Fix resx test name mangling logic (#39569)

src/libraries/System.Resources.Extensions/tests/TestData.cs

@@ -303,11 +303,8 @@ public override void BindToName(Type serializedType, out string assemblyName, ou
                 // workaround for https://github.com/dotnet/runtime/issues/31289
                 assemblyQualifiedTypeName = assemblyQualifiedTypeName.Replace(s_coreAssemblyName, s_mscorlibAssemblyName);
 
-                int pos = assemblyQualifiedTypeName.IndexOf(',');
-                if (pos > 0 && pos < assemblyQualifiedTypeName.Length - 1)
+                if (TryDeconstructFullyQualifiedTypeName(assemblyQualifiedTypeName, out string newTypeName, out assemblyName))
                 {
-                    assemblyName = assemblyQualifiedTypeName.Substring(pos + 1).TrimStart();
-                    string newTypeName = assemblyQualifiedTypeName.Substring(0, pos);
                     if (!string.Equals(newTypeName, serializedType.FullName, StringComparison.InvariantCulture))
                     {
                         typeName = newTypeName;
@@ -322,6 +319,48 @@ public override Type BindToType(string assemblyName, string typeName)
                 // We should never be using this binder during Deserialization
                 throw new NotSupportedException($"{nameof(TypeNameManglingSerializationBinder)}.{nameof(BindToType)} should not be used during testing.");
             }
+
+            private static bool TryDeconstructFullyQualifiedTypeName(string assemblyQualifiedTypeName, out string typeName, out string assemblyName)
+            {
+                // Skip over all generic arguments in the assembly-qualified type name.
+
+                int genericDepth = 0;
+                int i;
+                for (i = 0; i < assemblyQualifiedTypeName.Length; i++)
+                {
+                    switch (assemblyQualifiedTypeName[i])
+                    {
+                        case '[':
+                            checked { genericDepth++; }
+                            break;
+
+                        case ']':
+                            checked { genericDepth--; }
+                            break;
+
+                        case ',' when genericDepth == 0:
+                            goto AfterLoop;
+
+                        default:
+                            continue;
+                    }
+                }
+
+            AfterLoop:
+
+                if (i < assemblyQualifiedTypeName.Length - 1)
+                {
+                    // Found a proper fully-qualified type name with assembly!
+                    typeName = assemblyQualifiedTypeName.Substring(0, i);
+                    assemblyName = assemblyQualifiedTypeName.Substring(i + 1).Trim();
+                    return true;
+                }
+
+                // Couldn't find an assembly after the type name.
+                typeName = default;
+                assemblyName = default;
+                return false;
+            }
         }
     }
 }