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copied!<p><a href="http://weblogs.asp.net/scottgu/archive/2007/04/08/new-orcas-language-feature-lambda-expressions.aspx" rel="noreferrer">Lambda expressions</a> are a simpler syntax for anonymous delegates and can be used everywhere an anonymous delegate can be used. However, the opposite is not true; lambda expressions can be converted to expression trees which allows for a lot of the magic like LINQ to SQL. </p> <p>The following is an example of a <a href="http://msdn.microsoft.com/en-us/library/bb397919.aspx" rel="noreferrer">LINQ to Objects</a> expression using anonymous delegates then lambda expressions to show how much easier on the eye they are:</p> <pre><code>// anonymous delegate var evens = Enumerable .Range(1, 100) .Where(delegate(int x) { return (x % 2) == 0; }) .ToList(); // lambda expression var evens = Enumerable .Range(1, 100) .Where(x => (x % 2) == 0) .ToList(); </code></pre> <p>Lambda expressions and anonymous delegates have an advantage over writing a separate function: they implement <a href="http://en.wikipedia.org/wiki/Closure_(computer_science)" rel="noreferrer">closures</a> which can allow you to <a href="http://www.srtsolutions.com/Looking-Inside-C-Closures" rel="noreferrer">pass local state to the function without adding parameters</a> to the function or creating one-time-use objects.</p> <p><a href="http://www.interact-sw.co.uk/iangblog/2005/09/30/expressiontrees" rel="noreferrer">Expression trees</a> are a very powerful new feature of C# 3.0 that allow an API to look at the structure of an expression instead of just getting a reference to a method that can be executed. An API just has to make a delegate parameter into an <code>Expression<T></code> parameter and the compiler will generate an expression tree from a lambda instead of an anonymous delegate:</p> <pre><code>void Example(Predicate<int> aDelegate); </code></pre> <p>called like:</p> <pre><code>Example(x => x > 5); </code></pre> <p>becomes:</p> <pre><code>void Example(Expression<Predicate<int>> expressionTree); </code></pre> <p>The latter will get passed a representation of the <a href="http://en.wikipedia.org/wiki/Abstract_syntax_tree" rel="noreferrer">abstract syntax tree</a> that describes the expression <code>x > 5</code>. LINQ to SQL relies on this behavior to be able to turn C# expressions in to the SQL expressions desired for filtering / ordering / etc. on the server side.</p>

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