C# LINQ to Entities 无法识别方法“System.String get_Item (System.String)”，
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LINQ to Entities does not recognize the method 'System.String get_Item (System.String)',
提问by mcamara
How can I solve this problem?
我怎么解决这个问题？
Here is my code:
这是我的代码：
DateTime dtInicio = new DateTime();
DateTime dtFim = new DateTime();
Int32 codStatus = 0;
if(!string.IsNullOrEmpty(collection["txtDtInicial"]))
dtInicio = Convert.ToDateTime(collection["txtDtInicial"]);
if(!string.IsNullOrEmpty(collection["txtDtFinal"]))
dtFim = Convert.ToDateTime(collection["txtDtFinal"]);
if (!string.IsNullOrEmpty(collection["StatusCliente"]))
Convert.ToInt32(collection["StatusCliente"]);
var listCLientResult = (from c in db.tbClientes
orderby c.id
where (c.effdt >= dtInicio || string.IsNullOrEmpty(collection["txtDtInicial"]) &&
(c.effdt <= dtFim || string.IsNullOrEmpty(collection["txtDtFinal"])) &&
(c.cod_status_viagem == codStatus || string.IsNullOrEmpty(collection["StatusCliente"])))
select c);
return View(listCLientResult);
The error I am getting is:
我得到的错误是：
LINQ to Entities does not recognize the method 'System.String get_Item (System.String)', which can not be converted into an expression of the repository.
LINQ to Entities 无法识别方法“System.String get_Item (System.String)”，该方法无法转换为存储库的表达式。
采纳答案by Thomas Levesque
Linq queries performed against a database are translated to SQL before they can be executed; but collection["txtDtInicial"]can't be translated to SQL, because there is no equivalent SQL syntax, and anyway the database doesn't have access to collection. You need to extract collection["txtDtInicial"]to a variable first, and use only this variable in the query.
对数据库执行的 Linq 查询在执行之前被转换为 SQL；但collection["txtDtInicial"]无法转换为 SQL，因为没有等效的 SQL 语法，而且无论如何数据库无法访问collection. 您需要先提取collection["txtDtInicial"]到一个变量，并在查询中仅使用该变量。
Here's what I would do:
这是我会做的：
DateTime dtInicio = DateTime.MinValue;
DateTime dtFim = DateTime.MaxValue;
Int32 codStatus = 0;
if(!string.IsNullOrEmpty(collection["txtDtInicial"]))
dtInicio = Convert.ToDateTime(collection["txtDtInicial"]);
if(!string.IsNullOrEmpty(collection["txtDtFinal"]))
dtFim = Convert.ToDateTime(collection["txtDtFinal"]);
if (!string.IsNullOrEmpty(collection["StatusCliente"]))
codStatus = Convert.ToInt32(collection["StatusCliente"]);
var listCLientResult = (from c in db.tbClientes
orderby c.id
where (c.effdt >= dtInicio) &&
(c.effdt <= dtFim) &&
(c.cod_status_viagem == codStatus)
select c);
return View(listCLientResult);
By initializing dtInicioand dtFimto MinValue and MaxValue, you don't need to check whether they are defined in the query.
通过将dtInicioand初始化dtFim为 MinValue 和 MaxValue，您无需检查它们是否在查询中定义。
回答by ravi saini
The Linq query is ultimately transformed into an SQL query and LINQ doesn't know what to do with Session["UserName"] (that gets the "UserName" item).
Linq 查询最终转换为 SQL 查询，而 LINQ 不知道如何处理 Session["UserName"]（获取“UserName”项）。
A common way to workaround this is just to use a local variable to which you'll assign Session["UserName"] and that you'll use in your Linq query...
解决此问题的常用方法是使用局部变量，您将为其分配 Session["UserName"] 并且您将在 Linq 查询中使用该变量...
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string loggedUserName = Session["LogedUsername"].ToString();
var userdetail =
dc.faculties.Where(a => a.F_UserName.Equals(loggedUserName)).FirstOrDefault();
string loggingUserName = Session["LogedUsername"].ToString();
var userdetail = dc.faculties.Where(a => a.F_UserName.Equals(loggedUserName)).FirstOrDefault();
reference http://mvc4asp.blogspot.in/
回答by Amit Kadam
In one line...
在一行...
DO NOT USE STRING CONVERSION FUNCTIONS in Linq(Entity) Query!
不要在 Linq(Entity) 查询中使用字符串转换函数！
Wrong:
错误的：
user = db.Users.Where(u => u.Name == dt.Rows[i]["Name"].ToString()).FirstOrDefault();
Correct:
正确的：
string Name = dt.Rows[i]["Name"].ToString();
user = db.Users.Where(u => u.Name == Name).FirstOrDefault();
