Creating a generic filter system to pass into an API request

Question

I am building an application that will interact with an API. My service layer will call the repository which will request data from the API. I want to be able to pass filters to the repository and came up with the following. I am just curious if there are better ways of doing this?

First, I created an interface called IFilter:

public interface IFilter
{
 string GetFilterName();
 string GetFilterValue();
}

I then have a bunch of filters, for example:

public class CreatedAfterFilter : IFilter
{
 readonly string _name = "CreatedAfter";
 DateTime _value;
 public CreatedAfterFilter(DateTime value)
 {
 this._value = value;
 }
 public string GetFilterValue()
 {
 return _value.ToString("yyyy-MM-ddTHH:mm:ss+00:00");
 }
 public string GetFilterName()
 {
 return _name; ;
 }
}

...or

public class OrdersFilter : IFilter
{
 readonly string _name = "OrderIdList";
 List<Order> _value;
 public OrdersFilter(List<Order> value)
 {
 this._value = value;
 }
 public string GetFilterValue()
 {
 return '[' + String.Join(",", _value.Select(x => x.OrderId)) + ']';
 }
 public string GetFilterName()
 {
 return _name; ;
 }
}

When I call my repository, I do something like this:

var filters = new List<IFilter>();
filters.Add(new CreatedAfterFilter(DateTime.UtcNow.AddDays(-110)));
filters.Add(new OrderFilter(1709359)); 
var orders = orderRepository.GetSingleOrder(filters);

Then in my code that calls the API, I do this to create the query string:

foreach (var filter in filters)
{
 getParams.Add(new KeyValuePair<string, string>(filter.GetFilterName(), filter.GetFilterValue()));
}

Am I on the right track here? This is my first real time in writing C# an using Interfaces.

• koelkastfilosoof: You might want to put something in place that decouples the Filters from the Repository you are using. E.g. if you are interacting with a database using SQL, the translation of the filter is different than translating it to a REST call where OData filters are used.

Answer 1

Am I on the right track here?

I think you are. Here's how you can use a few tricks to make it even shorter.

---

Instead of the interface you can implement what is common in an abstract class.

public abstract class Filter
{
 public virtual string Name => Regex.Replace(GetType().Name, $"{nameof(Filter)}$", string.Empty);
 public static implicit operator string(Filter filter) => filter.ToString();
}

This means the base class will get the name of the filter from its type and it'll be able to implicitly cast it into a string by calling ToString. Now everything the derived class needs to do is to implement its own ToString:

public class CreatedAfterFilter : Filter
{
 private DateTime _value;
 public CreatedAfterFilter(DateTime value)
 {
 _value = value;
 }
 public override string ToString()
 {
 return _value.ToString("yyyy-MM-ddTHH:mm:ss+00:00");
 } 
}

So later in the loop you can just do this:

foreach (var filter in filters)
{
 getParams.Add(new KeyValuePair<string, string>(filter.Name, filter));
}

---

If you make the Name property virtual you can override it in a derived class if the name of the filter cannot be derived from its type name:

public class OrdersFilter : Filter
{
 public override string Name => "OrderIdList";
 public override string ToString()
 {
 ...
 }
}

• Lock: How cool is that?! Such a good way to learn by getting feedback. Thankyou so much! I really like these ideas.
• Lock: Just a question.. Why is the implicit operator string have to be static?
• t3chb0t: @Lock I'm sorry, I don't have an explanation for that other than the language requires it that way :-( sometimes it's disturbing but I don't know the design reason. However I think there is probably a logical answer somewhere :-)
• Lock: Is there a way to put in the abstract class an override of the ToString for Date types so it always does 'ToString("yyyy-MM-ddTHH:mm:ss+00:00");' on the date object?