let clause (C# Reference)

In a query expression, it is sometimes useful to store the result of a sub-expression in order to use it in subsequent clauses. You can do this with the let keyword, which creates a new range variable and initializes it with the result of the expression you supply. Once initialized with a value, the range variable cannot be used to store another value. However, if the range variable holds a queryable type, it can be queried.

In the following example let is used in two ways:

  1. To create an enumerable type that can itself be queried.

  2. To enable the query to call ToLower only one time on the range variable word. Without using let, you would have to call ToLower in each predicate in the where clause.

class LetSample1
{
    static void Main()
    {
        string[] strings = 
        {
            "A penny saved is a penny earned.",
            "The early bird catches the worm.",
            "The pen is mightier than the sword." 
        };

        // Split the sentence into an array of words 
        // and select those whose first letter is a vowel. 
        var earlyBirdQuery =
            from sentence in strings
            let words = sentence.Split(' ')
            from word in words
            let w = word.ToLower()
            where w[0] == 'a' || w[0] == 'e'
                || w[0] == 'i' || w[0] == 'o'
                || w[0] == 'u'
            select word;

        // Execute the query. 
        foreach (var v in earlyBirdQuery)
        {
            Console.WriteLine("\"{0}\" starts with a vowel", v);
        }

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
/* Output:
    "A" starts with a vowel
    "is" starts with a vowel
    "a" starts with a vowel
    "earned." starts with a vowel
    "early" starts with a vowel
    "is" starts with a vowel
*/
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