Dynamic Linq Library Help

Question

I have the following class:

public class Item
{
    public Dictionary<string, string> Data
    {
        get;
        set;
    }
}

and a list of it:

List<Item> items;

I need to filter and order this list dynamically using SQL-Like strings. The catch is, that I need to order it by the Data dictionary.

For example: Order By Data["lastname"] or Where Data["Name"].StartsWith("a"). I thought to use the dynamic linq library, but is there any way that my clients can write without the Data[]? For example:

Name.StartsWith("abc")

instead of

Data["Name"].StartsWith("abc")

?

Answer 1

You could add a property like this:

public class Item
{
    public Dictionary<string, string> Data
    { get; set; }

    public string Name { get { return Data["lastname"]; } }
}
//Call by: i.Name.StartsWith("abc");

Or an extension method:

public static class ItemExtensions 
{
  public static string Name(this Item item)
  {
    return item.Data["lastname"];
  }
}
//Call by: i.Name().StartsWith("abc");

Or if it's a very commonly used method, you could add something like a .NameStartsWith():

public static string NameStartsWith(this Item item, stirng start)
{
  return item.Data["lastname"].StartsWith(start);
}
//Call by: i.NameStartsWith("abc");

Answer 2

I assume that you don't know the names of the properties at compile-time (in which case, you could simply define properties and wouldn't have this problem). I have two suggestions that you could try, but I didn't implement any of them myself, so I can't guarantee that it will work.

• If you can use .NET 4.0, you could inherit from DynamicObject and implement TryGetMember method (which is called when you use o.Foo on an object that is declared as dynamic). Assuming that Dynamic LINQ works with DLR, it should automatically invoke this method for objects that inherit from DynamicObject. Inside the TryGetMember method, you would get a name of the accessed property, so you could perform a dictionary lookup. (However, this solution would work only if Dynamic LINQ integrates well with DLR).

• In any case, you could do some basic parsing of the string entered by the user and replace for example Name with Data["Name"]. This would definitely work, but it may be a bit difficult (because you should probably at least check that you're doing the replace in correct context - e.g. not inside a string constant).

Regarding extension methods - I'm not sure if Dynamic LINQ handles extension methods (but, I don't think so, because that would require searching all referenced assemblies)

Answer 3

This doesn't have anything to do with the Linq Dynamic Query unit. That unit is for when you have actual fields/properties and the names of them will be given to you at runtime. In other words, you have a class like this:

public class Person
{
    public int ID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

And you want to be able to write a query like this:

var sortedPeople = people.OrderBy("FirstName");

You are trying to do the exact opposite of this - you have a class that does not have any actual properties, just an attribute dictionary, and you want compile-time safety. You can't have it; there's no way to guarantee that an item will be in the dictionary, especially when the dictionary is public and anyone can add/remove directly from it!

If there's some reason that you must use that specific class design, then you could conceivably write some wrappers as Nick has presented, but I wouldn't even bother - they're not actually providing any encapsulation because the Data dictionary is still wide open to the whole world. Instead, I would just provide a single safe getter method or indexer property and create a few constants (or an enum) with the names of properties you expect to be in there.

public class Item
{
    public Dictionary<string, string> Data { get; set; }

    public string GetValue(string key)
    {
        if (Data == null)
            return null;
        string result;
        Data.TryGetValue(key, out result);
        return result;
    }
}

public class ItemKeys
{
    public const string Name = "Name";
    public const string Foo = "Foo";
}

And so on. Really the ItemKeys isn't that important, the safe GetValue method is what's important, because otherwise you run the risk of a NullReferenceException if Data hasn't been assigned, or a KeyNotFoundException if even one Item instance doesn't have that property. Using the GetValue method here will succeed no matter what:

var myItems = items.OrderBy(i => i.GetValue(ItemKeys.Name));

If you find you're writing a lot of repetitive code for the same attributes, then start worrying about adding shortcut properties or extension methods to the class.