Is there a pattern using Linq to dynamically create a filter?

Question

Is there a pattern using Linq to dynamically create a filter?

I have the need to create custom filtering on a list, in the past I would just dynamically create the SQL...it doesn't seem like this is possible with Linq.

Pellmell

Answer 1

Check out the Dynamic Linq Library from ScottGu's blog:

For example, below is a standard type-safe LINQ to SQL VB query that retrieves data from a Northwind database and displays it in a ASP.NET GridView control:

Dim Northwind As New NorthwindDataContext
Dim query = From q In Northwind.Products Where p.CategoryID = 2 And p.UnitPrice > 3 Order By p.SupplierID Select p

Gridview1.DataSource = query
GridView1.DataBind()

Using the LINQ DynamicQuery library I could re-write the above query expression instead like so

Dim Northwind As New NorthwindDataContext
Dim query = Northwind.Products .where("CategoryID=2 And UnitPrice>3") . OrderBy("SupplierId")
Gridview1.DataSource = query
GridView1.DataBind()

Notice how the conditional-where clause and sort-orderby clause now take string expressions instead of code expressions. Because they are late-bound strings I can dynamically construct them. For example: I could provide UI to an end-user business analyst using my application that enables them to construct queries on their own (including arbitrary conditional clauses).

Flowerage

Does this work? I get this : Error 28 Overload resolution failed because no accessible 'OrderBy' can be called with these arguments: Data type(s) of the type parameter(s) cannot be inferred from these arguments. Specifying the data type(s) explicitly might correct this error. I can't see 'String' as a parameter option in the .OrderBy clause. – Nashua

@Nashua - It works. I use it in many projects. Double check that the dynamic.cs is in your project and that you have it referenced properly in your using declarations. – Flowerage

That is great. I never knew you could do that. That will solve my problem with advanced search query building. Nice one +1 – Walloping

Answer 2

Dynamic Linq is one way to go.

It may be overkill for your scenario. Consider:

IQueryable<Customer> query = db.Customers;

if (searchingByName)
{
  query = query.Where(c => c.Name.StartsWith(someletters));
}
if (searchingById)
{
  query = query.Where(c => c.Id == Id);
}
if (searchingByDonuts)
{
  query = query.Where(c => c.Donuts.Any(d => !d.IsEaten));
}
query = query.OrderBy(c => c.Name);
List<Customer> = query.Take(10).ToList();

Abash

Answer 3

Dynamically Composing Expression Predicates

Uel

Answer 4

something like this?

var myList = new List<string> { "a","b","c" };
var items = from item in db.Items
            where myList.Contains(item.Name)
            select item;

that would create a sql statement like

SELECT * FROM Items [t0] where Name IN ('a','b','c')

Vachell

Interesting concept but a bit confusing – Walloping