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copied!<p>There is no simple way to do this. You could workaround this by defining a C# type that can store either one value or a sequence of values - using the F# notation it would be:</p> <pre><code>type EnumerationResult<'a> = | One of 'a | Seq of seq<'a> </code></pre> <p>(translate this to C# in any way you like :-))</p> <p>Now, you could write something like:</p> <pre><code>static IEnumerable<EnumerationResult<string>> Visitor (string root, string filter) { foreach (var file in Directory.GetFiles(root, filter)) yield return EnumerationResult.One(file); foreach (var subdir in Directory.GetDirectories(root)) yield return EnumerationResult.Seq(Visitor(subdir, filter)) } } </code></pre> <p>To use it, you'd have to write a function that flattens EnumerationResult, which could be an extension method in C# with the following signature:</p> <pre><code>IEnumerable<T> Flatten(this IEnumerable<EnumerationResult<T>> res); </code></pre> <p>Now, this is a part where it gets tricky - if you implemented this in a straighforward way, it would still contain "forach" to iterate over the nested "Seq" results. However, I believe that you could write an optimized version that wouldn't have quadratic complexity.</p> <p>Ok.. I guess this is a topic for a blog post rather than something that could be fully described here :-), but hopefully, it shows an idea that you can try following!</p> <p><em>[EDIT: But of course, you can also use naive implementation of "Flatten" that would use "SelectMany" just to make the syntax of your C# iterator code nicer]</em></p>

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