efcore 新特性 SaveChanges Events

efcore 新特性 SaveChanges Events

Intro

昨天早上看到之前关注的一个 efcore 的 issue 被 closed ，于是看了一眼， ef core 新合并了一个 PR，在 DbContext 中增加了 SaveChanges 相关的几个事件，具体的变更可以参数 PR https://github.com/dotnet/efcore/pull/21862

Events

之前写过两篇关于 EF Core 做自动审计的文章

第一次的实现需要显式继承一个 AuditDbContext ，在有些需要没办法修改 DbContext 或者原有 DbContext 已经有继承某一个类，就没有办法用了，可以参考 https://www.cnblogs.com/weihanli/p/auto-audit-for-entity-framework.html

后面结合 AOP 改进了一版，通过一个审计切面逻辑完成自动审计，但是需要引入 AOP 组件支持，对于不想引入额外组件的项目来说也并非特别友好，可以参考 https://www.cnblogs.com/weihanli/p/12819634.html

在这个 PR 合并之后，我们可以通过 SavingChanges 事件获取保存之前 DbContext 的状态，通过 SavedChanges 事件来获取保存成功后的 DbContext 信息，SaveChangesFailed 事件获取保存失败信息

事件定义如下：

/// 

///     An event fired at the beginning of a call to  or 
/// 
public event EventHandler SavingChanges;

/// 

///     An event fired at the end of a call to  or 
/// 
public event EventHandler SavedChanges;

/// 

///     An event fired if a call to  or  fails with an exception.
/// 
public event EventHandler SaveChangesFailed;

事件参数定义如下：

/// 

///     Base event arguments for the  and  events.
/// 
public abstract class SaveChangesEventArgs : EventArgs
{
    /// 

    ///     Creates a base event arguments instance for 
    ///     or  events.
    /// 
    ///  The value passed to SaveChanges. 
    protected SaveChangesEventArgs(bool acceptAllChangesOnSuccess)
    {
        AcceptAllChangesOnSuccess = acceptAllChangesOnSuccess;
    }

    /// 

    ///     The value passed to  or .
    /// 
    public virtual bool AcceptAllChangesOnSuccess { get; }
}

/// 

///     Event arguments for the  event.
/// 
public class SavingChangesEventArgs : SaveChangesEventArgs
{
    /// 

    ///     Creates event arguments for the  event.
    /// 
    ///  The value passed to SaveChanges. 
    public SavingChangesEventArgs(bool acceptAllChangesOnSuccess)
        : base(acceptAllChangesOnSuccess)
    {
    }
}

/// 

///     Event arguments for the  event.
/// 
public class SavedChangesEventArgs : SaveChangesEventArgs
{
    /// 

    ///     Creates a new  instance with the given number of entities saved.
    /// 
    ///  The value passed to SaveChanges. 
    ///  The number of entities saved. 
    public SavedChangesEventArgs(bool acceptAllChangesOnSuccess, int entitiesSavedCount) : base(acceptAllChangesOnSuccess)
    {
        EntitiesSavedCount = entitiesSavedCount;
    }

    /// 

    ///     The number of entities saved.
    /// 
    public virtual int EntitiesSavedCount { get; }
}

/// 

///     Event arguments for the  event.
/// 
public class SaveChangesFailedEventArgs : SaveChangesEventArgs
{
    /// 

    /// Creates a new  instance with the exception that was thrown.
    /// 
    ///  The value passed to SaveChanges. 
    ///  The exception thrown. 
    public SaveChangesFailedEventArgs(bool acceptAllChangesOnSuccess, [NotNull] Exception exception)
        : base(acceptAllChangesOnSuccess)
    {
        Exception = exception;
    }

    /// 

    /// The exception thrown during or .
    /// 
    public virtual Exception Exception { get; }
}

More

除了上面的审计，你也可以使用通过这些事件，实现保存之前的自动更新数据库字段的值，比如 Add 或 Update 操作数据时自动设置更新时间等信息

本文提到的特性还未正式发布，预计会在 .net5 下一个预览版中发布，如果想现在要尝试，请使用 efcore 的 daily build 的包，可以参考 https://github.com/dotnet/aspnetcore/blob/master/docs/DailyBuilds.md

Reference

• https://github.com/dotnet/efcore/issues/15910
• https://github.com/dotnet/efcore/pull/21862