为什么实体框架不能识别数据库中的现有记录？

Question

I have created a database system for a library, and when a user registers the program checks if they already have an account, but this is always returning false even when I can see in my DBMS that the record exists.

 public static bool UserExists(string email)
 {
            using (var context = new LibraryDbContext())
            {
                if (context.Users.Any(d => d.Email == email))
                {
                    return true;
                }
                else
                {
                    return false;
                }
            }
 }

Also, this is my Dbcontext class

public class LibraryDbContext : DbContext
{
    public LibraryDbContext()
        : base()
    {
    }

    public DbSet<Book> Books { get; set; }

    public DbSet<User> Users { get; set; }

    public DbSet<ReturnsLog> Returns { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseInMemoryDatabase("LibraryProjectDbo");
    }
}

Answer 1

Typically issues like this when connecting to an existing database for the first time are due to EF being pointed at a database you don't expect. (Ie creating one via Code First rather than connecting to your existing DB, or creating tables you don't expect.)

When using database first, you should always disable the Code First initialization:

public LibraryDbContext()
{
    Database.SetInitializer<LibraryDbContext>(null);
}

This will generate an exception if the DbContext goes and tries to create a schema which will help direct you to the correction.

By default EF will be looking for a connection string called "LibraryDbContext" in your web.confing/.exe.config file in the runtime location. When using connection strings I generally like to be explicit with the connection string name to reflect a Database rather than the DbContext. Often I will utilize multiple contexts to split up behaviour in a system that all point to the same database. So for example I'd call the main database connection string something like "AppConnection" and pass that through the base constructor.

public LibraryDbContext()
    : base ("AppConnection")
{
    Database.SetInitializer<LibraryDbContext>(null);
}

Alternatively to test your connection out you can pass a Connection String itself to the base constructor.