in the textbox,button and required field validator set the validation group.
example:
button:
<asp:button id="Button5" runat="server" onclick="Button5_Click" text="Submit" font-bold="true" validationgroup="GroupToValidate" xmlns:asp="#unknown" />
textbox
<asp:textbox id="TextBox1" runat="server" validationgroup="GroupToValidate" xmlns:asp="#unknown"></asp:textbox>
required field validator
<asp:requiredfieldvalidator id="RequiredFieldValidator1" runat="server" validationgroup="btn" errormessage="Insert the Category" controltovalidate="TextBox1" xmlns:asp="#unknown"></asp:requiredfieldvalidator>
note: button, textbox and requiredFielValidator should have the same validation group text (ex. ValidationGroup="
GroupToValidate")
hope it helps
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