ASP.NET NullReferenceException - asp.net

Question

I am trying to write the user login details to the Database.
When I click the submit button Im getting a NullReferenceException.
There are 4 TextBoxes
Username, Email, Password and ConfirmPassword.
protected void Button1_Click(object sender, EventArgs e)
{
if ((RegisterUserWizardStep.FindControl("Password") as TextBox).Text == (RegisterUserWizardStep.FindControl("ConfirmPassword") as TextBox).Text)
{
//call the method to execute insert to the database
ExecuteInsert((RegisterUserWizardStep.FindControl("UserName") as TextBox).Text,
(RegisterUserWizardStep.FindControl("Email") as TextBox).Text,
(RegisterUserWizardStep.FindControl("Password") as TextBox).Text);
Response.Write("Record was successfully added!");
ClearControls(Page);
}
else
{
Response.Write("Password did not match");
(RegisterUserWizardStep.FindControl("Password") as TextBox).Focus();
}
}
Thank you.

Answer 1

You mention there are four controls - Username, Email, Password and ConfirmPassword
The null exception you are seeing is almost certainly because FindControl(X) is returning null
A better way of checking is to do something like:
TextBox myTextBox = RegisterUserWizardStep.FindControl(X) as TextBox;
if(myTextBox != null){
//Continue
}
else{
//Write out some error information - now you know what the problem is.
}
Further, and this isn't related to your immediate error, but then you feed the contents of each of the text boxes directly into your ExecuteInsert method - you'd be better off doing some validation, too, just to check you have expected values.

Answer 2

Its likely that FindControl didn't find the control you are after, possibly because the TextBoxes are nested under another child control like a panel etc.
Instead of
if ((RegisterUserWizardStep.FindControl("Password") as TextBox).Text
try
TextBox passwordTextBox = RegisterUserWizardStep.FindControl("Password") as TextBox;
// .. same for username and email
if ((passwordTextBox != null) && (usernameTextBox != null) ... )
{
// Do something with the textboxes
}
// else you have a bug
This will also prevent you repeating the FindControl code on the same control (DRY principle)

Answer 3

In your description you've said that you have a Username TextBox.
The code is looking for RegisterUserWizardStep.FindControl("UserName").
Is this a typo in the question? otherwise it could be the cause of the exception.

Answer 4

Code like RegisterUserWizardStep.FindControl("UserName") as TextBox will return null either if there is no control named UserName or if the control named UserName can't be cast to a TextBox. This is most likely the source of your exception because you attempt to get the property Text of a reference that might be null.
To better understand where the problem is you can define an extension function:
static class ControlExtensions {
public T Find(this Control parent, String name) where T : Control {
var control = parent.FindControl(name);
if (control == null)
throw new ArgumentException(String.Format("Cannot find control named '{0}'.", name);
var t = control as T;
if (t == null)
throw new ArgumentException(String.Format("Control named '{0}' does not have type '{1}.", name, typeof(T).Name);
return t;
}
}
You can then get the Text property of the UserName control:
RegisterUserWizardStep.Find<TextBox>("UserName").Text
This call will throw a more descriptive exception if the control isn't found.