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    <p>The difference between <code>Func</code> and <code>Action</code> is simply whether you want the delegate to return a value (use <code>Func</code>) or not (use <code>Action</code>).</p> <p><code>Func</code> is probably most commonly used in LINQ - for example in projections:</p> <pre><code> list.Select(x =&gt; x.SomeProperty) </code></pre> <p>or filtering:</p> <pre><code> list.Where(x =&gt; x.SomeValue == someOtherValue) </code></pre> <p>or key selection:</p> <pre><code> list.Join(otherList, x =&gt; x.FirstKey, y =&gt; y.SecondKey, ...) </code></pre> <p><code>Action</code> is more commonly used for things like <code>List&lt;T&gt;.ForEach</code>: execute the given action for each item in the list. I use this less often than <code>Func</code>, although I <em>do</em> sometimes use the parameterless version for things like <code>Control.BeginInvoke</code> and <code>Dispatcher.BeginInvoke</code>.</p> <p><code>Predicate</code> is just a special cased <code>Func&lt;T, bool&gt;</code> really, introduced before all of the <code>Func</code> and most of the <code>Action</code> delegates came along. I suspect that if we'd already had <code>Func</code> and <code>Action</code> in their various guises, <code>Predicate</code> wouldn't have been introduced... although it <em>does</em> impart a certain meaning to the use of the delegate, whereas <code>Func</code> and <code>Action</code> are used for widely disparate purposes.</p> <p><code>Predicate</code> is mostly used in <code>List&lt;T&gt;</code> for methods like <code>FindAll</code> and <code>RemoveAll</code>.</p>
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    1. COI rather like seeing `Predicate` in a function signature. It illustrates that the passed method makes a decision rather than just returns a success code or a different sort of `bool`.
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      1. USRon Warholic
    2. COIs there any design guideline on whether to prefer `Predicate<T>` or `Func<T,bool>`? I like the expressiveness of `Predicate`, but I've seen recommendations for `Func` too.
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    3. CO@Ron: Yes, that's pretty much what my penultimate paragraph was about.
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