Apply last non empty string to empty string?

Your example has one more issue but currently I'll focus only on gathering the "last non empty" string.

If you look at your example you can spot few things that could potentially help you finding solution. These are for loop and reference to original list that stays intact.

For my example I'll use Linq because it will be much easier.

First of all I'll copy all from before for loop ( if that makes sense :D ) :

var sLines = File.ReadAllLines(cboPartProgram.Text)
     .Where(s => !s.StartsWith("'"))
     .Select(s => new
     {
         SValue = Regex.Match(s, "(?<=S)[\\d.]*").Value,
     })
     .ToArray();

string LastSValue = "";
string Value = "";

Just because it's okay and will work for now.

With your for loop I'll make modifications :

for (int i = 0; i < sLines.Count(); i++)
{
    // `i` is representing current "index" of processed "word"
    // we can use this to find last "valid" element
    // string notEmpty = sLines.Take(i).LastOrDefault(word => !string.IsNullOrEmpty(word));
    // but since you want to assign this to `Value` and there can be not empty string at `i` index
    // we can make it in one line :
    Value = string.IsNullOrEmpty(sLines[i]) ? sLines.Take(i).LastOrDefault(word => !string.IsNullOrEmpty(word)) : sLines[i].SValue;
    // instead of your previous logic :
    //if (sLines[i].SValue == "")
    //{
    //    LastSValue = "***";
    //    Value = LastSValue;
    //}
    //else
    //{
    //    Value = (sLines[i].SValue);
    //}
 }

Another problem which I think you'll face is that first value ( judging by the input ) can also be empty. Which will throw exception in my example. This will also be impossible to fit this kind of solution because there's no previous value ( at all ).

From what I understand, if you want to store the result in Value and do something else with it inside the loop (instead of changing it in the array), what you probably want is this:

for (int i = 0; i < sLines.Count(); i++)
{
    if (sLines[i].SValue == "")
    {
        Value = LastSValue;
    }
    else
    {
        Value = (sLines[i].SValue);
        LastSValue = Value;
    }

    // use Value
}

I would also suggest using sLines.Length instead of Count(), which is made for sequences where the length isn't known in advance - it's supposed to literally count the elements one by one. In this case it would probably be optimized but if you know you're dealing with an array, it's a good idea to ask for the length directly.

EDIT:

To get "NA" if there's no previous number, just initialize LastSValue to this value before the loop:

string LastSValue = "NA";

That way, if Value is empty and there was not LastSValue set before, it will still be "NA".

EDIT2: A solution similar to the one from @Cubi, to change it in place:

for (int i = 0; i < sLines.Length; i++)
{
     if (sLines[i].SValue == "")
         sLines[i].SValue = i > 0 ? sLines[i-1].SValue : "NA";
}