Bugfix: System.Xml.Serialization: Compiler.GetTempAssemblyName is not…

… persistent under .NET Core

The implementation of String.GetHashCode() was persistent by default in .NET Framework. allowing to predictably name an XmlSerializers.{HashCode}.dll containing the pre-generated serializers.
In .NET Core / .NET 5, String.GetHashCode() is no longer persistent, so the value of ns.GetHashCode() will change during each run, preventing it from picking up the XmlSerializers dll correctly.

src/libraries/System.Private.Xml/src/System/Xml/Serialization/Compiler.cs

@@ -87,7 +87,25 @@ internal TextWriter Source
 
         internal static string GetTempAssemblyName(AssemblyName parent, string? ns)
         {
-            return parent.Name + ".XmlSerializers" + (ns == null || ns.Length == 0 ? "" : "." + ns.GetHashCode());
+            return parent.Name + ".XmlSerializers" + (ns == null || ns.Length == 0 ? "" : "." + GetPersistentHashCode(ns));
+        }
+
+        /// <remarks>
+        /// Returns the same value returned by .NET Framework 64-bit String.GetHashCode()
+        /// </remarks>
+        private static int GetPersistentHashCode(string value)
+        {
+            int hash1 = 5381;
+            int hash2 = hash1;
+
+            for (int i = 0; i < value.Length && value[i] != '\0'; i += 2)
+            {
+                hash1 = ((hash1 << 5) + hash1) ^ value[i];
+                if (i == value.Length - 1 || value[i + 1] == '\0')
+                    break;
+                hash2 = ((hash2 << 5) + hash2) ^ value[i + 1];
+            }
+            return hash1 + (hash2 * 1566083941);
         }
     }
 }