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copied!<p>So, to start off with the scary definition - a lambda is another way of defining an anonymous method. There has (since C# 2.0 I believe) been a way to construct anonymous methods - however that syntax was very... inconvinient.</p> <p>So what is an anonymous method? It is a way of defining a method inline, with no name - hence being anonymous. This is useful if you have a method that takes a delegate, as you can pass this lambda expression / anonymous method as a parameter, given that the types match up. Take IEnumerable.Select as an example, it is defined as follows:</p> <pre><code>IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector); </code></pre> <p>If you were to use this method normally on say a List and select each element twice (that is concatenated to itself):</p> <pre><code>string MyConcat(string str){ return str + str; } ... void myMethod(){ IEnumerable<string> result = someIEnumerable.Select(MyConcat); } </code></pre> <p>This is a very inconvinient way of doing this, especially if you wish to perform many of these operations - also typically these kinds of methods you only use once. The definition you showed (parameters => expression) is very consise and hits it spot on. The interesting thing about lambda is that you do not need to express the type of the parameters - <strong>as long as they can be infered from the expression</strong>. Ie. in Select's case - we <em>know</em> the first parameter must be of type TSource - because the definition of the method states so. Further more - if we call the method just as foo.Select(...) then the return value of the expression will define TResult. </p> <p>An interesting thing as well is that for one-statement lambda's the return keyword is not needed - the lambda will return whatever that one expression evaluates to. However if you use a block (wrapped in '{' and '}') then you must include the return keyword like usual.</p> <p>If one wishes to, it is still 100% legal to define the types of the parameters. With this new knowledge, let's try and rewrite the previous example:</p> <pre><code>void myMethod(){ IEnumerable<string> result = someIEnumerable.Select(s => s + s); } </code></pre> <p>Or, with explicit parameters stated</p> <pre><code>void myMethod(){ IEnumerable<string> result = someIEnumerable.Select((string s) => s + s); } </code></pre> <p>Another interesting feature of lambda's in C# is their use to construct <a href="http://msdn.microsoft.com/en-us/library/bb397951.aspx" rel="nofollow noreferrer">expression trees</a>. That is probably not "beginner" material though - but in short an expression tree contains all the meta data about a lambda, rather than executable code.</p>

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