c# Reflect the ModelState which ony has get method in asp.net MVC

I want to change the property Controller.ModelState.IsValid manually for testing.

During reflection, I found that the definition of ModelState and ModelStae.IsValid is just {get;}. So when I use PropertyInfo to reflect it , the SetMethod is null.

Then I tried to get backfield of it via GetRunningField, but what I found is 3 object: one "_innerdictionary", and two "XXX<>_backfieldMethod".That's not the same to the normal reflection for readonly property, there's no "<propertyname>K_backfield" in it.

Is anyone who can help? I just want to make my test work easier.