- #2836
Part 1: Use existing property values when recursing in `MutableObjectModelBinder`
- remove `ComplexModelDTO` because that indirection made fixing this issue more difficult and doesn't add value
- started with an old closed PR (#2241) which did some of this work
- correct `MutableObjectModelBinderTest` tests that exercised behaviour that can't occur
- the old `dto.Results` dictionary was never incomplete; nor could it contain `null` values
Part 2: Change `MutableObjectModelBinder` to pass complex property values into binding system
- model binding no longer trounces nodes in the model tree that aren't bound
- create model instances less often in `TryUpdateModel` scenarios
- refactor `EnsureModel()` to `GetModel()` and use appropriately
- reorder logic in `SetProperty()` and `AddToProperty()` to avoid copying to / from the same collection
- also cleans up some code duplication
nits:
- clean up `MutableObjectModelBinderTest`
- fix odd line wrappings and indentation
- use `nameof()` more
- use `string.Empty` more
- simplify a couple of `Returns()` expressions
- make assertions in `TryUpdateModelIntegrationTest` more readable
- no need to work through `ModelStateDictionary.Keys`
- also use `Length` instead of `Count()`; test code but we don't need Linq at all in that test class
