How can we implement the `let` keyword in C#

[gordonwatts]

The let keyword is syntatic sugar, but it makes expressions a lot more readable (see LINQ documentation).

Here is an example from their documentation:

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();
    }
}

We ran into this when translating the CMS Higgs stuff - and wanted to re-write the following C++ code into LINQ:

      for (int i = 0; i < p.numberOfHits(); i++) 
	{
	  uint32_t hit = p.getHitPattern(i);
	  // if the hit is valid and in pixel
	  if (p.validHitFilter(hit) && p.pixelHitFilter(hit)) {GM_PixelHits++;}
	  if (p.validHitFilter(hit)) {GM_ValidHits++;}
	}