performance - C# Merging Two or more Text Files side by side

Question

using (StreamWriter writer = File.CreateText(FinishedFile))
{
    int lineNum = 0;
    while (lineNum < FilesLineCount.Min())
    {
        for (int i = 0; i <= FilesToMerge.Count() - 1; i++)
        {
            if (i != FilesToMerge.Count() - 1)
            {
                var CurrentFile = File.ReadLines(FilesToMerge[i]).Skip(lineNum).Take(1);
                string CurrentLine = string.Join("", CurrentFile);
                writer.Write(CurrentLine + ",");
            }
            else
            {
                var CurrentFile = File.ReadLines(FilesToMerge[i]).Skip(lineNum).Take(1);
                string CurrentLine = string.Join("", CurrentFile);
                writer.Write(CurrentLine + "
");
            }
        }
        lineNum++;
    }
}

The current way i am doing this is just too slow. I am merging files that are each 50k+ lines long with various amounts of data.

for ex: File 1
1
2
3
4

File 2
4
3
2
1

i need this to merge into being a third file
File 3
1,4
2,3
3,2
4,1

P.S. The user can pick as many files as they want from any locations.
Thanks for the help.

Answer 1

You approach is slow because of the Skip and Take in the loops.

You could use a dictionary to collect all line-index' lines:

string[] allFileLocationsToMerge = { "filepath1", "filepath2", "..." };
var mergedLists = new Dictionary<int, List<string>>();
foreach (string file in allFileLocationsToMerge)
{
    string[] allLines = File.ReadAllLines(file);
    for (int lineIndex = 0; lineIndex < allLines.Length; lineIndex++)
    {
        bool indexKnown = mergedLists.TryGetValue(lineIndex, out List<string> allLinesAtIndex);
        if (!indexKnown)
            allLinesAtIndex = new List<string>();
        allLinesAtIndex.Add(allLines[lineIndex]);
        mergedLists[lineIndex] = allLinesAtIndex;
    }
}

IEnumerable<string> mergeLines = mergedLists.Values.Select(list => string.Join(",", list));
File.WriteAllLines("targetPath", mergeLines);