为什么要为每个测试步骤实例化新的 DbContext

Question

在有关使用 SQLite 进行测试的实体框架核心文档中，示例代码为测试的每个步骤实例化一个新DbContext。这样做有什么原因吗？

// Copied from the docs:
[Fact]
public void Add_writes_to_database()
{
// In-memory database only exists while the connection is open
var connection = new SqliteConnection("DataSource=:memory:");
connection.Open();
try
{
var options = new DbContextOptionsBuilder<BloggingContext>()
.UseSqlite(connection)
.Options;
// Create the schema in the database
using (var context = new BloggingContext(options))
{
context.Database.EnsureCreated();
}
// Run the test against one instance of the context
using (var context = new BloggingContext(options))
{
var service = new BlogService(context);
service.Add("http://sample.com");
context.SaveChanges();
}
// Use a separate instance of the context to verify correct data was saved to database
using (var context = new BloggingContext(options))
{
Assert.Equal(1, context.Blogs.Count());
Assert.Equal("http://sample.com", context.Blogs.Single().Url);
}
}
finally
{
connection.Close();
}
}
// Why not do this instead:
[Fact]
public void Add_writes_to_database()
{
// In-memory database only exists while the connection is open
var connection = new SqliteConnection("DataSource=:memory:");
connection.Open();
try
{
var options = new DbContextOptionsBuilder<BloggingContext>()
.UseSqlite(connection)
.Options;
// Create the schema in the database
using (var context = new BloggingContext(options))
{
context.Database.EnsureCreated();
var service = new BlogService(context);
service.Add("http://sample.com");
context.SaveChanges();
Assert.Equal(1, context.Blogs.Count());
Assert.Equal("http://sample.com", context.Blogs.Single().Url);
}
}
finally
{
connection.Close();
}
}

为什么不实例化上下文一次，并在整个测试方法中使用该实例，如第二个代码示例所示？

Answer 1

因为这就是上下文的使用方式。应根据请求创建并释放它们。

一个实际原因是确保每次都返回到数据源，而不仅仅是查看上下文中的状态。