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IsNull

property IsNull: Boolean read write;

Example​

procedure ScriptEvent(var Value: variant);
var
  Text: TJSONString;
  Item: TJSONValue;
begin
  Text := TJSONString.Create;
  try
    Item := Text;
    Item.AsString := 'retained';
    Item.IsNull := True;
    Value := Item.IsNull;  // True; Text.Value remains retained.
  finally
    Text.Free;
  end;
end;

Usage​

IsNull reads or writes scalar null state while fixed container and JSON-null getters ignore the stored flag.

Additional Technical Info​

For an ordinary Boolean, String or correctly initialized numeric value, IsNull reads/writes a private flag separate from the payload and Name. Setting true does not erase the stored value; it makes null-aware AsString blank and causes an enclosing object to emit JSON null. Setting false reveals the retained payload again. Assigning a valid scalar payload normally sets the flag false automatically.

Two class families override only the getter:

  • TJSONNull.IsNull always returns True. Writing false changes hidden base storage but has no visible effect.
  • TJSONText (therefore objects and arrays) always returns False. Writing true changes hidden base storage but the public getter and null-aware serialization still read false. Containers cannot represent semantic null through this property.

Thus the property is not a generic reset operation. Use Clear to follow runtime-class behavior, and use a real TJSONNull child when JSON null is required. Array serialization has additional null-marked scalar defects documented by the scalar/container pages.

The PascalScript NotNull registration is wrong and calls this same reader. Both script properties return the same Boolean; write not Item.IsNull for a non-null test. State access is constant-time and unsynchronized. A parent-owned child remains borrowed regardless of null state.

External references

Created 2026-07-15