Question

What is the difference between .Select() and .Where() in Entity Framework?
Eg

return ContextSet().Select(x=> x.FirstName == "John")

vs

ContextSet().Where(x=> x.FirstName == "John")

When should I use .Select vs .Where?

Answer 1

Select is a projection, so what you get is the expression x=> x.FirstName == "John" evaluated for each element in ContextSet() on the server. i.e. lots of true/false values (the same number as your original list). If you look the select will return something like IEnumerable<bool> (because the type of x=> x.FirstName == "John" is a bool).

Where filters the results, returning an enumerable of the original type (no projection).

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So, use Select when you want to keep all results, but change their type (project them).

Use Where when you want to filter your results, keeping the original type