Better way to check if a Path is a File or a Directory?

public bool IsDirectory(string path) {
    return string.IsNullOrEmpty(Path.GetFileName(path)) || Directory.Exists(path);
}

Checks if the path file name is an empty string, or if the directory exists. This way you won't have the file attributes error while still providing redundancies for a possible exists failure.

I see, I'm 10 years too late to the party. I was facing the situation, where from some property I can receive either a file name or a full file path. If there is no path provided, I have to check the file-existence by attaching a "global" directory-path provided by another property.

In my case

var isFileName = System.IO.Path.GetFileName (str) == str;

did the trick. Ok, it's not magic, but perhaps this could save someone a few minutes of figuring out. Since this is merely a string-parsing, so Dir-names with dots may give false positives...

Maybe for UWP C#

public static async Task<IStorageItem> AsIStorageItemAsync(this string iStorageItemPath)
    {
        if (string.IsNullOrEmpty(iStorageItemPath)) return null;
        IStorageItem storageItem = null;
        try
        {
            storageItem = await StorageFolder.GetFolderFromPathAsync(iStorageItemPath);
            if (storageItem != null) return storageItem;
        } catch { }
        try
        {
            storageItem = await StorageFile.GetFileFromPathAsync(iStorageItemPath);
            if (storageItem != null) return storageItem;
        } catch { }
        return storageItem;
    }

Very late to the party here but I've found the Nullable<Boolean> return value to be quite ugly - IsDirectory(string path) returning null doesn't equate to a non-existent path without verbose commenting, so I've come up with the following:

public static class PathHelper
{
    /// <summary>
    /// Determines whether the given path refers to an existing file or directory on disk.
    /// </summary>
    /// <param name="path">The path to test.</param>
    /// <param name="isDirectory">When this method returns, contains true if the path was found to be an existing directory, false in all other scenarios.</param>
    /// <returns>true if the path exists; otherwise, false.</returns>
    /// <exception cref="ArgumentNullException">If <paramref name="path"/> is null.</exception>
    /// <exception cref="ArgumentException">If <paramref name="path"/> equals <see cref="string.Empty"/></exception>
    public static bool PathExists(string path, out bool isDirectory)
    {
        if (path == null) throw new ArgumentNullException(nameof(path));
        if (path == string.Empty) throw new ArgumentException("Value cannot be empty.", nameof(path));

        isDirectory = Directory.Exists(path);

        return isDirectory || File.Exists(path);
    }
}

This helper method is written to be verbose and concise enough to understand the intent the first time you read it.

/// <summary>
/// Example usage of <see cref="PathExists(string, out bool)"/>
/// </summary>
public static void Usage()
{
    const string path = @"C:\dev";

    if (!PathHelper.PathExists(path, out var isDirectory))
        return;

    if (isDirectory)
    {
        // Do something with your directory
    }
    else
    {
        // Do something with your file
    }
}

Just adding a fringe case - "Folder Selection." in path

In my app I get recently opened paths passed to me, some of which have "Folder Selection." at the end.

Some FileOpenDialogs and WinMerge add "Folder Selection." to paths (it's true).

But under Windows OS "Folder Selection." is not an advised file or folder name (as in don't do it, ever - shakes fist). As said here: http://msdn.microsoft.com/en-us/library/aa365247%28VS.85%29.aspx

Do not end a file or directory name with a space or a period. Although the underlying file system may support such names, the Windows shell and user interface does not. However, it is acceptable to specify a period as the first character of a name. For example, ".temp".

So whilst "Folder Selection." shouldn't be used, it can be. (awesome).

Enough explanation - my code (I like enums a lot):

public static class Utility
{
    public enum ePathType
    {
        ePathType_Unknown = 0,
        ePathType_ExistingFile = 1,
        ePathType_ExistingFolder = 2,
        ePathType_ExistingFolder_FolderSelectionAdded = 3,
    }

    public static ePathType GetPathType(string path)
    {
        if (File.Exists(path) == true) { return ePathType.ePathType_ExistingFile; }
        if (Directory.Exists(path) == true) { return ePathType.ePathType_ExistingFolder; }

        if (path.EndsWith("Folder Selection.") == true)
        {
            // Test the path again without "Folder Selection."
            path = path.Replace("\\Folder Selection.", "");
            if (Directory.Exists(path) == true)
            {
                // Could return ePathType_ExistingFolder, but prefer to let the caller known their path has text to remove...
                return ePathType.ePathType_ExistingFolder_FolderSelectionAdded;
            }
        }

        return ePathType.ePathType_Unknown;
    }
}