Exception when trying to convert TextBox content

Question

It is possible to write the following condition?

if (textbox.text != null)

Because when I have written this condition in my program, it throws an exception.

If the data type of textbox is string then it is also nullable ?

The line will only throw an exception, if `textbox` itself is null. But you did not mention what exception. So what *is* your problem? What exception are you getting? — nvoigt, Jul 10 '15 at 05:31
What exception did you get? Are you using Windows.Form TextBox or something else? — Liem Do, Jul 10 '15 at 05:31
if you typed as it is then i think there is a compile time error. it must be "textbox.Text". Second you should always check if(!string.IsNullOrEmpty(textbox.Text)). — Anupam Sharma, Jul 10 '15 at 05:37
Yes I am using windos.foam textbox. Basically my coding is .. ( Double? textt; If(textbox.text != null) { textt = convert.toDouble(textbox.text); // (input string is not in correct format)exeption in buttion click event if(textt > 0) { //do something } if(textt < 0) { //do something } if(textt == 0) { //do something } if(textt == null) { //do something } } — Muhammed Naqi, Jul 10 '15 at 05:42
Please edit the question and add this actual code. Clearly the exception is not on this row, so the question is wrong. — Sami Kuhmonen, Jul 10 '15 at 06:46

score 0 · Accepted Answer · edited Jul 10 '15 at 06:47

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The only reason that particular line of code should cause an error is if the textbox variable itself was actually null. What is the exact exception?

Also a handy tip: if you want to check for an empty text box I recommend String.IsNullOrWhitespace(textbox.text) which will return a bool. It's great for if statements like this.

edited Jul 10 '15 at 06:47

Sami Kuhmonen

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answered Jul 10 '15 at 05:31

psoshmo

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i think String.IsNullOrWhitespace will return true if your string is "abc pqr". so be careful using this. string.IsNullOrEmpty would be correct one. – Anupam Sharma Jul 10 '15 at 05:40
I do not believe that is the case. – psoshmo Jul 10 '15 at 05:42
@anupamsharma here's a link explaining. http://stackoverflow.com/questions/6976597/string-isnulloremptystring-vs-string-isnullorwhitespacestring – psoshmo Jul 10 '15 at 05:51
The unhandled exception was "Input string was not in correct format. " – Muhammed Naqi Jul 10 '15 at 05:52

score 0 · Answer 2 · answered Jul 10 '15 at 06:17

Based on the OP's commentary in the main post, and his other comment stating that the exception said "Input string was not in correct format", it looks like it's not the line of code above that threw the exception (although it should correctly be .Text instead of .text).

Rather, this FormatException indicates that it probably is the Convert.ToDouble(textbox.Text) that is unable to convert the content of textbox to a double. Are you sure you entered a correct double value? This issue could also be related to your culture settings.

Fabjan · Answer 3 · 2015-07-10T07:06:04.403

"if (textbox.Text != null)"
Because when I have written this condition in my program, it throws an exception.

If data type of textbox is string then it is also nullable ?

Yes. textbox is just a variable that holds a reference to a class instance so its default value is null; You should be careful accessing .Text property because if textbox value is set to null you will have NullReferenceException.

So i'd do something like this (assuming it's method or event handler) :

void MyMethod()
{
    if(textbox == null) return;

    if(String.IsNullOrWhiteSpace(textbox.Text)) return;

    // your code here if textbox.Text has valid value
}

P.S. if you're converting this string to Double it's different story:

double GetDouble()
{
    if(textbox == null) throw new NullReferenceException("textbox is null");

    if(String.IsNullOrWhiteSpace(textbox.Text)) throw new ArgumentException("textbox.Text contains null or white spaces");

    double result = 0;
    if(!Double.TryParse(textbox.Text, out result)) throw new ArgumentException("textbox.Text has invalid number");
    return result;
}

Exception when trying to convert TextBox content

3 Answers3