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// So you want to use anonymous interface implementations in C#? Does this lambda-based usage look ok?
// Ignore the fact that "list" and "projection" are undefined here, just note the lambdas that can close
// around any variables and procedures you want.
new AnonymousReadOnlyList<TOut>(
count: () => list.Count,
item: i => projection(list[i]),
iterator: list.AsEnumerable().Select(projection));
new AnonymousReadOnlyList<TOut>(
count: () => list.Count,
item: i => projection(list[i], i),
iterator: list.AsEnumerable().Select(projection));
new AnonymousReadOnlyList<T>(
count: () => list.Count,
item: i => list[list.Count - 1 - i]);
new AnonymousReadOnlyList<TOut>(
count: () => Math.Min(list1.Count, list2.Count),
item: i => projection(list1[i], list2[i]),
iterator: list1.AsEnumerable().Zip(list2, projection));
new AnonymousReadOnlyList<int>(
count: () => count,
item: i => i,
iterator: Enumerable.Range(0, count));
// ... and so on. This is how:
// 1. Define your interface.
public interface IReadOnlyList<out T> : IReadOnlyCollection<T> {
T this[int index] { get; }
// 2. Provide a single not-so-concrete implementation for all anonymous implementations to use.
// This also shows how to provide "default methods" - GetEnumerator() here.
public sealed class AnonymousReadOnlyList<T> : IReadOnlyList<T> {
private readonly Func<int> _count;
private readonly Func<int, T> _item;
private readonly IEnumerable<T> _iterator;
public AnonymousReadOnlyList(Func<int> count, Func<int, T> item, IEnumerable<T> iterator = null) {
if (count == null) throw new ArgumentNullException("count");
if (item == null) throw new ArgumentNullException("item");
this._count = count;
this._item = item;
this._iterator = iterator ?? DefaultIterator(count, item);
private static IEnumerable<T> DefaultIterator(Func<int> count, Func<int, T> item) {
var n = count();
for (var i = 0; i < n; i++)
yield return item(i);
public int Count {
get {
return _count();
public T this[int index] {
get {
return _item(index);
public IEnumerator<T> GetEnumerator() {
return _iterator.GetEnumerator();
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() {
return GetEnumerator();
// 3. Now you can create anonymous implementations with lambdas like the ones at the beginning of this gist!
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