Question

When page is to be posted back to the server, browser collects the current values of each control and pastes it together into a string. This postback data is then sent back to the server via HTTP POST.

Q1 – Besides control’s Text attributes and SelectedIndexchanged ( thus besides user input data), are there other attributes/values of a control which are saved by a browser as postback data?

Q2 – In case of GridView, which values are saved by a browser on a postback? Only those in a row which the user chooses to edit?

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Answer 1

The values of textarea, select, input and button fields are returned in the post. Each value is a key-value pair where the key is the name property of the element.

I think that I have got all the elements that include data in the post:

• textarea: The value propery is included, i.e. what’s typed in the textarea.

• select: The value property of the selected option is included. If the selected option doesn’t have a value property specified, the text of the option is used.

• input type='text': The value property is included, i.e. what’s typed in the input field.

• input type='password': The value property is included, i.e. what’s typed in the input field.

• input type='submit': If the button was used to send the form, the value property is included, i.e. the text of the button.

• input type='image': If the button was used to send the form, the coordinates of the mouse click within the image is sent in the post. Names for the x and y coordinates are created by adding ‘.x’ and ‘.y’ to the name of the element.

• input type='checkbox': If the checkbox is checked, the value property is included. If the element has no value property specified, the value ‘on’ is used.

• input type='radio': The value property is included from the selected item from each group. (A group is all radio buttons with the same name.)

• input type='file': The content of the selected file is included, along with the original file path (or only the file name, depending on browser and security settings).

• input type='hidden': The value property is included.

• button: If the button was used to send the form, the innerText property is included, i.e. the text of the button with any html markup removed.

A TextBox control is rendered either as an input type='text', an input type='password' or a textarea, depending on the TextMode property. A DropDownList control is rendered as a select element. A Button control is rendered as an input type='submit'. A CheckBox control is rendered as an input type='checkbox'. And so on… check the rendered html code to see what the actual html elements rendered are.

A GridView only includes any data in the post if it contains any editable form fields, or if it causes a postback (by navigating in the list for example). When doing a postback there is some information stored in a pair of hidden fields, so any control that causes a postback but doesn’t send any form data by itself (like a LinkButton for example) does include information about what caused the postback.

Controls may also put data in the ViewState, which is kept in a hidden field in the form. This is also included in the post, but it’s just sent to the browser and back again without being changed by the browser.