Question

How can I intelligently create a List<int> from an IEnumerable<CustomClass>, if CustomClass has a property of type int, called customProperty which I wish to use to create my List<int> ?

I know I can create a List<CustomClass> by calling instance.ToList<CustomClass>();

How can I extract the property info to populate my list? What is best practice here ?

Answer 1

Using Linq, you can project to the property and ToList() the resulting IEnumerable<int>:

var ints = customClasses.Select(c => c.customProperty).ToList();

If you have never seen Linq extension methods before, Select is a common “projection” method. It takes a lambda expression, in this case an expression taking a type CustomClass represented as c, it returns whatever you want it to, in this case an int. As Select is working on a set (enumerable of something) you actually get an IEnumerable<int> in response, it can infer the type of IEnumerable based on your Select lambda.

If you want a delay-run IEnumerable<int>, simply drop the ToList() call.

I never like to impart opinion on best practice, but this is a clean one-liner that clearly shows intent (assuming basic understanding of Linq) and I see it all over the place.

Linq Resources:

http://www.codeproject.com/Articles/84521/LINQ-for-the-Beginner

http://www.codeproject.com/Articles/19154/Understanding-LINQ-C

Lambda Expression Resources:

http://msdn.microsoft.com/en-us/library/bb397687.aspx

Alternatively, these topics are very easily searched with terms like “LINQ tutorials” or “lambda expression tutorials”.