Re-use EF Core Expressions to avoid redundant queries

I often see snippets in EF Core code reviews such as the following:

1dbContext.Users.Where( user => user.Id == id );

The query filter user => user.Id == id is contained directly in the Where - often not just in one place but sometimes in dozens of places. Here I ask myself: why is this not simply outsourced to a central place? It’s so simple!

Especially when queries become more complex, with multiple and- or or-conditions, the duplications are a potential gateway for future bugs; when at one place of the query is updated and other places are forgotten.

Static query classes

The query filter of Lambda queries are technically nothing more than Expressions . So why not just define the Expressions statically and allow reusability:

1public static class UserEntityQueries
2{
3    public static Expression<Func<UserEntity, bool>> HasId(Guid id)
4        => user => user.Id == id;
5}

This static expression can now be used directly:

1dbContext.Users.Where( UserEntityQueries.HasId(id) );

With very little effort, we not only have direct reusability and thus better maintainability, but have also created a very simple basis for unit testing through this construct.