Average extension method in Linq for default value

Question

Anyone know how I can set a default value for an average? I have a line like this...

dbPlugins = (from p in dbPlugins                 select new { Plugin = p, AvgScore = p.DbVersions.Average(x => x.DbRatings.Average(y => y.Score)) })             .OrderByDescending(x => x.AvgScore)             .Select(x => x.Plugin).ToList(); 

which throws an error becase I have no ratings yet. If I have none I want the average to default to 0. I was thinking this should be an extension method where I could specify what the default value should be.

Answer 1

There is: DefaultIfEmpty.

I 'm not sure about what your DbVersions and DbRatings are and which collection exactly has zero items, but this is the idea:

var emptyCollection = new List<int>(); var average = emptyCollection.DefaultIfEmpty(0).Average(); 

Update: (repeating what's said in the comments below to increase visibility)

If you find yourself needing to use DefaultIfEmpty on a collection of class type, remember that you can change the LINQ query to project before aggregating. For example:

class Item {     public int Value { get; set; } }  var list = new List<Item>(); var avg = list.Average(item => item.Value); 

If you don't want to/can not construct a default Item with Value equal to 0, you can project to a collection of ints first and then supply a default:

var avg = list.Select(item => item.Value).DefaultIfEmpty(0).Average(); 

Answer 2

My advice would to create a reusable solution instead of a solution for this problem only.

Make an extension method AverageOrDefault, similar to FirstOrDefault. See extension methods demystified

public static class MyEnumerableExtensions {     public static double AverageOrDefault(this IEnumerable<int> source)     {         // TODO: decide what to do if source equals null: exception or return default?         if (source.Any())             return source.Average();         else             return default(int);     } } 

There are 9 overloads of Enumerable.Average, so you'll need to create an AverageOrDefault for double, int?, decimal, etc. They all look similar.

Usage:

// Get the average order total or default per customer var averageOrderTotalPerCustomer = myDbContext.Customers     .GroupJoin(myDbContext.Orders,     customer => customer.Id,     order => order.CustomerId,     (customer, ordersOfThisCustomer) => new     {         Id = customer.Id,         Name  = customer.Name,         AverageOrder = ordersOfThisCustomer.AverageOrDefault(),     });