Question

It’s recently been pointed out to me that various Linq extension methods (such as Where, Select, etc) return an IEnumerable<T> that also happens to be IDisposable. The following evaluates to True

new int[2] {0,1}.Select(x => x*2) is IDisposable

Do I need to dispose of the results of a Where expression?

Whenever I call a method returning IEnumerable<T>, am I (potentially) accepting responsibility for calling dispose when I’ve finished with it?

Answer 1

No, you don’t need to worry about this.

The fact that they return an IDisposable implementation is an implementation detail – it’s because iterator blocks in the Microsoft implementation of the C# compiler happen to create a single type which implements both IEnumerable<T> and IEnumerator<T>. The latter extends IDisposable, which is why you’re seeing it.

Sample code to demonstrate this:

using System;
using System.Collections.Generic;

public class Test 
{
    static void Main() 
    {
        IEnumerable<int> foo = Foo();
        Console.WriteLine(foo is IDisposable); // Prints True
    }

    static IEnumerable<int> Foo()
    {
        yield break;
    }
}

Note that you do need to take note of the fact that IEnumerator<T> implements IDisposable. So any time you iterate explicitly, you should dispose of it properly. For example, if you want to iterate over something and be sure that you’ll always have a value, you might use something like:

using (var enumerator = enumerable.GetEnumerator())
{
    if (!enumerator.MoveNext())
    {
        throw // some kind of exception;
    }
    var value = enumerator.Current;
    while (enumerator.MoveNext())
    {
        // Do something with value and enumerator.Current
    }
}

(A foreach loop will do this automatically, of course.)