public void TestImmutableKeys() {
      ParseUser user = new ParseUser();
      string[] immutableKeys = new string[] {
        "sessionToken", "isNew"
      };

      foreach (var key in immutableKeys) {
        Assert.Throws<InvalidOperationException>(() =>
          user[key] = "1234567890"
        );

        Assert.Throws<InvalidOperationException>(() =>
          user.Add(key, "1234567890")
        );

        Assert.Throws<InvalidOperationException>(() =>
          user.AddRangeUniqueToList(key, new string[] { "1234567890" })
        );

        Assert.Throws<InvalidOperationException>(() =>
          user.Remove(key)
        );

        Assert.Throws<InvalidOperationException>(() =>
          user.RemoveAllFromList(key, new string[] { "1234567890" })
        );
      }

      // Other special keys should be good
      user["username"] = "******";
      user["password"] = "******";
    }