SA1649 alerts for Razor Pages model classes

[omccully]

SA1649 - "File name should match first type name"

The standard template is it creates a file named MyPage.cshtml.cs" with a class MyPageModel`. The analyzer warns on this. I think it should have an exception for these Razor Pages model classes, since it is standard.

[bjornhellander]

You can easily suppress these files yourself (#2590 (comment)), but I also think that it would be good if this could be automatically handled by this rule. Maybe even detect this case and expect the file name to match the convention?