System.InvalidOperationException: The LINQ expression.... 'could not be translated. ASP.NET 6 ...Translation of method 'DecodeFrom64' failed

Question

In ASP.NET Core 6 Web API Project, I'm trying to retrieve and decode the password from the database before comparing it with the entered password.

I have this code:

public async Task<User> GetUser(string email, string password)
{
     return await _context.Set<User>().Where(e => e.Email == email
                    && DecodeFrom64(e.Password) == password).FirstOrDefaultAsync();
}

I got this error:

 System.InvalidOperationException: The LINQ expression 'DbSet<User>().Where(u => u.Email == __email_0 && DbHelper.DecodeFrom64(u.Password) == __password_1)' could not be translated. Additional information: Translation of method 'UserOnboardingApi.Model.DbHelper.DecodeFrom64' > failed.

How do I get this resolved?

Thanks

Expected to retrieve and decode the password from the database and compare to what the user entered

Answer 1

EF translates LINQ (when working with IQueryable ) into SQL and it does not know anything about your method DecodeFrom64 and can't translate it. Options are

• Encoding password and checking it server-side (db side):

public async Task<User> GetUser(string email, string password)
{
     return await _context.Set<User>()
          .Where(e => e.Email == email
                    && e.Password == YourEncodeMethod(password))
          .FirstOrDefaultAsync();
}

• Fetching the user by email (I suppose it should be unique) and check password client-side. Something along this lines:

public async Task<User> GetUser(string email, string password)
{
     var user = await context.Set<User>()
          .FirstOrDefaultAsync(e => e.Email == email);
     if(user is not null && DecodeFrom64(user.Password) == password)
     {
          return user;
     }
     return null;
}

• Implementing the decode function on the db side and mapping it .

But in general you should consider storing password hashes ( see this question ), not the encoded ones and check them.