Build lambda expression (predicate) with dynamic comparison operators

Question

Take Northwind as example.

My goal is to use Linq to Entity to search Employees dynamically.

My expression maybe:

Expression<Func<Employee, bool>> exp= em => em.EmployeeID > 2;

Here the operator '>' is selected by combobox at runtime, and it may be '<', '=', '<>', 'contains' and so on.

My question is, how to build the expression dynamically with the operator selected?

One important thing, no selective statements allowed, such as 'if' and 'switch'.

Any answer is appreciated.

@Raphaël Althaus, that one is good, I will examin it first. Thanks! — Lei Yang, Aug 28 '13 at 16:07
@LeiYang of course, this is one of mines ;) To be honest, the comparison operators are ok, but the "contains" part is missing. — Raphaël Althaus, Aug 28 '13 at 16:07
@Raphaël Althaus, the second duplicate is exactly what I want, but my question, especially the title, is much better than that one, don't you think so? What is the best title? — Lei Yang, Aug 29 '13 at 02:11
Well, I tend to agree with you. I would probably use something like "Build lambda expression (predicate) with dynamic comparison operators". But that's a detail. You may change the other post title, maybe... — Raphaël Althaus, Aug 29 '13 at 05:57

Sam I am says Reinstate Monica · Answer 1 · 2013-08-29T14:10:52.260

0

If conditionals inside your lambda look unsightly to you, you can use a function

Expression<Func<Employee, bool>> exp= em => foo(em, otherParam);

public static bool foo(Employee em, otherType otherParam)
{
    ...
}

edited Aug 29 '13 at 14:10

answered Aug 28 '13 at 16:05

Sam I am says Reinstate Monica

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Not understand, sorry, how does it run with ExpressionType? – Lei Yang Aug 29 '13 at 06:30

score 0 · Answer 2 · answered Aug 28 '13 at 16:11

0

I would create a class that has a Func<Employee, bool> and overrides the ToString method to represent the operator that the function is using. Then, I would use an ObservableCollection of that class as the ItemsSource of your ComboBox.

Call your class Operator, and your code to perform the operation becomes:

var op = dropDown.SelectedItem as Operator;

if (op != null)
{
  op.Function(employee);
}

answered Aug 28 '13 at 16:11

Luke Willis

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Thanks, but could you explain type of "Operator"? I my case, it's ExpressionType. – Lei Yang Aug 29 '13 at 06:13

Build lambda expression (predicate) with dynamic comparison operators

2 Answers2