Big-endian fix for JsonClassInfo.GetKey (dotnet#49708)

* Big-endian fix for JsonClassInfo.GetKey

* Skip assertion in JsonClassInfo.GetKey on big-endian
  (code will not ensure the properties tested here)

* Update src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonClassInfo.Cache.cs

Co-authored-by: Stephen Toub <[email protected]>

Author: uweigand

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonClassInfo.Cache.cs

@@ -474,19 +474,23 @@ public static ulong GetKey(ReadOnlySpan<byte> name)
             }
 
             // Verify key contains the embedded bytes as expected.
-            const int BitsInByte = 8;
-            Debug.Assert(
-                // Verify embedded property name.
-                (name.Length < 1 || name[0] == ((key & ((ulong)0xFF << BitsInByte * 0)) >> BitsInByte * 0)) &&
-                (name.Length < 2 || name[1] == ((key & ((ulong)0xFF << BitsInByte * 1)) >> BitsInByte * 1)) &&
-                (name.Length < 3 || name[2] == ((key & ((ulong)0xFF << BitsInByte * 2)) >> BitsInByte * 2)) &&
-                (name.Length < 4 || name[3] == ((key & ((ulong)0xFF << BitsInByte * 3)) >> BitsInByte * 3)) &&
-                (name.Length < 5 || name[4] == ((key & ((ulong)0xFF << BitsInByte * 4)) >> BitsInByte * 4)) &&
-                (name.Length < 6 || name[5] == ((key & ((ulong)0xFF << BitsInByte * 5)) >> BitsInByte * 5)) &&
-                (name.Length < 7 || name[6] == ((key & ((ulong)0xFF << BitsInByte * 6)) >> BitsInByte * 6)) &&
-                // Verify embedded length.
-                (name.Length >= 0xFF || (key & ((ulong)0xFF << BitsInByte * 7)) >> BitsInByte * 7 == (ulong)name.Length) &&
-                (name.Length < 0xFF || (key & ((ulong)0xFF << BitsInByte * 7)) >> BitsInByte * 7 == 0xFF));
+            // Note: the expected properties do not hold true on big-endian platforms
+            if (BitConverter.IsLittleEndian)
+            {
+                const int BitsInByte = 8;
+                Debug.Assert(
+                    // Verify embedded property name.
+                    (name.Length < 1 || name[0] == ((key & ((ulong)0xFF << BitsInByte * 0)) >> BitsInByte * 0)) &&
+                    (name.Length < 2 || name[1] == ((key & ((ulong)0xFF << BitsInByte * 1)) >> BitsInByte * 1)) &&
+                    (name.Length < 3 || name[2] == ((key & ((ulong)0xFF << BitsInByte * 2)) >> BitsInByte * 2)) &&
+                    (name.Length < 4 || name[3] == ((key & ((ulong)0xFF << BitsInByte * 3)) >> BitsInByte * 3)) &&
+                    (name.Length < 5 || name[4] == ((key & ((ulong)0xFF << BitsInByte * 4)) >> BitsInByte * 4)) &&
+                    (name.Length < 6 || name[5] == ((key & ((ulong)0xFF << BitsInByte * 5)) >> BitsInByte * 5)) &&
+                    (name.Length < 7 || name[6] == ((key & ((ulong)0xFF << BitsInByte * 6)) >> BitsInByte * 6)) &&
+                    // Verify embedded length.
+                    (name.Length >= 0xFF || (key & ((ulong)0xFF << BitsInByte * 7)) >> BitsInByte * 7 == (ulong)name.Length) &&
+                    (name.Length < 0xFF || (key & ((ulong)0xFF << BitsInByte * 7)) >> BitsInByte * 7 == 0xFF));
+            }
 
             return key;
         }