LINQ ASP.NET中嵌套lambda表达式中的参数引用错误

Question

For some reason I'm getting an error in Microsoft Visual Studio 2012 over the following code:

students.Where(s => 
    foreignStudents.Any(f => 
        s.FirstName == f.FirstName && 
        s.LastName  == f.LastName
    )
);

student is a list of students with various attributes including FirstName and LastName and foreignStudents is a list containing only the FirstName and LastName of students. I've changed the variable names to make it easier to understand the problem.

It says that IEnumerable does not contain a definition for 'Any' and the best extension method overload Enumerable.Any<TSource>(IEnumerable<TSource>, Func<TSource,bool>) has some invalid arguments.

Switching it to f => true or f => f.FirstName == "Sarah" removes the error.

Any insight would be much appreciated!

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Edit: Actual Code

// Retreives orders from the database using parameters from the URL
string orderQuery = "SELECT * FROM EventOrders WHERE EventID = @0 AND AccountNum = @1";
var orders = db.Query(orderQuery, Request.Form["id"], Request.Form["accountnum"]);

// Parses order IDs from the URL
// Where Order IDs are in the form: <orderNum>-<orderLine>[,...]
var orderIDs = Request.QueryString["orderids"].Split(',')
    .Select(orderID => {
        var components = orderID.Split('-');
        return new { 
            OrderNum  = components[0].AsInt(), 
            OrderLine = components[1].AsInt() 
        };
    });

var quantityList = orders
    .Where(o => orderIDs.Any(i => o.OrderNum == i.OrderNum && o.OrderLine ==             i.OrderLine))
    .OrderByDescending(o => o.Quantity)
    .Select(o => new { o.OrderNum, o.OrderLine, o.Quantity })

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Edit 2: So I think it might just be a problem with Visual Studio now. It seems work after debugging the rest of the code. Visual Studio still has it underlined in red indicating an error though.

Answer 1

What you're really looking to do here is a Join . By joining the two tables you can find all of the students in one table that exist in the other. This is also an operation that can be done much more efficiently that what you're describing in which you search the entire set of data for each item . If it's known that you're doing a Join it can be very heavily optimized.

var query = from student in students
            join foreignStudent in foreignStudents
                on new { student.FirstName, student.LastName }
                equals new { foreignStudent.FirstName, foreignStudent.LastName }
                into joinedStudents
            where joinedStudents.Count() > 0
            select student;