How to change RadzenTemplateForm items, RadzenTextBox, to ReadOnly in code?

At the same time to turn background color of mentioned ReadOnly fields to gray.

I know I can set variable to RadzenTextBox one by one.
Is there a way like WinForm to loop controls using the code?

You can set the ReadOnly property of all text boxes to the same page property which you can toggle. For example ${textBoxesReadOnly}. Use a Set Property action to change textBoxesReadOnly to true or false.