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copied!<p>If you're looking to <em>conditionally</em> project a Foo to another Foo (leaving others untouched), you can do something like:</p> <pre><code>IEnumerable<Foo> foos = ... var transformed = foos.Select(foo => myCondition(foo) ? transform(foo) : foo); </code></pre> <p>On the other hand, if you only want to project Foos that match the condition:</p> <pre><code>var transformed = foos.Where(foo => myCondition(foo)) .Select(foo => transform(foo)); </code></pre> <p>Do note that both of these return <em>new</em> sequences - LINQ isn't normally used to <em>modify</em> existing collections. You could of course materialize the results into a collection, overwriting an existing variable if necessary.</p> <pre><code>// assuming the transform is from Foo -> Foo foos = foos.Select(foo => transform(foo)).ToList(); </code></pre> <p>Since you specifically mention <em>lists</em>, there is another non-LINQ immediate-execution alternative to the first query - the <a href="http://msdn.microsoft.com/en-us/library/73fe8cwf.aspx" rel="noreferrer"><strong><code>List<T>.ConvertAll</code></strong></a> method:</p> <pre><code>List<Foo> foos = ... // implicitly List<Foo> assuming the transform is from Foo -> Foo var transformed = foos.ConvertAll (foo => myCondition(foo) ? transform(foo) : foo); </code></pre> <hr> <p><strong>EDIT</strong>: Sounds like you're looking for a "ReplaceWhere" method - as far as I know, there is no <em>direct</em> framework method that replaces the elements of a list based on a predicate. It's easy to write one yourself though:</p> <pre><code>/// <summary> /// Replaces items in a list that match the specified predicate, /// based on the specified selector. /// </summary> public static void ReplaceWhere<T>(this IList<T> list, Func<T, bool> predicate, Func<T, T> selector) { // null-checks here. for (int i = 0; i < list.Count; i++) { T item = list[i]; if (predicate(item)) list[i] = selector(item); } } </code></pre> <p><strong>Usage</strong>:</p> <pre><code>List<int> myList = ... myList.ReplaceWhere(i => i > 0, i => i * i); </code></pre>

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