如何字符串转换可空INT LINQ [英] How to convert string to nullable int LINQ

问题描述

我看到了很多这样的问题，但似乎我不能找到任何关系彼此，找到我的解决方案。我使用MVC 4应用程序，我有里面的一些值一个DropDownList。

I saw a lot of questions like this but seems I cant find anything to relate one to another and find solution for mine. I am using MVC 4 application where I have a dropdownlist with some values inside.

我使用viewbag来填充它。

I am using viewbag to populate it.

ViewBag.Roles = new SelectList(db.Roles, "RoleId", "RoleName");

在查看我的下拉列表看起来是这样的。

In the view my dropdown looks like this.

@Html.DropDownList("selectedRole", ViewBag.Roles as SelectList, String.Empty)

字符串 selectedRole 从视图中获得角色ID 值。

在此之后，我做了一些LINQ到数据库 selectedRole 它说，我不能将字符串转换我的LINQ里面INT，我真的想这样我就可以返回更好的结果

After this I am doing some LINQ to the database with selectedRole which says that I cannot convert string to Int inside my LINQ which I really want so I can return better results.

     public ActionResult SearchResults(string selectedRole, string selectedCourse, string SearchParam)
        {
            var members = (from m in db.Members
                           select m);

            if (!String.IsNullOrEmpty(selectedRole))
            {
                members = (from m in db.Members
                           where m.UserRole == Int32.Parse(selectedRole)
                           select m);


            }
        //some more if's here...
        return View(members.ToList());
}

所以，问题是如何将这个 selectedRole 转换为整数。有没有什么规则在LINQ转换时要遵循？

So the question is how to convert this selectedRole to Integer. Are there any rules to follow when converting in LINQ?

PS：我也试过 Convert.ToInt32（selectedRole）返回相同的错误

PS: I tried also Convert.ToInt32(selectedRole) which returns same error.

推荐答案

在这种情况下，这个问题是不相关的...

In this case the question is irrelevant...

if (!String.IsNullOrEmpty(selectedRole))
{
    int selectedRole2 = Int32.Parse(selectedRole);

    members = (from m in db.Members
               where m.UserRole == selectedRole2
               select m);

您不这样做在LINQ，你外面去做。

You don't do it in linq, you do it outside.

在一般情况下，你是不是只使用LINQ中，你正在使用LINQ到SQL或LINQ到实体，因为成员是一个表。可悲的是，至少LINQ到实体没有任何投法。您可以获得通过使用用户定义的函数类似的东西，但它是一个有点复杂。 LINQ到SQL，而不是应该支持 Convert.ToInt32 （见的 https://msdn.microsoft.com/en-us/library/vstudio/bb882655.aspx ）

In general, you aren't using only Linq, you are using Linq-to-Sql or Linq-to-Entities, because Members is a table. Sadly at least Linq-to-Entities doesn't have any cast method. You can obtain something similar by using a user defined function, but it is a little complex. Linq-to-Sql instead should support Convert.ToInt32 (see https://msdn.microsoft.com/en-us/library/vstudio/bb882655.aspx)

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