Pass a Control to a Method

Question

I'm trying to create a method that will custom validate any TextBox control passed to it.

Here's what I've got so far:

A Custom Validator

protected void CustomValidatorDelLN_ServerValidate(object sender, ServerValidateEventArgs args)
    {
        CustomValidator ThisValidator = sender as CustomValidator;
        TextBox MyBox = FindControl(ThisValidator.ControlToValidate) as TextBox;

        args.IsValid = isValid(MyBox);
    }

Validation Method

protected bool isValid(System.Web.UI.WebControls.TextBox MyBox)
    {
        bool is_valid = MyBox.Text != "";
        MyBox.BackColor = is_valid ? System.Drawing.Color.White : System.Drawing.Color.LightPink;
        return is_valid;
    }

The code compiles OK but I get a

NullReferenceException was unhandled by user code

on

bool is_valid = MyBox.Text != "";

When I run the validation.

I know I'm close (well I think I am) but where am I going wrong?

Line 'TextBox MyBox = FindControl(ThisValidator.ControlToValidate) as TextBox;' - MyBox is null — Tomas Voracek, Sep 23 '11 at 14:39
Look at FinControl() it's not recursive. I'm not sure why you're using FindControl() there anyway, are you on the page? If so you can put a runat="server" in that sucka and use it by name. — EJC, Sep 23 '11 at 14:41
Sorry, I see you are using it from the custom validator, so you can validate any TextBox. — EJC, Sep 23 '11 at 14:43
@EJC Thanks for your comments which helped the penny drop. All sussed and working. — ComfortablyNumb, Sep 23 '11 at 14:56
Can your firstly separate the code by `bool is_valid; if(MyBox.Text != "") is_valid = true; else is_valid=false;` — SUN Jiangong, Sep 23 '11 at 14:59
@ComfortablyNumb no problem :) sorry for scrutinizing your usage of `FinControl()` at first :) — EJC, Sep 23 '11 at 14:59
@EJC That's OK. I'm on a steep learning curve so any help is very appreciated! — ComfortablyNumb, Sep 23 '11 at 17:33

score 1 · Answer 1 · answered Sep 23 '11 at 14:47

1

you first need to check that the object itself exist, after casting:

bool is_valid = MyBox != null;

and after that you can check its text property

answered Sep 23 '11 at 14:47

Shlomi Komemi

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score 1 · Accepted Answer · answered Sep 23 '11 at 14:51

Your problem is the FindControl() method is not recursive, therefore MyBox is null. You'll have to write a recursive FindControl() method like the one here if you want it to work correctly.

You'll probably also want to check if MyBox is null and return out of the method if it is.

score 0 · Answer 3 · answered Sep 23 '11 at 14:56

For completeness this code did the trick for me:

protected void CustomValidatorDelLN_ServerValidate(object sender, ServerValidateEventArgs args)
    {
        args.IsValid = isValid(txtDeliveryLastName);
    }

    protected bool isValid(System.Web.UI.WebControls.TextBox MyBox)
    {
        bool is_valid = MyBox.Text != "";
        MyBox.BackColor = is_valid ? System.Drawing.Color.White : System.Drawing.Color.LightPink;
        return is_valid;
    }

score -4 · Answer 4 · answered Sep 23 '11 at 14:44

-4

You are trying to validate an empty text box. You can't validate an empty string.

answered Sep 23 '11 at 14:44

Botonomous

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2

An empty string will not cause a `NullReferenceException`. – Anthony Pegram Sep 23 '11 at 14:47
bool is_valid = MyBox.Text != ""; This will not cause NullReferenceException ? Why not do bool is_valid = MyBox.Text != "null" like the above poster – Botonomous Sep 23 '11 at 14:49
The problem isn't the empty string. It's accessing the `.Text` property of a null object. Look at it another way. Is an empty string a null reference? (No.) Or could the textbox variable contain a null reference? (Yes.) Then ask if you can access a property of a null reference. – Anthony Pegram Sep 23 '11 at 14:52
I guess i said it wrong...I meant the null object, not a null string. Yes a string can be empty and not null. Thanks for the correction. – Botonomous Sep 23 '11 at 14:58

Pass a Control to a Method

4 Answers4