C# 概念拆箱

Question

I'm trying to learn C# Boxing and Unboxing Concept. I filled the list of objects with integers and then I wanted to summarize them as result in console output.

List<object> listOfObjects = new List<object>();
//var listOfObjects = new List<object>();

//adding first string to list
listOfObjects.Add("First string");

//adding integers to list
for (int j = 0; j < 5; j++)
{
    listOfObjects.Add(j);
}
listOfObjects.Add("Second string");
for (int k = 5; k < 10; k++)
{
    listOfObjects.Add(k);
}
foreach (var obj in listOfObjects)
{
    Console.WriteLine(obj);
}
var sum = 0;
for (var l = 0;  l < 4;  l++)
{
    sum += (int)listOfObjects[l];            
}
Console.WriteLine(sum);

but output throws exception

Exception thrown: 'System.InvalidCastException' in ConsoleApp.exe An unhandled exception of type 'System.InvalidCastException' occurred

sum += (int)listOfObjects[l] // this unboxing cause compile error

Someone knows why? I used the example from MSDN resource .

Answer 1

The problem is lines like these:

listOfObjects.Add("First string");

This adds a string to the list and it makes no sense to try to convert that to an int .

Remove lines such as this and you should get an output.

Answer 2

Since your listOfObjects is a List whose contents are of type object , you can add anything to it. You first add a string , which is an object. Then you add a bunch of numbers of type int , which is a value type , so the numbers are boxed into an object before they are added to the list. Then you add another string . Then you add another bunch of numbers of type int , again boxing as before.

Your problem is in the last loop, where you loop over the first 4 elements. The first element is a string , so you cannot cast it to an int.

Answer 3

'System.InvalidCastException' -> says you that you have made wrong cast.

So if we will take special range of items from list, after cast them to ( int ), then we get result of unboxing .

Because boxing has been made when you added numbers to Lis.

Hope this detailed response will be more friendfull.

List listOfObjects = new List(); //var listOfObjects = new List();

        //adding first string to list
        listOfObjects.Add("First string");

        //adding integers to list
        for (int j = 0; j < 5; j++)
        {
            listOfObjects.Add(j);
        }
        listOfObjects.Add("Second string");
        for (int k = 5; k < 10; k++)
        {
            listOfObjects.Add(k);
        }
        foreach (var obj in listOfObjects)
        {
            Console.WriteLine(obj);
        }
        var sum = 0;
        for (var l = 0; l < 4; l++)
        {

            var a = listOfObjects[l].GetType();

            if (a == typeof(int))
            {
                sum += (int)listOfObjects[l];
                Console.WriteLine("I've get an integer type.");

            }
            Console.WriteLine("I've get only string type.");
        }
        Console.WriteLine(sum);

        Console.Read();

Hope this modified sample will help you. Good Luck!