Find common parent-path in list of files and directories

First sort the list with the paths to inspect. Then you can split and compare the first and the last item - if they are same proceed to the next dimension until you find a difference.

So you just need to sort once and then inspect two items.

A function to get the longest common prefix may look like this:

public static string GetLongestCommonPrefix(string[] s)
{
    int k = s[0].Length;
    for (int i = 1; i < s.Length; i++)
    {
        k = Math.Min(k, s[i].Length);
        for (int j = 0; j < k; j++)
            if (s[i][j] != s[0][j])
            {
                k = j;
                break;
            }
    }
    return s[0].Substring(0, k);
}

Then you may need to cut the prefix on the right hand. E.g. we want to return c:/dir instead of c:/dir/file for

c:/dir/file1
c:/dir/file2

You also may want to normalize the paths before processing. See Normalize directory names in C#.

I dont know whether this is the best performing solution (probably not), but it surely is very easy to implement.

• Sort your list alphabetically
• compare the first entry in that sorted list to the last in that list, character by character, and terminate when you find a difference (the value before the termination is the longest shared substring of both those strings)

Sample Fiddle

Sample code:

List<string> paths = new List<string>();

paths.Add(@"C:/Hello/World/This/Is/An/Example/Bla.cs");
paths.Add(@"C:/Hello/World/This/Is/Not/An/Example/");
paths.Add(@"C:/Hello/Earth/Bla/Bla/Bla");

List<string> sortedPaths = paths.OrderBy(s => s).ToList();

Console.WriteLine("Most common path here: {0}", sharedSubstring(sortedPaths[0], sortedPaths[sortedPaths.Count - 1]));

And that function of course:

public static string sharedSubstring(string string1, string string2)
{
    string ret = string.Empty;

    int index = 1;
    while (string1.Substring(0, index) == string2.Substring(0, index))
    {
        ret = string1.Substring(0, index);
        index++;
    }

    return ret;
} // returns an empty string if no common characters where found