(GH-853) Handle null values better

When a path is empty or null, handle it with either better error
messaging or work around the issue.

- Do not allow combining paths when the left side is null.
- When attempting to get files from a directory, if the directory value
   is empty, do not perform the search.
- When splitting strings, attempt to always remove empty entries.
- Check null or empty for strings

src/chocolatey/infrastructure.app/configuration/EnvironmentSettings.cs

@@ -198,12 +198,14 @@ public static void update_environment_variables()
 
         private static IDictionary convert_to_case_insensitive_dictionary(IDictionary originalDictionary)
         {
-            return new Hashtable(originalDictionary, StringComparer.InvariantCultureIgnoreCase);
+            if (originalDictionary == null) return new Hashtable(new Dictionary<string, string>(), StringComparer.OrdinalIgnoreCase);
+
+            return new Hashtable(originalDictionary, StringComparer.OrdinalIgnoreCase);
         }
 
         private static void refresh_environment_variables(IDictionary environmentVariables)
         {
-            foreach (DictionaryEntry variable in environmentVariables)
+            foreach (DictionaryEntry variable in environmentVariables.or_empty_list_if_null())
             {
                 Environment.SetEnvironmentVariable(variable.Key.to_string(), variable.Value.to_string());
             }

src/chocolatey/infrastructure.app/services/ChocolateyPackageService.cs

@@ -809,7 +809,7 @@ private void uninstall_cleanup(ChocolateyConfiguration config, PackageResult pac
 
             if (config.Force)
             {
-                var packageDirectory = _fileSystem.combine_paths(packageResult.InstallLocation);
+                var packageDirectory = packageResult.InstallLocation;
 
                 if (string.IsNullOrWhiteSpace(packageDirectory) || !_fileSystem.directory_exists(packageDirectory)) return;
 
@@ -1010,7 +1010,7 @@ private void move_bad_package_to_failure_location(PackageResult packageResult)
         {
             _fileSystem.create_directory_if_not_exists(ApplicationParameters.PackageFailuresLocation);
 
-            if (packageResult.InstallLocation != null && _fileSystem.directory_exists(packageResult.InstallLocation))
+            if (!string.IsNullOrWhiteSpace(packageResult.InstallLocation) && _fileSystem.directory_exists(packageResult.InstallLocation))
             {
                 FaultTolerance.try_catch_with_logging_exception(
                  () => _fileSystem.move_directory(packageResult.InstallLocation, packageResult.InstallLocation.Replace(ApplicationParameters.PackagesLocation, ApplicationParameters.PackageFailuresLocation)),

src/chocolatey/infrastructure.app/services/NugetService.cs

@@ -328,7 +328,7 @@ public ConcurrentDictionary<string, PackageResult> install_run(ChocolateyConfigu
 
             //todo: handle all
 
-            SemanticVersion version = config.Version != null ? new SemanticVersion(config.Version) : null;
+            SemanticVersion version = !string.IsNullOrWhiteSpace(config.Version) ? new SemanticVersion(config.Version) : null;
             if (config.Force) config.AllowDowngrade = true;
 
             IList<string> packageNames = config.PackageNames.Split(new[] { ApplicationParameters.PackageNamesSeparator }, StringSplitOptions.RemoveEmptyEntries).or_empty_list_if_null().ToList();
@@ -513,7 +513,7 @@ public ConcurrentDictionary<string, PackageResult> upgrade_run(ChocolateyConfigu
             _fileSystem.create_directory_if_not_exists(ApplicationParameters.PackagesLocation);
             var packageInstalls = new ConcurrentDictionary<string, PackageResult>(StringComparer.InvariantCultureIgnoreCase);
 
