Using interfaces and dependency injection for inversion of control in ASP.NET Core 3.1 Razor Pages projects built with the MVVM design pattern is crucial for reducing class coupling and improving modifiability. The Model-View-ViewModel (MVVM) design paradigm provides a comprehensive architecture, but using concrete classes that implement repositories causes rigid structure issues, making it difficult to modify and test applications as business requirements evolve. By introducing C# interfaces and .NET dependency injection, developers can decouple their code, improve maintainability, and enhance unit testing capabilities. This post demonstrated how to refactor an existing application to utilize interfaces and dependency injection, providing a more extensible and scalable architecture for ASP.NET Core 3.1 Razor Pages projects.