What is Entity Framework Core Query Types in C#?

Entity Framework Core Query Types provide a flexible way to map database views or custom SQL queries to read-only entity types in your C# application. This feature is particularly useful when you need to work with data that doesn’t fit neatly into your entity model or when you want to optimize performance for read-heavy operations.

Here’s how Entity Framework Core Query Types work:

1. Mapping to Non-Entity Types: Unlike regular entity types that map to database tables, query types represent data that isn’t backed by a table. You define a query type in your DbContext using the `HasNoKey` method to indicate that it doesn’t have a primary key and should be treated as a read-only entity.

2. Defining Query Types: To create a query type, you define a C# class with properties that match the columns or computed values from your database view or custom SQL query. Decorate the class with `[Keyless]` to inform Entity Framework Core that it’s a query type.

3. Configuration: In your DbContext class, you configure the query type using the `OnModelCreating` method. You use the `Query` method to specify the query type and map it to a database view or custom SQL query.

4. Querying: Once you’ve defined and configured a query type, you can query it using LINQ or Entity SQL, just like regular entity types. Entity Framework Core will automatically project the results of your queries into instances of the query type.

5. Benefits: Query types are read-only, which means you can’t perform insert, update, or delete operations on them. This read-only nature is perfect for scenarios where you need to display data from complex database views, join multiple tables, or perform computationally intensive operations without affecting the underlying data.

6. Performance: Query types can be used to optimize read-heavy operations by mapping them to precomputed views or queries, reducing the need for complex joins and calculations in your application code.

Entity Framework Core Query Types are a powerful tool for handling read-only data that doesn’t neatly fit into your entity model. They allow you to work with database views and custom SQL queries efficiently, improving the performance and maintainability of your data access code.