You can use the RenderAllHiddenFieldsAtTopOfForm property to specify that some system-generated hidden fields are rendered at the end of the form.
In versions of ASP.NET earlier than the .NET Framework 3.5 SP1, ASP.NET renders some hidden fields (for example, __VIEWSTATE) at the top of the form. Other hidden fields (for example, __EVENTVALIDATION) are rendered near the bottom of the form, just before the closing </div> tag. By default, in ASP.NET 3.5 SP1, all system-generated hidden fields are rendered at the top of the page. This makes sure that the information in these fields is sent to the server even if a postback is performed before the page has finished loading. If RenderAllHiddenFieldsAtTopOfForm is set to false, performing a postback before the page has finished loading can cause an "Invalid postback or callback argument" error.
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Although the property name indicates "all hidden fields," user-created hidden fields are not affected by settings for this property.
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