不能将lambda表达式转换为类型'string'，因为它不是委托类型[复制]

This question already has an answer here:

这个问题已经有了答案:

Entity Framework - Cannot convert lambda expression to type 'string' because it is not a delegate type 13 answers
实体框架-不能将lambda表达式转换为类型“string”，因为它不是一个委托类型13的答案。

I am using a LINQ lambda expression like so:

我用的是LINQ lambda表达式

int Value = 1;
qryContent objContentLine;

using (Entities db = new Entities())
{
    objContentLine = (from q in db.qryContents
                      where q.LineID == Value
                      orderby q.RowID descending
                      select q).FirstOrDefault();
}

However, I am getting the following error:

然而，我有以下错误:

Cannot convert lambda expression to type 'string' because it is not a delegate type

不能将lambda表达式转换为类型“string”，因为它不是一个委托类型。

5 个解决方案

#1

184

I think you are missing using System.Linq; from this system class.

我认为你是在使用System.Linq;从这个系统类。

and also add using System.Data.Entity; to the code

并添加使用System.Data.Entity;代码

#2

In my case, I had to add using System.Data.Entity;

在我的例子中，我必须添加使用System.Data.Entity;

#3

My case it solved i was using

我的案子解决了我的问题。

@Html.DropDownList(model => model.TypeId ...)

using

使用

@Html.DropDownListFor(model => model.TypeId ...)

will solve it

将解决这个问题

#4

If it's not related to missing using directives stated by other users, this will also happen if there is another problem with your query.

如果您的查询还有其他问题，如果没有使用其他用户声明的指令，这也会发生。

Take a look on VS compiler error list : For example, if the "Value" variable in your query doesn't exist, you will have the "lambda to string" error, and a few errors after another one more related to the unknown/erroneous field.

看一下VS编译器错误列表:例如，如果您的查询中“值”变量不存在，那么您将会有“lambda to string”错误，并且会出现一些与未知/错误字段相关的错误。

In your case it could be :

在你的情况下，它可能是:

objContentLine = (from q in db.qryContents
                  where q.LineID == Value
                  orderby q.RowID descending
                  select q).FirstOrDefault();

Errors:

错误:

Error 241 Cannot convert lambda expression to type 'string' because it is not a delegate type

错误241不能将lambda表达式转换为类型“string”，因为它不是一个委托类型。

Error 242 Delegate 'System.Func<..>' does not take 1 arguments

错误242代表“System.Func < . .>不接受1个参数。

Error 243 The name 'Value' does not exist in the current context

错误243当前上下文不存在名称“值”。

Fix the "Value" variable error and the other errors will also disappear.

修正“值”变量错误，其他错误也会消失。

#5

For people just stumbling upon this now, I resolved an error of this type that was thrown with all the references and using statements placed properly. There's evidently some confusion with substituting in a function that returns DataTable instead of calling it on a declared DataTable. For example:

对于刚刚遇到这种情况的人，我解决了这个类型的错误，该错误被抛出了所有的引用，并正确地使用了语句。显然，在一个返回DataTable的函数中替换而不是在已声明的DataTable上调用它，会造成一些混淆。例如:

This worked for me:

这工作对我来说:

DataTable dt = SomeObject.ReturnsDataTable();

List<string> ls = dt.AsEnumerable().Select(dr => dr["name"].ToString()).ToList<string>();

But this didn't:

但这没有:

List<string> ls = SomeObject.ReturnsDataTable().AsEnumerable().Select(dr => dr["name"].ToString()).ToList<string>();

I'm still not 100% sure why, but if anyone is frustrated by an error of this type, give this a try.

我仍然不能百分之百地确定为什么，但是如果有人因为这种类型的错误而感到沮丧，请尝试一下。

#1

184