As casting

13y1.6kVisual Basic .NET

In this example following way casting is done. string value = list[i] as string;

Similar output is obtained with varied casting way string value = (string)list[i];

My question is whether casting is correct either way whatever data types are.

using System;
using System.Collections;

class Program
{
static void Main()
{
ArrayList list = new ArrayList();
list.Add("man");
list.Add("woman");
list.Add("plant");

for (int i = 0; i < list.Count; i++)
{
string value = list[i] as string;
Console.WriteLine(value);
}
Console.ReadKey();
}
}

4 Replies

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Vulpes13y ago
Accepted answer
The 'as' operator only works with certain types of conversions, namely reference type conversions, boxing conversions and nullable value type conversions.

It doesn't work with non-nullable value type conversions or with user-defined conversions for which you must use a cast instead.

The following program illustrates these points:

using System;

class MyClass
{
public string MyString {get; set;}

public MyClass (string s)
{
MyString = s;
}

public static explicit operator MyClass(string s)
{
return new MyClass(s);
}
}

class Test
{
static void Main()
{
object o = "Hello";
string s = o as String; // works with reference types

int i = 3;
o = i as object; // works when boxing value types

int j = 4;
int? k = j as int?; // works with nullable value types

long l = 5;
int m = l as int; // doesn't work with non-nullable value types

int n = (int)l; // works fine with cast

string t = "Goodbye";
MyClass mc = t as MyClass; // doesn't work with user defined conversions

MyClass mc2 = (MyClass)t; // works fine with cast

Console.ReadKey();
}
}
13y ago
Thank you for your explanation.
Iftikar Hussain13y ago
Hi,
The difference between below two type casting is

string value = list[i] as string; - This is Safe Casting
string value = (string)list[i]; - This is a normal Casting

The Safe Casting will not throw you InvalidCastException, instead it will return you null if the cast fails. Where as normal Casting will throw you the InvalidCastException error
So you need to use Safe Casting when you are not sure about the desired casting type else you can use normal Casting.
In your example just modify your code and check

By Using Safe Casting

using System;
using System.Collections;

class Program
{
static void Main()
{
ArrayList list = new ArrayList();
list.Add(new Object());
list.Add("woman");
list.Add("plant");

for (int i = 0; i < list.Count; i++)
{
string value = list[i] as string;
Console.WriteLine(value);
}
Console.ReadKey();
}
}

The output will be
(null)
woman
plant

By Using normal Casting

using System;
using System.Collections;

class Program
{
static void Main()
{
ArrayList list = new ArrayList();
list.Add("man");
list.Add("woman");
list.Add("plant");

for (int i = 0; i < list.Count; i++)
{
string value =(string) list[i];
Console.WriteLine(value);
}
Console.ReadKey();
}
}

The output
It will throw InvalidcastException

Regards,
Iftikar
Jignesh Trivedi13y ago
hi,

As per my knowledge, when you cast using (string)list[i] it forcing compiler to cast it gave runtime exception if cast type is not match.

when you cast using "as" keyword it try cast object, if object is not cast to target type then it return null value or default value. it means, if the conversion isn't possible, as returns null instead of raising an exception. internally perform following operation.

expression is type ? (type)expression : (type)null

hope this will help you.