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题记:在用EF Core的内存数据库进行单元测试的时候遇到“无法访问已释放的对象”的错误怎么办?
之前在EF Core 1.0中使用Include的小技巧中简单谈到了使用EF Core内存数据库进行单元测试的方法。不过这个方法有个小问题,就是容易出现“无法访问已释放的对象”的错误。
在之前的示例代码中(http://git.oschina.net/ike/codes/jtu9dnsk3pe6x24clbq50),单元测试能够顺利通过,是因为db和db2两个实例对象并没有放到using里面让其自动disposed掉。可在大部分情况下,正确的写法应该是使用using的,就算不使用using,如果一次性执行多个测试,那么就会遇到这个“无法访问已释放的对象”错误。
根据这里的讨论(https://github.com/aspnet/EntityFramework/issues/4092),这一错误的根源是EF Core的内存数据库设计出来的行为和真实的关系数据库是一致的,同时又受到ServiceProvider作用域的控制。换句话说,就是在同一个ServiceProvider之下,内存数据库的数据和关系数据库的数据一样是保持“持久”的(当然是在内存中)。由此导致了,使用内存数据库进行单元测试,需要考虑两种情况:
1,单元测试涉及的每次数据操作,数据都完全隔离。
这种情况下,可以采用官方的文档(https://github.com/aspnet/EntityFramework.Docs/issues/95)中的说明,实例化或者注入ServiceCollection,然后每次操作数据库都要通过新的ServiceProvider获得DbContext实例。具体代码如下:
using Microsoft.Data.Entity;
using Microsoft.Data.Entity.Infrastructure;
using Microsoft.Extensions.DependencyInjection;
using System;
using System.Linq;namespace ConsoleApp1
{
public class Program
{
public static void Main(string[] args)
{
var serviceCollection = new ServiceCollection();
serviceCollection
.AddEntityFramework()
.AddInMemoryDatabase()
.AddDbContext<SampleContext>(c => c.UseInMemoryDatabase()); using (var db = serviceCollection.BuildServiceProvider().GetService<SampleContext>())
{
db.Blogs.Add(new Blog { Url = "Test" });
db.SaveChanges();
Console.WriteLine(db.Blogs.Count());
} using (var db = serviceCollection.BuildServiceProvider().GetService<SampleContext>())
{
db.Blogs.Add(new Blog { Url = "Test" });
db.SaveChanges();
Console.WriteLine(db.Blogs.Count());
}
}
} public class SampleContext : DbContext
{
public DbSet<Blog> Blogs { get; set; }
} public class Blog
{
public int BlogId { get; set; }
public string Url { get; set; }
}
}
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.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
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width: 100%;
margin: 0em;
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2,单元测试涉及的每次数据操作,数据需要模拟真实的唯一数据库。
详细说来,就是我需要在一个TestFixture中,先插入一些示例数据,然后随后的每个单元测试都以这些示例数据为准。在这种情况下,为了避免遇到“无法访问已释放的对象”,就需要通过统一的ServiceProvider(上面已经解释了为什么需要统一的)来获得在Scoped中分别获取DbContext以避免之前的DbContext被释放。这种方式,官方也有参考的代码,具体我就不贴了,见(https://github.com/aspnet/MusicStore/blob/dev/src/MusicStore/Models/SampleData.cs)。下面是我给出的一个示例代码(代码片段见:http://git.oschina.net/ike/codes/t069no34dfu8p2zeahrsv):
public class EFCoreInMemoryTest
{
[Fact]
public async Task WillSuccessWithWrongApproach()
{
var serviceCollection = new ServiceCollection();
serviceCollection
.AddEntityFramework()
.AddInMemoryDatabase()
.AddDbContext<MarketDbContext>(options => options.UseInMemoryDatabase()); var serviceProvider = serviceCollection.BuildServiceProvider(); var db = serviceProvider.GetRequiredService<MarketDbContext>();
SampleData.Create(db); var db2 = serviceProvider.GetRequiredService<MarketDbContext>(); var products = await db2.Products.ToListAsync(); Assert.Equal(3, products.Count); var promotions = await db2.Promotions.ToListAsync(); Assert.Equal(2, promotions.Count);
} [Fact]
public async Task WillFaild()
{
var serviceCollection = new ServiceCollection();
serviceCollection
.AddEntityFramework()
.AddInMemoryDatabase()
.AddDbContext<MarketDbContext>(options => options.UseInMemoryDatabase()); var serviceProvider = serviceCollection.BuildServiceProvider(); using (var db = serviceProvider.GetRequiredService<MarketDbContext>())
{
SampleData.Create(db);
} using (var db2 = serviceProvider.GetRequiredService<MarketDbContext>())
{
var products = await db2.Products.ToListAsync(); Assert.Equal(3, products.Count); var promotions = await db2.Promotions.ToListAsync(); Assert.Equal(2, promotions.Count);
} } [Fact]
public async Task WillSuccessWithRightApproach()
{
var serviceCollection = new ServiceCollection();
serviceCollection
.AddEntityFramework()
.AddInMemoryDatabase()
.AddDbContext<MarketDbContext>(options => options.UseInMemoryDatabase()); var serviceProvider = serviceCollection.BuildServiceProvider(); DoDbActionInScoped(serviceProvider, (db) =>
{
SampleData.Create(db);
}); await DoDbActionInScopedAsync(serviceProvider, async (db2) =>
{
var products = await db2.Products.ToListAsync(); Assert.Equal(3, products.Count); var promotions = await db2.Promotions.ToListAsync(); Assert.Equal(2, promotions.Count);
});
} private void DoDbActionInScoped(IServiceProvider serviceProvider, Action<MarketDbContext> action)
{
using (var serviceScope = serviceProvider.GetRequiredService<IServiceScopeFactory>().CreateScope())
{
using (var db = serviceScope.ServiceProvider.GetRequiredService<MarketDbContext>())
{
action(db);
}
}
} private async Task DoDbActionInScopedAsync(IServiceProvider serviceProvider, Func<MarketDbContext, Task> action)
{
using (var serviceScope = serviceProvider.GetRequiredService<IServiceScopeFactory>().CreateScope())
{
using (var db = serviceScope.ServiceProvider.GetRequiredService<MarketDbContext>())
{
await action(db);
}
}
}
}