如何使用实体框架在Xamarin中查询Sqlite? [英] How can I query Sqlite in Xamarin using Entity Framework?

问题描述

我正在使用Sqlite和EF插件(Microsoft.Enttityframeworkcore.sqlite)的Xamarin.Forms项目(C#).发生什么情况取决于:

I'm in a Xamarin.Forms project (C#) using Sqlite and EF plug-in (Microsoft.Enttityframeworkcore.sqlite). What happens depends:

方案1:我使用"Database.EnsureDeletedAsync"

Scenario 1: I use "Database.EnsureDeletedAsync"

• 我可以将数据放入数据库
• 在相同的"using"语句和"dbContext"对象中，我可以使用LINQ查询检索数据而没有错误.

方案2:我不使用"Database.EnsureDeletedAsync"

Scenario 2: I do not use "Database.EnsureDeletedAsync"

• 数据库已经存在
• 我可以向其中添加数据
• 在相同的"using"语句和"dbContext"对象中，任何使用LINQ查询检索数据的尝试(我尝试了多次 方式)，导致错误(System.NullReferenceException:对象 引用未设置为对象的实例).

• the database already exists
• I can add data to it
• within the same "using" statement and "dbContext" object, any attempt to retrieve the data using LINQ query (I've tried multiple ways), results in error (System.NullReferenceException: Object reference not set to an instance of an object).

在两种情况下，我都可以ADB从Android模拟器中提取数据库并进行验证

In BOTH scenarios, I am able to ADB pull the db from the Android emulator and verify

1)数据库存在并且 2)我的数据正在处理中.

1) the db exists and 2) my data are going into it.

仅供参考-DbPath已验证正确(再次，我可以拉出数据库并查看其中的数据).

FYI - the DbPath is verified to be correct (again, I can pull the DB and see data in it).

// POCO class

public class Participant
{
        [Key]
        public int Id { get; set; }
        public string Fname { get; set; }
}


// DB Context

public class DatabaseContext : DbContext
{
        string _dbPath;

        public DbSet<Participant> Participants { get; set; }

        public DatabaseContext(string dbPath)
        {
            _dbPath = dbPath;

            //Database.EnsureDeletedAsync();
            //Database.EnsureCreatedAsync();
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlite($"Filename={_dbPath}");
        }
}


// Main page code-behind

InitializeComponent ();

// Create Database & Tables
using (var db = new DatabaseContext(App.DbPath))
{
    // Insert Data
    db.Add(new Participant() { Fname = "time: " + DateTime.Now.ToString() });
    db.SaveChanges();

    // Retrieve Data
    // Next two lines receive error in scenario 2
    var result2 = db.Participants.Where(xyz => xyz.Id == 1).ToList();
    IEnumerable<Participant> p = db.Participants.ToList();
}

推荐答案

问题已解决！事实证明，这是dbContext以及定义的方式/位置与尝试使用它的位置.代码已经过重组，以确保在执行读写操作之前不会丢失上下文范围.

Problem solved! It turned out to be the dbContext and how/where that was defined vs. where it was attempted to be used. Code has been restructured to ensure not to lose the context scope before performing reads and writes.

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