Refactor known folder GUIDs to use static properties

src/libraries/System.Private.CoreLib/src/System/Environment.Android.cs

@@ -15,6 +15,8 @@ public static partial class Environment
 
         private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOption _ /*option*/)
         {
+            // No need to validate if 'folder' is defined; GetSpecialFolder handles this check.
+
             if (s_specialFolders == null)
             {
                 Interlocked.CompareExchange(ref s_specialFolders, new Dictionary<SpecialFolder, string>(), null);
@@ -92,7 +94,9 @@ private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOptio
                     return "/usr/share";
 
                 default:
-                    return string.Empty;
+                    if (!Enum.IsDefined(folder))
+                        throw new ArgumentOutOfRangeException(nameof(folder), folder, SR.Format(SR.Arg_EnumIllegalVal, folder));
+                    return null;
             }
         }
     }

src/libraries/System.Private.CoreLib/src/System/Environment.GetFolderPathCore.Unix.cs

@@ -19,9 +19,9 @@ public static partial class Environment
     {
         private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOption option)
         {
-            // Get the path for the SpecialFolder
-            string path = GetFolderPathCoreWithoutValidation(folder) ?? string.Empty;
-            Debug.Assert(path != null);
+            // No need to validate if 'folder' is defined; GetSpecialFolder handles this check.
+
+            string path = GetSpecialFolder(folder) ?? string.Empty;
 
             // If we didn't get one, or if we got one but we're not supposed to verify it,
             // or if we're supposed to verify it and it passes verification, return the path.
@@ -46,7 +46,7 @@ private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOptio
             return path;
         }
 
-        private static string? GetFolderPathCoreWithoutValidation(SpecialFolder folder)
+        private static string? GetSpecialFolder(SpecialFolder folder)
         {
             // First handle any paths that involve only static paths, avoiding the overheads of getting user-local paths.
             // https://www.freedesktop.org/software/systemd/man/file-hierarchy.html
@@ -143,8 +143,11 @@ private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOptio
 #endif
             }
 
+            if (!Enum.IsDefined(folder))
+                throw new ArgumentOutOfRangeException(nameof(folder), folder, SR.Format(SR.Arg_EnumIllegalVal, folder));
+
             // No known path for the SpecialFolder
-            return string.Empty;
+            return null;
         }
 
         private static string GetXdgConfig(string home)

src/libraries/System.Private.CoreLib/src/System/Environment.Win32.cs

@@ -216,16 +216,14 @@ private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOptio
             //
             // The only SpecialFolderOption defines we have are equivalent to KnownFolderFlags.
 
-            string folderGuid;
+            ReadOnlySpan<byte> folderGuid;
             string? fallbackEnv = null;
             switch (folder)
             {
                 // Special-cased values to not use SHGetFolderPath when we have a more direct option available.
                 case SpecialFolder.System:
                     // This assumes the system directory always exists and thus we don't need to do anything special for any SpecialFolderOption.
                     return SystemDirectory;
-                default:
-                    return string.Empty;
 
                 // Map the SpecialFolder to the appropriate Guid
                 case SpecialFolder.ApplicationData:
@@ -374,6 +372,9 @@ private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOptio
                     folderGuid = Interop.Shell32.KnownFolders.Windows;
                     fallbackEnv = "windir";
                     break;
+                default:
+                    Debug.Assert(!Enum.IsDefined(folder), $"Unexpected SpecialFolder value: {folder}. Please ensure all SpecialFolder enum values are handled in the switch statement.");
+                    throw new ArgumentOutOfRangeException(nameof(folder), folder, SR.Format(SR.Arg_EnumIllegalVal, folder));
             }
 
             Guid folderId = new Guid(folderGuid);

src/libraries/System.Private.CoreLib/src/System/Environment.cs

@@ -148,15 +148,20 @@ public static string ExpandEnvironmentVariables(string name)
             return ExpandEnvironmentVariablesCore(name);
         }
 
-        public static string GetFolderPath(SpecialFolder folder) => GetFolderPath(folder, SpecialFolderOption.None);
+        public static string GetFolderPath(SpecialFolder folder) => GetFolderPathCore(folder, SpecialFolderOption.None);
 
         public static string GetFolderPath(SpecialFolder folder, SpecialFolderOption option)
         {
-            if (!Enum.IsDefined(folder))
-                throw new ArgumentOutOfRangeException(nameof(folder), folder, SR.Format(SR.Arg_EnumIllegalVal, folder));
+            // No need to validate if 'folder' is defined; GetFolderPathCore handles this check.
 
-            if (option != SpecialFolderOption.None && !Enum.IsDefined(option))
-                throw new ArgumentOutOfRangeException(nameof(option), option, SR.Format(SR.Arg_EnumIllegalVal, option));
+            if (option is not SpecialFolderOption.None and not SpecialFolderOption.Create and not SpecialFolderOption.DoNotVerify)
+            {
+                // Use a throw helper so that if 'option' is a constant,
+                // the JIT can inline this method and remove the validation check entirely.
+                Throw(option);
+                static void Throw(SpecialFolderOption option) =>
+                    throw new ArgumentOutOfRangeException(nameof(option), option, SR.Format(SR.Arg_EnumIllegalVal, option));
+            }
 
             return GetFolderPathCore(folder, option);
         }

src/libraries/System.Private.CoreLib/src/System/Environment.iOS.cs

@@ -20,6 +20,8 @@ public static partial class Environment
 
         private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOption _ /*option*/)
         {
+            // No need to validate if 'folder' is defined; GetSpecialFolder handles this check.
+
             if (s_specialFolders == null)
             {
                 Interlocked.CompareExchange(ref s_specialFolders, new Dictionary<SpecialFolder, string>(), null);
@@ -91,7 +93,9 @@ private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOptio
                     return "/usr/share";
 
                 default:
-                    return string.Empty;
+                    if (!Enum.IsDefined(folder))
+                        throw new ArgumentOutOfRangeException(nameof(folder), folder, SR.Format(SR.Arg_EnumIllegalVal, folder));
+                    return null;
             }
 
             static string CombineSearchPath(NSSearchPathDirectory searchPath, string subdirectory)