Internal

Program.cs

using Microsoft.EntityFrameworkCore;

public abstract class EntityBase<TKey> where TKey : IEquatable<TKey>
{
    public abstract TKey Id { get; set; }
}

public abstract class RepositoryBase<TDbContext, TEntity, TKey>
    where TDbContext : DbContext, new()
    where TEntity : EntityBase<TKey>
    where TKey : IEquatable<TKey>
{
    public async ValueTask<TEntity?> GetAsync(TKey id)
    {
        var dbContext = new TDbContext();
        IQueryable<TEntity> queryable = GetQueryable(dbContext!)!;
        TEntity? result = await queryable.FirstOrDefaultAsync(x => Equals(x.Id, id));
        return result;
    }

    protected abstract IQueryable<TEntity> GetQueryable(TDbContext context);
}


internal class ApplicationDbContext : DbContext
{
}

public class User : EntityBase<Guid>
{
    public override Guid Id { get; set; } = Guid.NewGuid();
}

// The consuming app doesn't need ApplicationDbContext, just User and UserRepository,
// so why can't I declare the class of this type?
// and why can't I override a protected method considering the class is sealed so
// a consuming app cannot override it?
public sealed class UserRepository : RepositoryBase<ApplicationDbContext, User, Guid>
{
    protected override IQueryable<User> GetQueryable(ApplicationDbContext context)
    {
        throw new NotImplementedException();
    }
}