>In my form i have placed three text boxes 1.txtnum1 2.txtnum2 3.txtresult and one command button "cmdaddition". user has to enter two number and then click on the command button to produce an answer in txtresult. suppose user want to answer will be calculated every time either of the two text boxes values are changed with out click on the command button.
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>how to do this?

You can call a form's method from each Textbox InteractiveChange event.