Need more context, don't even know what platform etc

For me it's Avalonia.

You're right. I'll elaborate a bit :)

Imagine that you have a Name field, usually handled by a TextBox.
When the view loads, this field is empty, just showing a placeholder text.

[Name]

If you add a rule requiring the field to be something, the user experience becomes a bit weird because, as soon as the view loads, the TextBox is surrounded by a red border and a validation error saying "Name cannot be empty" when the user hasn't even typed anything.

What most users expect is not this, but to receive data validation errors once they click a button. Then, the validation is triggered, and remains active from then on.

I've run into the same issue and I'm kind of surprised the samples do not cover this.
The way the samples work, and any other examples I can find provides poor UX in my opinion.

Ideally there would be some kind of way to clear all errors, giving you a clean state.
Then as the user inputs data they'll dirty the state, either providing valid data or produce a new error for just the property they've just affected.

Lastly in your submit method you can eagerly run a check on all of them using GetIsValid, ensuring anything the user did not touch also gets validated. If that returns false you can bail out early and show feedback on the fields they've missed.

So in summary, I believe all we need is some kind of ClearValidationErrors on the ValidationContext which clears all errors until they're rechecked.

Right now I have an imperfect solution where I don't add rules until the user tries to submit the form. That way they only get feedback if anything is wrong after they've clicked Submit once, but at least they don't start off with a form full of error messages.

 private void ExecuteContinue()
 {
 if (!IsDirty)
 {
 this.ValidationRule(vm => vm.Name, s => !string.IsNullOrWhiteSpace(s), "Name cannot be empty.");
 this.ValidationRule(vm => vm.Summary, s => !string.IsNullOrWhiteSpace(s), "Summary cannot be empty.");
 this.ValidationRule(vm => vm.Description, s => !string.IsNullOrWhiteSpace(s), "Description cannot be empty.");
 this.ValidationRule(vm => vm.Categories, Categories.ToObservableChangeSet().AutoRefresh(c => c.IsSelected).Filter(c => c.IsSelected).IsEmpty()
 .CombineLatest(this.WhenAnyValue(vm => vm.IsDirty), (empty, dirty) => !dirty || !empty),
 "At least one category must be selected"
 );
 IsDirty = true;
 
 // The ValidationContext seems to update asynchronously, so stop and schedule a retry
 Dispatcher.UIThread.Post(ExecuteContinue);
 return;
 }
 
 if (!ValidationContext.GetIsValid())
 return;
 State.Name = Name;
 State.Summary = Summary;
 State.Description = Description;
 }