Re: problem converting to object - C# Discussion Boards

14-May-08 7:46

When I uncomment the last four lines I get an error. "Cannot convert from int/char to object" Can anyone help. Thanx in advance.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Ch07Structs
{
    public class Employee
    {
        private int emp;
        public Employee(int i)
        {
            emp = i;
        }

        public override string ToString()
        {
            return emp.ToString();
        }
    }
    class Program
    {
        public static void Display(object[] o)
        {
            
            foreach (object f in o)
                Console.WriteLine("{0} ",f.ToString());
            Console.WriteLine();
        }

        static void Main()
        {
            Employee[] emp = new Employee[5];
            for (int i = 0; i < emp.Length; i++)
            {
                emp[i] = new Employee(i + 5);
            }
            Display(emp);

            string[] st = { "hello" };
            Display(st);
            //int[] i = { 2, 2, 2, 2 };
            //Display(i);
            //char[] stArray = { 'h','e','l','l','o' };
            //Display(stArray);
            Console.ReadKey();
        }
    }
}

Csharp™14-May-08 8:30

Csharp™

14-May-08 8:30

you need to cast the int / char array's before calling Display.
eg:

<br />
char[] stArray = { 'h','e','l','l','o' };<br />
object[] obj = (object[])stArray;<br />
Display(obj);<br />

<font color="blue">Csharp</font> me = (<font color="blue">Csharp</font>)Dynamic_Sysop; <font color="#008000">// My Signature.</font><br><font color="#008000">$twins</font> = <font color="#008000">me.twins</font>(<font color="#800080">"Taylor"</font> , <font color="#800080">"Delta"</font>); <font color="#C0C0C0">// My twin daughters.</font><br>echo <font color="#008000">$twins</font>;</br></br>

Guffa14-May-08 9:35

14-May-08 9:35

Csharp™ wrote:
you need to cast the int / char array's before calling Display.
eg:

char[] stArray = { 'h','e','l','l','o' };
object[] obj = (object[])stArray;

You can't cast a char array to an object array.

Despite everything, the person most likely to be fooling you next is yourself.

Guffa14-May-08 9:40

14-May-08 9:40

You misread the error message. If you check again, it says that you can't cast int[] or char[] to object[]. You can't cast an int array or a char array into an object array. You have to cast each item in the array:

int[] i = { 2, 2, 2, 2 };
object[] oi = new object[i.Length];
for (int j = 0; j < i.Length; j++) io[j] = i[j];
Display(oi);
char[] stArray = { 'h','e','l','l','o' };
object[] ostArray = new object[stArray.Length];
for (int j = 0; j < stArray.Length; j++) ostArray[j] = stArray[j];
Display(ostArray);

Despite everything, the person most likely to be fooling you next is yourself.

humayunlalzad14-May-08 19:57

14-May-08 19:57

Thanx now I understand that you have to convert the values of the array and not the array itself. But I still dont understand how come the string and the Employee arrays are being accepted by the display method. Has it got something to do with reference and value types. Because when I changed the Employee class to a struct I got an error. If you explain. How is it the object array is giving preference to ref types and not to value types, if all have been derived from object.

Guffa15-May-08 0:55

15-May-08 0:55

All value types (built-in value types and structs) inherit from the abstract class ValueType, which is treated differently by the compiler by storing the data inline instead of using a reference.

When you declare an array of strings, the array contains references to the strings, and the strings are stored outside the array. When you declare an array of ints, the array contains the int values themselves.

Despite everything, the person most likely to be fooling you next is yourself.

humayunlalzad15-May-08 5:09

15-May-08 5:09

Yes but what I dont understand is, why is the object array, accepting string[] and not int[].
where as an object variable will accept a string or an int or anything. Sorry if I am being trouble.

Guffa15-May-08 6:53

15-May-08 6:53

humayunlalzad wrote:
Yes but what I dont understand is, why is the object array, accepting string[] and not int[]. where as an object variable will accept a string or an int or anything.

When you store a string reference in an object reference variable, it's simply cast into an object reference. As the compiler knows that it's safe to store a string reference in an object reference variable, there is actually no conversion at all, the reference is just stored in the variable.

When you store an int value in an object reference variable, the value itself is not stored in the variable. Instead a new object is created on the heap, the int value is stored in this object, and the reference to the object is stored in the object reference variable. This is called boxing.

A string array can be handled as an object array, as they both contain references. An int array can not be handled as an object array, as it doesn't contain references. Instead each int in the array has to be boxed, and the references placed in an object array.

Boxing of single value types is done automatically, but there is no automatic boxing of all items in an array, as this could get very inefficient. Creating a value type array is efficient as it's still only one object, but if you could trigger an automatic boxing of all items in the array, you could accidentally create thousands of new objects.

Despite everything, the person most likely to be fooling you next is yourself.

humayunlalzad15-May-08 7:33

Re: problem converting to object [modified]

15-May-08 7:33

Thank you so much, now everything is crystal clear

Peter Josefsson Sweden14-May-08 12:01

Peter Josefsson Sweden

14-May-08 12:01

Like others have said, you can't cast an array of something into an array of something else. Ever. You have to loop, one way or the other. This is part of the philosophical heritage of C# - stuff that's very expensive to execute shouldn't be so simple to write that it routinely fools people into thinking it's inexpensive.

However, the .NET Framwork doesn't necessarily play by those rules, so a look in the class library can sometimes help you avoid writing the loop yourself. Look at System.Array.Copy and System.Array.ConstrainedCopy, for instance (or ConvertAll).

In your particular example I would suggest a generic version of Display(), though, as in:

// new generic version:
public static void Display<T>(T[] arr)
{
    foreach (T e in arr)
    {
        Console.WriteLine("{0} ", e.ToString());
    }
    Console.WriteLine();
}

// reimplementation of object array version, using generic version:
public static void Display(object[] oarr)
{
    Display<object>(oarr);
}

Provided that you rename the array called "i" (name clashes with the for loop counter, at least in VS2005) you can remove the comments and it will work like a charm. Note that you DON'T need to specify the type (as in Display<int>(intArray);), as it is inferred by the compiler.

For added value, you can also insert the keyword "params" before the parameter declarations, as in:

public static void Display<T>(params T[] arr) //...
public static void Display(params object[] oarr) //...

That way, you can invoke them like this:

Display(1, "hello", 2, "word"); // object version
Display<int>(1, 2, 3, 4); // generic version for ints

If you do anything more complex than ToString() (or something else that's inhereted from System.Object) on the elements, you will probably have to constrain T (using the where keyword) to make sure that it implements some interface or other.

I still think generics are sexy... wonder how long that will last...

Later,

--
Peter

modified on Wednesday, May 14, 2008 6:19 PM

humayunlalzad14-May-08 19:48