question

Hi all,

How would I go about using an Expression Tree to dynamically create a predicate that looks something like...

(p.Length== 5) && (p.SomeOtherProperty == "hello")

So that I can stick the predicate into a lambda expression like so...

q.Where(myDynamicExpression)...

I just need to be pointed in the right direction.

Thanks.

Edit: Sorry folks, I left out the fact that I want the predicate to have multiple conditions as above. Sorry for the confusion.

Answer 1

Original

Like so:

    var param = Expression.Parameter(typeof(string), "p");
    var len = Expression.PropertyOrField(param, "Length");
    var body = Expression.Equal(
        len, Expression.Constant(5));

    var lambda = Expression.Lambda<Func<string, bool>>(
        body, param);

---

Updated

re (p.Length== 5) && (p.SomeOtherProperty == "hello"):

var param = Expression.Parameter(typeof(SomeType), "p");
var body = Expression.AndAlso(
       Expression.Equal(
            Expression.PropertyOrField(param, "Length"),
            Expression.Constant(5)
       ),
       Expression.Equal(
            Expression.PropertyOrField(param, "SomeOtherProperty"),
            Expression.Constant("hello")
       ));
var lambda = Expression.Lambda<Func<SomeType, bool>>(body, param);

Answer 2

You could instantiate the expression and look at it with an Expression Tree visualizer. There is one in the Visual studio samples - you can compile it and then put it in a specific folder.

That will give you a nice little tree that shows you how an expression is made up. Then you could construct such an expression with the static methods of the Expression object.

Answer 3

To combine several predicates with the && operator, you join them together two at a time.

So if you have a list of Expression objects called predicates, do this:

Expression combined = predicates.Aggregate((l, r) => Expression.AndAlso(l, r));

Answer 4

Use the predicate builder.

http://www.albahari.com/nutshell/predicatebuilder.aspx

Its pretty easy!