如何处理Entity Framework中的DbUpdateConcurrencyException异常

1. Concurrency的作用

场景

有个修改用户的页面功能,我们有一条数据User, ID是1的这个User的年龄是20, 性别是female(数据库中的原始数据)

正确的该User的年龄是25, 性别是male

这个时候A发现User的年龄不对, 就给改成25, 那么在Entity Framework中,我们会这样做。

var user = dbConext.User.Find();

//B用户在这里完成修改了User的性别

user.age = ;

dbContext.SaveChanges();

但是加入在上面注释处,有个B用户发现性别不对,完成了对用户性别的修改,改成male. 会出现什么结果呢。

var user = dbConext.User.Find(1);

当A执行这个代码的时候,获取的性别是female

user.age = 25;

当A执行这个代码的时候, 不知道B已经修改了这个记录的性别,这个时候A的user的性别还是female

dbContext.SaveChanges();

保存修改的时候,会把female覆盖回去,这样B用户的修改就作废了。

但这不是A的本意,A其实只是想修改年龄而已。

Entity Framework使用[ConcurrencyCheck] 来解决这种问题, 当标记为[ConcurrencyCheck] 的Entity属性,如果发现在从数据库中取下来和提交的时候不一致,就会出现DbUpdateConcurrencyException异常,避免错误提交。

2. 如何正确处理DbUpdateConcurrencyException异常

2.1 数据库优先方式

原理是在出现异常的时候,重新加载数据库中的数据,覆盖Context本地数据

using (var context = new BloggingContext())
{
  var blog = context.Blogs.Find();
  blog.Name = "The New ADO.NET Blog";  
  bool saveFailed;
  do
  {
    saveFailed = false;
    try
    {
      context.SaveChanges();
    }
    catch (DbUpdateConcurrencyException ex)
    {
      saveFailed = true;
      // Update the values of the entity that failed to save from the store
      ex.Entries.Single().Reload();
    }
  } while (saveFailed);
}

 

2.2 客户端优先方式

以Context保存的客户端数据为主,覆盖数据库中的数据

using (var context = new BloggingContext())
{
  var blog = context.Blogs.Find();
  blog.Name = "The New ADO.NET Blog";
  bool saveFailed;
  do
  {
    saveFailed = false;
    try
    {
      context.SaveChanges();
    }
    catch (DbUpdateConcurrencyException ex)
    {
      saveFailed = true;
      // Update original values from the database
      var entry = ex.Entries.Single();
      entry.OriginalValues.SetValues(entry.GetDatabaseValues());
    }
  } while (saveFailed);
}

3.3 综合方式

有时候,不是非A即B的关系,我们希望综合数据库中的数据和context中修改的数据,再保存到数据库中

使用下面的CurrentValues, GetDatabaseValues(), 得到Context数据和数据库数据,重新构建一个正确的Entity,再更新到数据库中

using (var context = new BloggingContext())
{
  var blog = context.Blogs.Find();
  blog.Name = "The New ADO.NET Blog";
  bool saveFailed;
  do
  {
    saveFailed = false;
    try
    {
      context.SaveChanges();
    }
    catch (DbUpdateConcurrencyException ex)
    {
      saveFailed = true;
      // Get the current entity values and the values in the database
      var entry = ex.Entries.Single();
      var currentValues = entry.CurrentValues;
      var databaseValues = entry.GetDatabaseValues();
      // Choose an initial set of resolved values. In this case we
      // make the default be the values currently in the database.
      var resolvedValues = databaseValues.Clone();
      // Have the user choose what the resolved values should be
      HaveUserResolveConcurrency(currentValues, databaseValues, resolvedValues);
      // Update the original values with the database values and
      // the current values with whatever the user choose.
      entry.OriginalValues.SetValues(databaseValues);
      entry.CurrentValues.SetValues(resolvedValues);
    }
  } while (saveFailed);
}
public void HaveUserResolveConcurrency(DbPropertyValues currentValues,
DbPropertyValues databaseValues,
DbPropertyValues resolvedValues)
{
  // Show the current, database, and resolved values to the user and have
  // them edit the resolved values to get the correct resolution.
}

对上面方法的优化

使用DbPropertyValues总是别扭,使用Enttiy对象就会方便很多,下面就是转换成Entity对象操作的方法

using (var context = new BloggingContext())
{
  var blog = context.Blogs.Find();
  blog.Name = "The New ADO.NET Blog";
  bool saveFailed;
  do
  {
    saveFailed = false;
    try
    {
      context.SaveChanges();
    }
    catch (DbUpdateConcurrencyException ex)
    {
      saveFailed = true;
      // Get the current entity values and the values in the database
      // as instances of the entity type
      var entry = ex.Entries.Single();
      var databaseValues = entry.GetDatabaseValues();
      var databaseValuesAsBlog = (Blog)databaseValues.ToObject();
      // Choose an initial set of resolved values. In this case we
      // make the default be the values currently in the database.
      var resolvedValuesAsBlog = (Blog)databaseValues.ToObject();
      // Have the user choose what the resolved values should be
      HaveUserResolveConcurrency((Blog)entry.Entity,
        databaseValuesAsBlog,
        resolvedValuesAsBlog);
      // Update the original values with the database values and
      // the current values with whatever the user choose.
      entry.OriginalValues.SetValues(databaseValues);
      entry.CurrentValues.SetValues(resolvedValuesAsBlog);
    }
  } while (saveFailed);
}
public void HaveUserResolveConcurrency(Blog entity,
  Blog databaseValues,
  Blog resolvedValues)
{
// Show the current, database, and resolved values to the user and have
// them update the resolved values to get the correct resolution.
}

 

 
 
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