SELECT changes();
引发的异常:“Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException”(位于 Microsoft.EntityFrameworkCore.dll 中)
“Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException”类型的异常在 Microsoft.EntityFrameworkCore.dll 中发生，但未在用户代码中进行处理
Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions.

报了这个错，官方说是并发引起，就我自己，哪来并发？网上的其他答案也没解决我的问题。

最后我偶然解决了这个问题，作为引发此异常的原因之一，记录一下，帮助有需要的人。

我的数据库实体类定义了构造函数，然后没有声明默认构造函数，导致EF创建实体失败，导致了此异常。

解决方案：

在你自定义构造函数后面补充个默认构造函数

    public class KeyValue
        [Key]
        public string Key { get; set; }
        public string Value { get; set; }
        public string Type { get; set; }
        public KeyValue(string key, string value, string type)
            Key = type + ":" + key;
            Value = value;
            Type = type;
        public KeyValue()
                                    解决 VSCode 编辑 vue 项目报错 Expected indentation of 2 spaces but found 4解决 VSCode 编辑 vue 项目报错 Expected indentation of 2 spaces but found 4问题问题分析解决办法一解决办法二
解决 VSCode 编辑 vue 项目报错 Expected indentation of 2 spaces but found 4
系统：Win10
编辑器：VSCode
使用 VSCode 在写 Vue 项目的时候，出现报错
Expected indentation of 2 spaces 
                                    asp.net 更新数据时报错：Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded
原因：Check your object, does it pass...
                                    EFcore 保存出现并发被提前修改的报错
提示语言Database operation expected to affect 1 row(s) but actually affected 0 row(s).
   public static void EFNewExecute(Action<DbContext> executeAction, DbContext context, bool isCheckConcurrent = false)
                                    Postgre , The database operation was expected to affect 1 row(s), but actually affected 0 row
                                    Preface
Like any programmer, you need to be able to produce reliable code that conforms to a 
specification, which means that you need to test your code. In this book, you'll learn how to 
use techniques and Python tools that reduce the effort involved in testing, and at the same 
time make it more useful—and even fun.
You'll learn about several of Python's automated testing tools, and you'll learn about the 
philosophies and methodologies that they were designed to support, like unit testing and 
test-driven development. When you're done, you'll be able to produce thoroughly tested 
code faster and more easily than ever before, and you'll be able to do it in a way that doesn't 
distract you from your "real" programming.
What this book covers
Chapter 1: Testing for Fun and Profit introduces Python test-driven development and various 
testing methods.
Chapter 2: Doctest: The Easiest Testing Tool covers the doctest tool and teaches you how  
to use it.
Chapter 3: Unit Testing with Doctest introduces the ideas of unit testing and test-driven 
development, and applies doctest to create unit tests.
Chapter 4: Breaking Tight Coupling by using Mock Objects covers mock objects and the 
Python Mocker tool.
Chapter 5: When Doctest isn't Enough: Unittest to the Rescue introduces the unittest 
framework and discusses when it is preferred over doctest.
Chapter 6: Running Your Tests: Follow Your Nose introduces the Nose test runner, and 
discusses project organization.
This material is copyright and is licensed for the sole use by Betty Vaughan-Pope on 1st February 2010
2601 S Broadway St, Unit 29, La Porte, , 77571
Preface
[   ]
Chapter 7: Developing a Test-Driven Project walks through a complete test-driven 
development process.
Chapter 8: Testing Web Application Frontends using Twill applies the knowledge gained from 
previous chapters to web applications, and introduces the Twill tool.
Chapter 9: Integration Testing and System Testing teaches how to build from unit tests to 
tests of a complete software system.
Chapter 10: Other Testing Tools and Techniques introduces code coverage and continuous 
integration, and teaches how to tie automated testing into version control systems.
Appendix: Answers to Pop Quizes contains the answers to all pop quizes, chapter-wise.
What you need for this book
To use this book, you will need a working Python interpreter, preferably one of the 2.6 version 
series. You'll also need a source code editor, and occasional access to the internet. You will 
need to be comfortable enough using your operating system's textual interface—your DOS 
prompt or command shell—to do basic directory management and to run programs.
Who this book is for
If you are a Python developer and want to write tests for your applications, this book will get 
you started and show you the easiest way to learn testing.
You need to have sound Python programming knowledge to follow along. An awareness of 
software testing would be good, but no formal knowledge of testing is expected nor do you 
need to have any knowledge of the libraries discussed in the book.
                                    出现错误：The database operation was expected to affect 1 row(s), but actually affected 0 row(s);由于我们用的是主从库，在新增完后，再立即去更新数据，这时候同步过程还没完成，导致查出来的数据是从从库里查出来的，还是原来的数据，就会导致这个原因。解决方案就是这个方法只从主库里读取和写入。
                                    以上错误是我的 IDistributedEventHandler 实现类TahmEventHandler（用来消费mqtt的消息内容）的消息订阅事件 HandleEventAsync中，因为给服务器发送消息很快，执行的消息数据解析业务方法，执行数据操作时报出的错误。（原因我还没搞明白） ，所以我用ISingletonDependency来测试，然后用mqttx客户端使劲给服务器发送消息，我的业务系统就抛出了上面的错误。数据库操作预计影响1行，但实际影响0行;自实体加载以来，数据可能已被修改或删除。
                                    我的开源项目修改的类的源码，源码地址：**https://github.com/TerraMours/TerraMours_Gpt_Api/blob/main/TerraMours/TerraMours/Framework/Infrastructure/EFCore/FrameworkDbContext.cs。在这种情况下，数据不会丢失。在 Jane 单击“保存”之前，John 访问了相同页面，并将开始日期字段从 2007/1/9 更改为 2013/1/9。跟踪用户已修改的属性，并仅更新数据库中相应的列。
                                    I'm trying to insert records in two tables, but getting the exception. Could you please help me to resolve the issue.First I tried the below code.await _testRepository.InsertAsync(test);await _xyzRepo...
                                    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.F...
                                    ASP.NET Core 操作数据库提示“Microsoft.EntityFrameworkCore.DbUpdateException”，
本以为是程序方面的错误，然后找了一下信息，发现还是报错，通过一般般注释发现，在添加表字段信息时，提示这个错误
然后去数据库看了下字段的长度，觉得是长度的问题就把长度改大，再次调试下，通过了。
                                    package com.example.sharingcontact.db;import android.content.Context;import android.database.Cursor;import android.database.sqlite.SQLiteDatabase;import android.database.sqlite.SQLiteStatement;public