using LINQ to find the cumulative sum of an array of numbers in C#

There’s a time for generality, and there’s a time for solving the problem actually posed. This is one of the latter times. If you want to make a method that turns a sequence of doubles into a sequence of partial sums, then just do that:

public static IEnumerable<double> CumulativeSum(this IEnumerable<double> sequence)
{
    double sum = 0;
    foreach(var item in sequence)
    {
        sum += item;
        yield return sum;
    }        
}

Easy. No messing around with aggregates and complicated queries and whatnot. Easy to understand, easy to debug, easy to use:

textBox_f.Text
    .Split(new char[]{','})
    .Select(s => double.Parse(s))
    .CumulativeSum()
    .ToArray();

Now, I note that if that is user input then double.Parse can throw an exception; it might be a better idea to do something like:

public static double? MyParseDouble(this string s)
{
    double d;
    if (double.TryParse(s, out d))
        return d;
    return null;
}

public static IEnumerable<double?> CumulativeSum(this IEnumerable<double?> sequence)
{
    double? sum = 0;
    foreach(var item in sequence)
    {
        sum += item;
        yield return sum;
    }        
}
...
textBox_f.Text
    .Split(new char[]{','})
    .Select(s => s.MyParseDouble())
    .CumulativeSum()
    .ToArray();

and now you don’t get an exception if the user makes a typing mistake; you get nulls.