为什么列出. Find()方法将最后一个对象返回为null? [英] Why the List . Find() method return the last object as null ?

问题描述

代码:

the code:

List<Model.Sys_Permission> sys_permissions = new BLL.Sys_Permission().GetAll();
            List<Model.Sys_Permission> sys_permissionsNew = new List<Model.Sys_Permission>();
            
            var tempPermission = new Model.Sys_Permission();
            permissionCodes.ForEach(r => {
                tempPermission = sys_permissions.Find(c => c.PermissionCode == r);
                sys_permissionsNew.Add(tempPermission);
            });
            return sys_permissionsNew;

the list permissionCodes is a List<string> include 5 string object : '1001','1002','1003','1004','1005' . 

the list sys_permissions  includes 5 Sys_Permission model  objects, the values of the permissionCode are also  :  '1001','1002','1003','1004','1005' . 

the issue is why it returns null when the find() method runs to the last one  ? 
the find() method can return '1001','1002','1003','1004' . but it returns null when it finds the '1005' ? 
so that the sys_permissionsNew only fetch four values without the last one valued '1005' . 

who can tell me what's wrong with it ? thanks . 

推荐答案

如果您查看文档: ^ ]解释说:

If you look at the documentation: MSDN[^] it explains that:

Return Value
Type: T
The first element that matches the conditions defined by the specified predicate, if found; otherwise, the default value for type T.
...
Important note
When searching a list containing value types, make sure the default value for the type does not satisfy the search predicate. Otherwise, there is no way to distinguish between a default value indicating that no match was found and a list element that happens to have the default value for the type. If the default value satisfies the search predicate, use the FindIndex method instead.

基本上，当匹配用尽时，它将返回默认值-对于引用类型，它是null

Basically, it returns the default value when it runs out of matches - for a reference type, that''s null

谢谢大家.问题解决了.因为数据库中的PermissionCodes的值为"1005"，但是在数据库中，"sys_permissions"的值为"1005".

当我将许可代码插入db时，这是我的错.
谢谢原创.

thanks you all . the issue is solved . cause the values of permissionCodes in the database are ''1005 '' , but the value of ''sys_permissions'' is ''1005'' in the databse .

it is my fault when I insert the permission codes into the db .
thanks original .

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