为什么内存列表包含不区分大小写？答案

【问题标题】：Why memory list contains worked as case insensitive?为什么内存列表包含不区分大小写？
【发布时间】：2019-01-04 20:06:48
【问题描述】：

我有一个来自 MySQL 的内存列表和对象

requiredUserNames: 调用 "where users.Contains(us.Username)" 为什么它不区分大小写包含 where users 是内存列表对象（普通 c# 对象）而第二个工作正常，因为 C# 区分大小写：

var users = (from u in _dbContext.MyUser
             select new 
             {
                 UserName = u.UserName
             }).ToList();

var requiredUserNames = (from us in _dbContext.NextUsers
                    // users is a memory list and not a database list why executed as case insensitive
                   where users.Contains(us.Username)
                   select new { Username = us.Username}).ToList();


var requiredUserNames02 = (from us in _dbContext.NextUsers
                        // here working normally as case sensitive
                     where users.FindIndex((usr) => usr.Username.Equals(us.Username)) > -1
                   select new { Username = us.Username }).ToList();

【问题讨论】：

标签： c# mysql linq .net-core

【解决方案1】：

users 是一个本地列表，这是真的。但是，_dbContext.NextUsers 是远程的，您正在询问您的 MySQL 实例：

select us.* from NextUsers us where us.UserName in ('theNameFromLocalList','...')

IOW 是 MySQL 执行不敏感搜索。如果您阻止了这种情况，那么成本会更高，但是：

var requiredUserNames = (from us in _dbContext.NextUsers.AsEnumerable()
                    // users is a memory list and not a database list why executed as case insensitive
                   where users.Contains(us.Username)
                   select new { Username = us.Username}).ToList();

【讨论】：