Can I use LINQ to retrieve only “on change” values?

Question

What I\'d like to be able to do is construct a LINQ query that retrieved me a few values from some DataRows when one of the fields changes. Here\'s a contrived example to illus

Answer 1

This is one of those instances where the iterative solution is actually better than the set-based solution in terms of both readability and performance. All you really want Linq to do is filter and pre-sort the list if necessary to prepare it for the loop.

It is possible to write a query in SQL Server (or various other databases) using windowing functions (ROW_NUMBER), if that's where your data is coming from, but very difficult to do in pure Linq without making a much bigger mess.

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If you're just trying to clean the code up, an extension method might help:

public static IEnumerable Changed(this IEnumerable items,
    Func equalityFunc)
{
    if (equalityFunc == null)
    {
        throw new ArgumentNullException("equalityFunc");
    }
    T last = default(T);
    bool first = true;
    foreach (T current in items)
    {
        if (first || !equalityFunc(current, last))
        {
            yield return current;
        }
        last = current;
        first = false;
    }
}

Then you can call this with:

var changed = rows.Changed((r1, r2) =>
    r1.Field("Observation") == r2.Field("Observation"));