Difference Between ToString() and Casting to String

Question

string id = (string)result.Rows[0]["Id"];

Above line of code returns InvalidCastException. Why it is that so?

But if I change the code to this

string id = result.Rows[0]["Id"].ToString();

then it's working. Am I did anything wrong in my previous line of code?

Related: http://stackoverflow.com/q/1565100/447156 and http://stackoverflow.com/q/4152205/447156 — Soner Gönül, Jul 28 '15 at 07:41
note that `.ToString` method is for all objects. it may return name of the object not the property (`id`)you want. — M.kazem Akhgary, Jul 28 '15 at 07:46

score 5 · Accepted Answer · edited Jul 28 '15 at 08:28

5

it's not working because ID has a different Type. It's not string - so you can convert it but not cast it.

edited Jul 28 '15 at 08:28

Aslam Jiffry

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answered Jul 28 '15 at 07:39

fubo

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Fabjan · Answer 2 · 2015-07-28T08:08:51.933

Let's have a look at different operations like if it's some dialog between you and compiler:

    // here you say to compiler "hey i am 100% sure that it is possible 
    // to cast this `result.Rows[0]["Id]` to string
    // this results in error if cast operation failed

    string id = (string)result.Rows[0]["Id"];


    // here you say to compiler: "please try to cast it to 
    // string but be careful as i am unsure that this is possible"
    // this results in `null` if cast operation failed

    string id = result.Rows[0]["Id"] as string;


    // here you say to compiler: "please show me the string representation of 
    // this result.Rows[0]["Id"] or whatever it is"
    // this results in invoking object.ToString() method if type of result.Rows[0]["Id"]  
    // does not override .ToString() method.

    string id = result.Rows[0]["Id"].ToString();

score 2 · Answer 3 · answered Jul 28 '15 at 07:53

I guess your rows' indexer's type isn't string. A cast looks like this:

(TypeA)objB

This is only successful when

objB is of type TypeA,
objB is of type TypeC where TypeC is a subclass of TypeA,
objB is of type TypeC where TypeC is a superclass of TypeA and objB's declaring type is TypeA.

So, your code doesn't work.

However, since every type derives from the holy Object class, every type has a ToString method. Thus, whatever type Rows[0]["Id"] returns, it has or has not a custom implementation of the ToString method. The type of the return value of the ToString method is always, you guessed it, String. So that's why ToString works.

score 1 · Answer 4 · edited May 23 '17 at 11:43

ToString() does not simply cast your object, it calls its ToString-method providing a "string-representation". Casting however means that the object itself IS a string and therefor you can cast it.

Also have a look here: Casting to string versus calling ToString

EDIT: The ToString-method derived from object can be used to give a representation of any arbitrary object.

MyClass 
{
    int myInt = 3;
    public override string ToString() {
        return Convert.ToString(myInt);
    }
}

If ToString is not overridden within your class, than it´s default return-value is the class´ typename.

score 0 · Answer 5 · answered Jul 28 '15 at 07:40

0

With ToString() you are converting the row0's Id to string but in other case your casting as string which is impossible in current scenario.

answered Jul 28 '15 at 07:40

DeshDeep Singh

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Difference Between ToString() and Casting to String

5 Answers5