-            SemanticVersion version = config.Version != null ? new SemanticVersion(config.Version) : null;
+            SemanticVersion version = !string.IsNullOrWhiteSpace(config.Version) ? new SemanticVersion(config.Version) : null;
             if (config.Force) config.AllowDowngrade = true;
 
             var packageManager = NugetCommon.GetPackageManager(
@@ -1280,7 +1280,7 @@ private void set_package_names_if_all_is_specified(ChocolateyConfiguration confi
                 if (!string.IsNullOrWhiteSpace(config.UpgradeCommand.PackageNamesToSkip))
                 {
                     var packagesToSkip = config.UpgradeCommand.PackageNamesToSkip
-                        .Split(',')
+                        .Split(new [] {','}, StringSplitOptions.RemoveEmptyEntries)
                         .Where(item => !string.IsNullOrWhiteSpace(item))
                         .Select(p => p.trim_safe())
                         .ToList();

src/chocolatey/infrastructure/commandline/InteractivePrompt.cs

@@ -51,7 +51,7 @@ public static string prompt_for_confirmation(string prompt, IEnumerable<string>
                     c => c.Count() > 0,
                     (name, value) => { throw new ApplicationException("No choices passed in. Please ensure you pass choices"); });
 
-            if (defaultChoice != null)
+            if (!string.IsNullOrWhiteSpace(defaultChoice))
             {
                 Ensure
                     .that(() => choices)
@@ -117,7 +117,7 @@ public static string prompt_for_confirmation(string prompt, IEnumerable<string>
             var selection = timeoutInSeconds == 0 ? Console.ReadLine() : Console.ReadLine(timeoutInSeconds * 1000);
             if (shortPrompt) Console.WriteLine();
 
-            if (string.IsNullOrWhiteSpace(selection) && defaultChoice != null)
+            if (string.IsNullOrWhiteSpace(selection) && !string.IsNullOrWhiteSpace(defaultChoice))
             {
                 "chocolatey".Log().Info(ChocolateyLoggers.LogFileOnly, "Choosing default choice of '{0}'".format_with(defaultChoice.escape_curly_braces()));
                 return defaultChoice;

src/chocolatey/infrastructure/filesystem/DotNetFileSystem.cs

@@ -66,6 +66,8 @@ private static IEnvironment Environment
 
         public string combine_paths(string leftItem, params string[] rightItems)
         {
+            if (leftItem == null) throw new ApplicationException("Path to combine cannot be empty.");
+
             var combinedPath = Platform.get_platform() == PlatformType.Windows ? leftItem : leftItem.Replace('\\', '/');
             foreach (var rightItem in rightItems)
             {
@@ -166,11 +168,15 @@ public string get_current_assembly_path()
 
         public IEnumerable<string> get_files(string directoryPath, string pattern = "*.*", SearchOption option = SearchOption.TopDirectoryOnly)
         {
+            if (string.IsNullOrWhiteSpace(directoryPath)) return new List<string>();
+
             return Directory.EnumerateFiles(directoryPath, pattern, option);
         }
 
         public IEnumerable<string> get_files(string directoryPath, string[] extensions, SearchOption option = SearchOption.TopDirectoryOnly)
         {
+            if (string.IsNullOrWhiteSpace(directoryPath)) return new List<string>();
+
             return Directory.EnumerateFiles(directoryPath, "*.*", option)
                             .Where(f => extensions.Any(x => f.EndsWith(x, StringComparison.OrdinalIgnoreCase)));
         }

src/chocolatey/infrastructure/logging/Log4NetAppenderConfiguration.cs

@@ -54,7 +54,7 @@ public static void configure(string outputDirectory = null)
 
             XmlConfigurator.Configure(xmlConfigStream);
 
-            if (outputDirectory != null)
+            if (!string.IsNullOrWhiteSpace(outputDirectory))
             {
                 set_file_appender(outputDirectory);
             }

src/chocolatey/infrastructure/powershell/PoshHostUserInterface.cs

@@ -186,7 +186,7 @@ public override Dictionary<string, PSObject> Prompt(string caption, string messa
         private static string[] get_hotkey_and_label(string input)
         {
             var result = new[] { String.Empty, String.Empty };
-            string[] fragments = input.Split('&');
+            string[] fragments = input.Split(new char[] {'&'}, StringSplitOptions.RemoveEmptyEntries);
             if (fragments.Length == 2)
             {
                 if (fragments[1].Length > 0) result[0] = fragments[1][0].to_string().ToUpper(CultureInfo.CurrentCulture);