How to use LINQ Contains() to find a list of enums

问题:

I have an enum called OrderStatus, and it contains various statuses that an Order can be in:

• Created
• Pending
• Waiting
• Valid
• Active
• Processed
• Completed

What I want to do is create a LINQ statement that will tell me if the OrderStaus is Valid, Active, Processed or Completed.

Right now I have something like:

var status in Order.Status.WHERE(status => 
      status.OrderStatus == OrderStatus.Valid || 
      status.OrderStatus == OrderStatus.Active|| 
      status.OrderStatus == OrderStatus.Processed|| 
      status.OrderStatus == OrderStatus.Completed)

That works, but it's very "wordy". Is there a way to convert this to a Contains() statement and shorten it up a bit?

回答1:

Sure:

var status in Order.Status.Where(status => new [] {
        OrderStatus.Valid, 
        OrderStatus.Active, 
        OrderStatus.Processed,
        OrderStatus.Completed
    }.Contains(status.OrderStatus));

You could also define an extension method In() that would accept an object and a params array, and basically wraps the Contains function:

public static bool In<T>(this T theObject, params T[] collection)
{
    return collection.Contains(theObject);
}

This allows you to specify the condition in a more SQL-ish way:

var status in Order.Status.Where(status => 
    status.OrderCode.In(
        OrderStatus.Valid, 
        OrderStatus.Active, 
        OrderStatus.Processed,
        OrderStatus.Completed));

Understand that not all Linq providers like custom extension methods in their lambdas. NHibernate, for instance, won't correctly translate the In() function without additional coding to extend the expression parser, but Contains() works just fine. For Linq 2 Objects, no problems.

回答2:

I have used this extension:

    public static bool IsIn<T>(this T value, params T[] list)
    {

                     return list.Contains(value);           
    }

You may use this as the condition:

   Where(x => x.IsIn(OrderStatus.Valid, ... )

回答3:

If that set of statuses has some meaning, for example those are statuses for accepted orders, you can define an extension method on your enum and use that in your linq query.

public static class OrderStatusExtensions
{
    public static bool IsAccepted(this OrderStatuses status)
    {
        return status == OrderStatuses.Valid
            || status == OrderStatuses.Active
            || status == OrderStatuses.Processed
            || status == OrderStatuses.Completed;
    }
}

var acceptedOrders = from o in orders
                     where o.Status.IsAccepted()
                     select o;

Even if you could not give the method a simple name, you could still use something like IsValidThroughCompleted. In either case, it seems to convey a little more meaning this way.

回答4:

Assumnig that the enum is defined in the order you specified in the question, you could shorten this by using an integer comparison.

var result = 
  Order.Status.Where(x => 
    (int)x >= (int)OrderStatus.Valid &
    & (int)x <= (int)OrderStatus.Completed);

This type of comparison though can be considered flaky. A simply re-ordering of enumeration values would silently break this comparison. I would prefer to stick with the more wordy version and probably clean up it up by refactoring out the comparison to a separate method.

回答5:

You could put these in a collection, and use:

OrderStatus searchStatus = new[] {
                OrderStatus.Valid,
                OrderStatus.Active,
                OrderStatus.Processed,
                OrderStatus.Completed };

var results = Order.Status.Where(status => searchStatus.Contains(status));