在使用Entity Framework 4.0时，我们遇到这样的Exception: New transaction is not allowed because there are other threads running in the session,是在这样的场景下面：


1:          [Test]
2:          [ExpectedException(typeof(System.Data.EntityException))]
3:          public void TestUnknowIssue2()
4:          {
5:              using (var db = new TestPerformaceDBEntities())
6:              {
7:                  var products = db.Products.Where(p => p.CategoryID == 1);
8:                  foreach (var product in products)
9:                  {
10:                      Debug.WriteLine("Get it");
11:
12:                      product.ProductName = "new name";
13:
14:                      //Here will throw :New transaction is not allowed because there are other threads running in the session.
15:                      db.SaveChanges();
16:                  }
17:              }
18:          }


我们使用是Northwind示例Database,你注意要上面代码中有foreach。主要原因是我们的foreach的循环时，我们不能在同一个连接中同时Reader读数据，又同时做Update，它还没有读完。这时你需要把这个集合对象转换成Array或List<T>，将代码修改为这样就可以了：


1:          [Test]
2:          public void TestUnknowIssue2()
3:          {
4:              using (var db = new TestPerformaceDBEntities())
5:              {
6:                  var products = db.Products.Where(p => p.CategoryID == 1).ToList();
7:                  foreach (var product in products)
8:                  {
9:                      Debug.WriteLine("Get it");
10:
11:                      product.ProductName = "new name";
12:                      db.SaveChanges();
13:                  }
14:              }
15:          }


注意返回的数量很大时，类似填充List方法将会消耗很多内存。此时，建议你使用skip方式对结果集进行数据分页处理。