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.