Level 2 s-function cannot update the dialog parameters

Hi, could anyone help me?

I have the trouble in changing the dialog parameters.

Pasted is my code about setting dialog parameters.
 % Register the parameters.
  block.NumDialogPrms = 1;
  block.DialogPrmsTunable = {'Tunable'};


  % Register all tunable parameters as runtime parameters.
  block.AutoRegRuntimePrms;

block.RegBlockMethod('ProcessParameters', @ProcessPrms);

function ProcessPrms(block)
 block.AutoUpdateRuntimePrms;

Above is my set about dialog parameters. But when I do block.DialogPrm(1).Data=1, it says it is only can be read and cannot be set.

Anybody can help me?

Thank you in advance.