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There are two problems with your code:

• The StateHasChanged method re-renders the component on which it was called and all child components, but does not propagate upwards to parent components. Instead, you want to use two-way data binding if you want a child component to update a property on the parent component, notifying the parent component of the change via an event.
• The car you pass is from model.Cars, which you try to remove from model2.Cars. Since there is no such car in model2.Cars, nothing will be deleted.

To resolve, first change the loop to iterate through model2.Cars:

This is a bug in MudBlazor. It will be fixed in the next release MudBlazor v5.0.8

Move the input box to another component. Then you can then hook into the lifecycle of the component and call .FocusAsync after it has rendered.

https://blazorrepl.com/repl/cluoEsvU59fl8zYM22

I'll try to explain why your approach is not working and then suggest ways to solve it. I hope I understand your intentions correctly.

First you need to change the to

When your method buttonClick is finished in your parent component, Blazor will call StateHasChanged on your component. It's part of the built-in logic of an EventHandler. This will trigger the component life cycle of your child component. During that cycle, the setter of your child component property testModel will be called, again. Blazor doesn't make any test for equality. (The only mighty check engine is the DiffierentialRenderTree at the end of a render cycle). That means a new EditContext will be created. This context, though, doesn't know about the validation error. Hence the message disappears. To prove that point, set a counter variable inside the setter and display it on the page. You will see this result.

.

To avoid this scenario, you create the EditContext once, when the parameters are set, for instance.

The following code describes how you can define a generic class deriving from the EditForm component...