The ModelState object represents the state of the model and its validation. It contains a collection of name and value pairs for each entry submitted in a form, as well as a collection of error messages for each entry that failed validation.
Here's why and when you might need ModelState.Clear():
Retaining Old Values: When you return a view after a POST action, the values that were posted are bound to the view. This means if you've made changes to the model in your action method after the POST, those changes might not be reflected in the view because the posted values take precedence. By calling ModelState.Clear(), you're clearing the old values and errors, ensuring that the view will display the current state of the model.
Avoiding Validation Issues: If you've made changes to the model in your action method and you want to return the view without re-validating the model, you might run into issues where the ModelState still contains validation errors from the initial POST. Clearing the ModelState ensures that these old validation errors don't interfere with the rendering of the view.
Redirecting to a Different Action: If you're redirecting to a different action after a POST (using the Post-Redirect-Get pattern), you typically don't need to clear the ModelState because a new GET request will be initiated, and the ModelState will be fresh. However, if you're returning a view directly after a POST without a redirect, that's when you might run into the issues mentioned above.
What would happen if you don't use it?
Stale Data: As mentioned, if you modify the model in your POST action and then return a view without redirecting, the view might display the old, posted values instead of the updated values from the model.
Validation Errors Persist: If there were validation errors during the POST, and you've corrected them in the action but haven't cleared the ModelState, the view might still display the old validation errors.
Confusion: If you're not aware of the behaviour of ModelState and how it retains old values and errors, it can lead to confusion and unexpected behaviour in your application.
Conclusion
While you don't always need to use ModelState.Clear(), it's a useful method to know about and can help avoid certain issues when returning views directly after a POST action.
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