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

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.

From stackoverflow

• 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);
  

  Hovito : Thanks, but stupidly I forgot to mention that I'd like the predicate to read like... (p.Length == 5) && (p.SomeOtherProperty == "hello"). In other words, how do I chain the conditions? Sorry for not having been clear

  Hovito : Thanks alot for the update. Seems to be what I was looking for. Thanks

• 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.

• 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));
  

  Marc Gravell : Actually, you mean Expression.AndAlso. Expression.And is the bitwise and - i.e. where 2 & 1 = 3

  Daniel Earwicker : Thanks, corrected it.

• Use the predicate builder.

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

  Its pretty easy!