MVC and auto persisting values

When you have a textbox in HTML (let’s say

<input name=”FirstName” type=”text” />

And it binds to “FirstName” Property of a Model, and in HttpPost Action you do modify the value and return the same view, the value shown in the textbox is the posted one, not the modified one. ( The first thought is that HttpPost is not executing – but it is a false impression!)
The solution is:

or, better

(and it’s not a bug, it’s a feature: classical example: numeric textbox/ numeric property and user enters “aaa” – the validation error appears and the textbox must have aaa, not 0 or default value for the property )