In C#, how do I combine more than two parts of a file path at once?

Here's a utility method you can use:

public static string CombinePaths(string path1, params string[] paths)
{
    if (path1 == null)
    {
        throw new ArgumentNullException("path1");
    }
    if (paths == null)
    {
        throw new ArgumentNullException("paths");
    }
    return paths.Aggregate(path1, (acc, p) => Path.Combine(acc, p));
}

Alternate code-golf version (shorter, but not quite as clear, semantics are a bit different from Path.Combine):

public static string CombinePaths(params string[] paths)
{
    if (paths == null)
    {
        throw new ArgumentNullException("paths");
    }
    return paths.Aggregate(Path.Combine);
}

Then you can call this as:

string path = CombinePaths(path1, path2, path3);

As others have said, in .NET 3.5 and earlier versions there hasn't been a way to do this neatly - you either have to write your own Combine method or call Path.Combine multiple times.

But rejoice - for in .NET 4.0, there is this overload:

public static string Combine(
    params string[] paths
)

There are also overloads taking 3 or 4 strings, presumably so that it doesn't need to create an array unnecessarily for common cases.

Hopefully Mono will port those overloads soon - I'm sure they'd be easy to implement and much appreciated.

Not simple, but clever :)

string str1 = "aaa", str2 = "bbb", str3 = "ccc";
string comb = new string[] { str1, str2, str3 }
    .Aggregate((x, y) => System.IO.Path.Combine(x, y));

Or:

string CombinePaths(params string[] paths)
{
    return paths.Aggregate((x,y) => System.IO.Path.Combine(x, y));
}

EDIT Order23's answer is actually up to date with current .NET https://stackoverflow.com/a/41148772/235648

Nope - you have to call Path.Combine() several times.

You could write a helper method that does it for you, though:

public static string CombinePaths(params string[] paths) {
    if (paths == null) {
        return null;
    }
    string currentPath = paths[0];
    for (int i = 1; i < paths.Length; i++) {
        currentPath = Path.Combine(currentPath, paths[i]);
    }
    return currentPath;
}