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I have now come across a few situations where using nested forms in ASP .NET causes problems. The typical error produced in this case is a 'Invalid postback or callback argument' error. This occurs because ASP .NET allows the rendering of multiple, nested forms but fails to validate the forms when a postback occurs. Thus when ASP .NET recognizes nested forms in a page, it marks the page as not valid (Page.IsValid returns false).

The reason that this error occurs is because multiple, nested forms cannot reside on a single aspx page .

The solution is very simple: The submit button for the user created form must contain the following attribute: onclick="this.form.submit();". A recent example I came across is below. The basic form that was developed and placed inside an aspx page as a nested form was as follows (only the beginning and ending of this form is shown. The content is not necessary or relevant):

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<form accept-charset="UTF-8" action="http://www.response-o-matic.com/mail.php" method="post" enctype="multipart/form-data">
<table>
<tbody>
...
<tr>
<td colspan="2" align="center">
<input value="Submit Form" type="submit" />
</td>
</tr>
</tbody>
</table>
</form>

The following code demonstrates the proper way of entering this form so that nested forms will work. Please note that I reiterated all of the attributes (action, method, enctype, etc...) of the form in the JavaScript call.

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<form accept-charset="UTF-8" action="http://www.response-o-matic.com/mail.php" method="post" enctype="multipart/form-data">
<table>
<tbody>
...
<tr>
<td>
<input onclick="this.form.action='http://www.response-o-matic.com/mail.php'; this.form.method='post';this.form.enctype='multipart/form-data';this.form.submit();" value=" Submit Form " type="submit" />
</td>
</tr>
</tbody>
</table>
</form>