Unity 3d : Get Selected Folder in Project Window

gistfile1.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.IO;

public static class UnityUtil
{
	public static string GetSelectedPathOrFallback()
	{
		string path = "Assets";
		
		foreach (UnityEngine.Object obj in Selection.GetFiltered(typeof(UnityEngine.Object), SelectionMode.Assets))
		{
			path = AssetDatabase.GetAssetPath(obj);
			if ( !string.IsNullOrEmpty(path) && File.Exists(path) ) 
			{
				path = Path.GetDirectoryName(path);
				break;
			}
		}
		return path;
	}
}

Stable cross-platform solution, supports folders and files. Default path is Application.dataPath

public static string GetProjectWindowFolder()
{
	string projectPath = new DirectoryInfo(Application.dataPath).Parent.FullName;
	string objectProjectPath = AssetDatabase.GetAssetPath(Selection.activeObject);
	string objectAbsolutePath = string.IsNullOrEmpty(objectProjectPath) ? Application.dataPath : $"{projectPath}/{objectProjectPath}";
	string objectCorrectAbsolutePath = objectAbsolutePath.Replace('/', Path.DirectorySeparatorChar).Replace('\\', Path.DirectorySeparatorChar);
	string folderAbsolutePath = File.Exists(objectCorrectAbsolutePath) ? Path.GetDirectoryName(objectCorrectAbsolutePath) : objectCorrectAbsolutePath;
	return Path.GetRelativePath(projectPath, folderAbsolutePath);
}