Question

我有以下两个陈述： -

var isadminByuser = tms.SecurityRoles.Where(a => a.Name.ToLower() == "administrator")
                                     .Select(a=>a.SecurityRoleUsers.Where(a2 => a2.UserName.ToLower() == user.ToLower()));

if (isadminByuser.Count() >= 1) { return true;}

＆安培;

var adminByGroup = tms.SecurityRoles.Where(a => a.Name == "Administrator")
                      .SingleOrDefault().Groups
                      .Select(a2 => a2.TMSUserGroups
                                      .Where(a3 => a3.UserName.ToLower() == user.ToLower()));

bool isadminByGroup = adminByGroup.Count() >= 1;

第一个var isadminByuser将始终包含元素（始终为＆gt; = 1），即使where子句a3.UserName.ToLower() == user.ToLower()))为false，而另一个var也有计数零是where子句（a3.UserName.ToLower() == user.ToLower())）是false。那么为什么第一个var永远不会有零？

由于

Answer 1

问题的答案是您在选择时选择IQueryable<SecurityRoleUsers> a.SecurityRoleUsers.Where(a2 => a2.UserName.ToLower() == user.ToLower())

可能有也可能没有0，但包含查询将为匹配IQueryables的每个SecurityRole返回其中一个Name = "administrator"，因此计数将始终为1 +如果至少有一个匹配SecurityRole

更新

// first get SecurityRoles
bool isadminByuser = tms.SecurityRoles.Where(a => a.Name.ToLower() == "administrator")
    // now you're only interested in the related SecurityRoleUsers
    .SelectMany( a => a.SecurityRoleUsers )
    // now check if any of the SecurityRoleUsers meet your criteria
    .Any( sru => sru.UserName.ToLower() == user.ToLower() );

Answer 2

您能否观察到ms-sql-profiler

的SQL查询差异

tms.SecurityRoles.Where(a => a.Name.ToLower() == "administrator")
                                     .Select(a=>a.SecurityRoleUsers.Where(a2 => a2.UserName.ToLower() == user.ToLower()));

***

tms.SecurityRoles.Select(a=>a.SecurityRoleUsers.Where(a2 => a2.UserName.ToLower() == user.ToLower())).Where(a => a.Name.ToLower() == "administrator");

。在哪里。选择，如何只选择匹配where子句的记录： -

2 个答案: