Let there be LINQ


Just a quick post regarding use of the let keyword in LINQ, which I find to be somewhat under-used by many people. Whilst one benefit of it can be readability (i.e. aliasing sections of complex queries to aid understanding of the query), the other benefit can be performance.

There is indeed a cost associated with using it i.e. every time you use it, you’re effectively creating a new anonymous type to hold that plus whatever the previous result in your query pipeline. So if you chain up lots of lets in a query, that’ll have an impact on the query. However, there is a case where let can give large performance benefits: –

image

Compare that code with the following: –

image

This will eliminate a massive number of calls to Convert.ToInt32() and reduce the time taken to process that query by around 40%; the former sample took ~1400ms to run whereas the latter took only around 800ms.

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