int.TryParse = null if not numeric?

Question

is there some way of return null if it can't parse a string to int?

with:

public .... , string? categoryID) 
{
int.TryParse(categoryID, out categoryID);

getting "cannot convert from 'out string' to 'out int'

what to do?

EDIT:

No longer relevant because of asp.net constraints is the way to solve problem

/M

Why do you parse it to an int when you want to store it in a string?? — Stefan Steinegger
– Stefan Steinegger, Commented Nov 13, 2009 at 14:17
By the way, string is a reference type, so you should not make it nullable. (does this compile anyway?) — Stefan Steinegger
– Stefan Steinegger, Commented Nov 13, 2009 at 14:17
I use this for asp.net mvc so the categoryID is mapped from the url, so I get error if i pass in string in url — Lasse Edsvik
– Lasse Edsvik, Commented Nov 13, 2009 at 14:20
@Stefan: My guess is that the OP is validating string format using this method as well as enforcing a fixed string format. Therefore, they can parse 10,000 and turn it into 10000, for example. Just a guess though. — Jeff Yates
– Jeff Yates, Commented Nov 13, 2009 at 14:29

Joey · Accepted Answer · 2014-01-20 08:23:08Z

28

First of all, why are you trying to parse a string to an int and stick the result back into a string?

The method signature is

bool int.TryParse(string, out int)

so you have to give a variable of type int as second argument. This also means that you won't get null if parsing fails, instead the method will simply return false. But you can easily piece that together:

int? TryParse2(string s) {
    int i;
    if (!int.TryParse(s, out i)) {
        return null;
    } else {
        return i;
    }
}

edited Jan 20, 2014 at 8:23

answered Nov 13, 2009 at 14:19

Joey

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2 Comments

Joey Over a year ago

Eep, thanks, Garry. Writing code without ever compiling it invites errors such as those :-)

cost Over a year ago

I bet this would make a nice extension method

jason · Accepted Answer · 2009-11-13 14:17:34Z

14

Here's a proper use of Int32.TryParse:

int? value;
int dummy;
if(Int32.TryParse(categoryID, out dummy)) {
    value = dummy;
}
else {
    value = null;
}
return value;

answered Nov 13, 2009 at 14:17

jason

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3 Comments

Dan Tao Over a year ago

Or: value = int.TryParse(categoryID, out dummy) ? dummy : null;

jason Over a year ago

Yes, absolutely but that's more opaque to a neophyte.

Konrad Rudolph Over a year ago

@Dan: won’t compile ;-) But yes, I prefer that solution (plus one cast, to make it compile).

DigitalNomad · Accepted Answer · 2009-11-13 14:20:38Z

10

How about this?

public int? ParseToNull(string categoryId)
{
    int id;
    return int.TryParse(categoryId, out id) ? (int?)id : null;
}

answered Nov 13, 2009 at 14:20

DigitalNomad

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5 Comments

Martin Clarke Over a year ago

This solution might benefit from being turned into an extension method.

DigitalNomad Over a year ago

It's already an extension method in my app but I didn't know what molgan wanted it for so I kept it as a method.

Khanzor Over a year ago

you don't need the cast on id there. An int or a null is a valid Nullable<int> :).

SLaks Over a year ago

@Khanzor: Actually, you do, unless you replace null with new int?(). Otherwise, the type of the conditional cannot be inferred.

Khanzor Over a year ago

@SLaKs - Ahh, so he does. If you don't use the ternary operator you can avoid the cast though.

John C · Accepted Answer · 2019-10-18 09:03:44Z

2

Simplest and one-liner...

int N = int.TryParse(somestring, out N) ? N : 0;

It works 'cause it's evaluated left to right. Null not so easy.

edited Oct 18, 2019 at 9:03

John C

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answered Jun 11, 2015 at 18:22

user5000501

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Comments

Konamiman · Accepted Answer · 2009-11-13 14:17:30Z

0

TryParse will return false if the string can't be parsed. You can use this fact to return either the parsed value, or null. Anyway I guess that you are intending to return int? from your method, then it would be something like this:

public int? ParseInt(string categoryID) 
{
    int theIntValue;
    bool parseOk = int.TryParse(categoryID, out theIntValue);
    if(parseOk) {
        return theIntValue;
    } else {
        return null;
    }
}

answered Nov 13, 2009 at 14:17

Konamiman

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Comments

Stefan Steinegger · Accepted Answer · 2009-11-13 14:19:18Z

0

Do you want to do something like this?

public int? Parse(string categoryID) 
{
  int value;
  if (int.TryParse(categoryID, out value))
  {
    return value;
  }
  else
  {
    return null;
  }
}

answered Nov 13, 2009 at 14:19

Stefan Steinegger

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Comments

score 0 · Accepted Answer · 2009-11-13 14:24:01Z

0

Int is a value type which means there is no such thing as a null int. So no, TryParse will never alter the out parameter so that it is null.

But the problem you're having is you're passing a string to the out parameter of TryParse when its expecting an integer.

You need something like this...

Int categoryID = 0;
string strCategoryID = "somestringmaybeitsaninteger";

int.TryParse(strCategoryID, out categoryID);

edited Nov 13, 2009 at 14:24

answered Nov 13, 2009 at 14:16

user6212

3 Comments

Dani Over a year ago

There is a nullable int: int?

Joey Over a year ago

Dani: TryParse however, returns int, not Nullable<int>.

jerhinesmith Over a year ago

int? is not actually an integer type ... it's shorthand for System.Nullable<int>.

Dani · Accepted Answer · 2009-11-13 14:38:32Z

0

** this answer was down-voted a lot ** Although it is a possible solution - it is a bad one performance wise, and probably not a good programming choice.

I will not delete it, as I guess many programmers might not be aware of this, so here is an example how not to do things:

use try and catch

try
{
res = Int32.Parse(strVAR)
}
catch(exception ex) 
{
 return null;
}

edited Nov 13, 2009 at 14:38

answered Nov 13, 2009 at 14:16

Dani

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17 Comments

Bomlin Over a year ago

Since C# 2.0 came out, TryParse is almost always better than doing exception catching.

Jon Skeet Over a year ago

It's an option, but it's a bad one - hence all the downvotes.

Dani Over a year ago

I will be happy to understand why it is bad (performance ?) ?

Bomlin Over a year ago

Exceptions should not, as a practice, be a part of the normal flow of code. They should be exactly as their name implies, an exception. If you are expecting certain things on a regular basis, you should try to code a solution rather than throwing an exception and handling it. An example is checking if a file exists prior to trying to open it rather than opening a file and catching a file not found exception. So in general, avoid exception programming if you don't need it. In other words, save it for the exceptions... :)

DigitalNomad Over a year ago

+1 Because although it's incorrect, it does show a valid way that someone could solve a problem such as this. Since the poster edited his answer to show that it was incorrect, and left comments as to why, I see no reason that it should be given such a low ranking.

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