将lambda表达式作为回调传递给扩展方法，并向其传递参数

Question

I am trying to understand the concept of lambda expression,extension method,Linq and IEnumerable interface. You can guess that i am farely new to c sharp.Here i've come up with a problem which will incorporate all the above mentioned concepts . Here i have a list which contain three object.I want to change the name property of a Students object in a specified index .I wrote an extension method which accept a callback function.Callback function accepts an integer index and a new Name string. It should change the name property and return the object .But my code failed to do so as i am not sure how to pass parameter to Func callback in extension method.I am in need of some assistant to understand the problem and fix errors from my code ?

class Program
{
    static void Main(string[] args)
    {
        List<Students> students = new List<Students>();

        students.Add(new Students(111443, "sakib"));
        students.Add(new Students(111445, "zami"));
        students.Add(new Students(111444, "habib"));

        var student = students.First();

        var changed1 = students.Change((int num,string newname) => { return students[num].s_name = newname;});


    }
}

public class Students
{
    public int s_id;

    public string s_name;

    public Students(int id, string name)
    {
        this.s_id = id;

        this.s_name = name;
    }
}

public static class LinqHelper
{
    public static IEnumerable<T> Change<T> (this IEnumerable<T> source, Func<int,string,T> callback)
    {
        var myList = new List<Students>();

        myList.Add(callback(1,"zami")); // i was passing parameter here which is not so helpful i guess !

        return myList;

    }
}

Answer 1

The Func < int, string, T > denotes a function that accepts an integer and string as inputs and T as the return type. The anonymous function you have used has a return type of "string":

var changed1 = students.Change((int num,string newname) => { return students[num].s_name = newname;});

You should return the student instance from the function to make it work. Try replacing the above code with the following:

var changed1 = students.Change((int index, string newname) => 
                                         {
                                             var studentObj = students[index];
                                             studentObj.s_name = newname;
                                             return studentObj;
                                         });

To allow the LinqHelper to accept the index and argument, use the following:

public static class LinqHelper
{
    public static IEnumerable<T> Change<T>(this IEnumerable<T> source, Func<int, string, T> callback, int index, string argument)
    {
        var myList = new List<T>();

        myList.Add(callback(index, argument)); // i was passing parameter here which is not so helpful i guess !

        return myList;

    }
}

And then, you could invoke the method as follows:

var changed1 = students.Change((int index, string newname) => 
                                         {
                                             var studentObj = students[index];
                                             studentObj.s_name = newname;
                                             return studentObj;
                                         }, 
                                         1, 
                                         "zami");

Answer 2

You haven't created the lambda that evaluated to Func<int, string, T> . Your call to Change extension should look like:

var changed1 = students.Change((num, newnam) => {  
                            students[num].s_name = newnam; 
                            return students[num];
                         });

(you should return T as Func requires).