As casting

Question

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();
}
}

Vulpes · 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(); } }

Maha · Answer

Thank you for your explanation.

Iftikar Hussain · Answer

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 { 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 { string value =( string) list[i] ; Console.WriteLine(value); } Console.ReadKey(); } } The output It will throw InvalidcastException Regards Iftikar

Jignesh Trivedi · Answer

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.