Question

I have the following LINQ Select which does not work.

Data.Select(d => d.Value.IsDirty = true); //-- Not working

My longer workaround does.

foreach (var d in Data)
    d.Value.IsDirty = true;

Why does my first code not work?

Answer 1

Select() returns an IEnumerable<…>, which has the ability to iterate over the input and invoke the code in question, but doesn’t actually do so until you enumerate it in some manner:

Data.Select(d => d.Value.IsDirty = true).ToList();

or

foreach (var _ in Data.Select(d => d.Value.IsDirty = true))
    ; // Do nothing

However, given that they perform side effects (obviously the intent here), both of the above are bad karma. Don’t use them. Your original foreach is the only sensible choice.