Improved Guid.ToUppercaseAsciiLetters() extension

Microsoft.Toolkit.Uwp.UI.Media/Extensions/System/GuidExtensions.cs

@@ -4,7 +4,6 @@
 
 using System;
 using System.Diagnostics.Contracts;
-using System.Linq;
 
 namespace Microsoft.Toolkit.Uwp.UI.Media
 {
@@ -19,12 +18,19 @@ namespace Microsoft.Toolkit.Uwp.UI.Media
         /// <param name="guid">The input <see cref="Guid"/> to process</param>
         /// <returns>A <see cref="string"/> representation of <paramref name="guid"/> only made up of letters in the [A-Z] range</returns>
         [Pure]
-        public static string ToUppercaseAsciiLetters(this Guid guid)
+        public static string ToUppercaseAsciiLetters(in this Guid guid)
         {
-            return new string((
-                from c in guid.ToString("N")
-                let l = char.IsDigit(c) ? (char)('G' + c - '0') : c
-                select l).ToArray());
+            // Composition IDs must only be composed of characters in the [A-Z0-9_] set,
+            // and also have the restriction that the initial character cannot be a digit.
+            // Because of this, we need to prepend an underscore to a serialized guid to
+            // avoid cases where the first character is a digit. Additionally, we're forced
+            // to use ToUpper() here because ToString("N") currently returns a lowercase
+            // hexadecimal string. Note: this extension might be improved once we move to
+            // .NET 5 in the WinUI 3 release, by using string.Create<TState>(...) to only
+            // have a single string allocation, and then using Guid.TryFormat(...) to
+            // serialize the guid in place over the Span<char> starting from the second
+            // character. For now, this implementation is fine on UWP and still fast enough.
+            return $"_{guid.ToString("N").ToUpper()}";
         }
     }
 }