Use the repository pattern to abstract data access and improve maintainability. The repository pattern is a design pattern that separates the data access code from the rest of the application. It provides a layer of abstraction between the application and the data access code, making it easier to change the data access code without affecting the rest of the application. The repository pattern is commonly used in .NET Core applications to improve maintainability and testability.

The repository pattern is useful because it allows you to change the data access code without affecting the rest of the application. For example, if you decide to switch from using a SQL Server database to a MongoDB database, you can simply change the implementation of the repository without changing the rest of the application. This makes it easier to maintain the application over time, as you can make changes to the data access code without worrying about breaking the rest of the application.

Here is an example of how to implement the repository pattern in a .NET Core application using C#:

public interface IRepository<T>
{
    Task<T> GetByIdAsync(int id);
    Task<IEnumerable<T>> GetAllAsync();
    Task AddAsync(T entity);
    Task UpdateAsync(T entity);
    Task DeleteAsync(T entity);
}

public class Repository<T> : IRepository<T> where T : class
{
    private readonly DbContext _dbContext;

    public Repository(DbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public async Task<T> GetByIdAsync(int id)
    {
        return await _dbContext.Set<T>().FindAsync(id);
    }

    public async Task<IEnumerable<T>> GetAllAsync()
    {
        return await _dbContext.Set<T>().ToListAsync();
    }

    public async Task AddAsync(T entity)
    {
        await _dbContext.Set<T>().AddAsync(entity);
        await _dbContext.SaveChangesAsync();
    }

    public async Task UpdateAsync(T entity)
    {
        _dbContext.Entry(entity).State = EntityState.Modified;
        await _dbContext.SaveChangesAsync();
    }

    public async Task DeleteAsync(T entity)
    {
        _dbContext.Set<T>().Remove(entity);
        await _dbContext.SaveChangesAsync();
    }
}

In this example, we define an interface called IRepository that defines the methods that we want to use to interact with the data. We then create a concrete implementation of the repository called Repository that implements the methods defined in the IRepository interface. The Repository class takes a DbContext object in its constructor, which is used to interact with the database.