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How would you go about validating a field and throw an error if the field is empty because this is a REQUIRED field (not null)?

I am developing an ASP.NET application using VS2015 and .NetFramework 4.6.1

I use the follow text box control:

How would you go about validating this field and throw an error if this field is empty because this is a REQUIRED field (not null?

<asp:Label ID="lblCheck" Text = "Check:" runat ="server" ForeColor="#00356A" Width="10%" />
<asp:TextBox ID="txtChecknum" Maxlength="6" runat="server" style="margin-left: 0px;text-align: left;" Width="15%" ></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator2" runat="server"
ErrorMessage="Check Number Invalid. Enter only numbers."
ControlToValidate="txtChecknum"
SetFocusOnError="true"
ValidationExpression="^\d+$">
</asp:RegularExpressionValidator>

zimmer9

ASKER

I tried the following but no error pops up if I leave the text box blank.

<asp:Label ID="lblCheck" Text = "Check:" runat ="server" ForeColor="#00356A" Width="10%" />
<asp:TextBox ID="txtChecknum" Maxlength="6" runat="server" style="margin-left: 0px;text-align: left;" Width="15%" ></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator2" runat="server"
ErrorMessage="Check Number Invalid. Enter only numbers." Display="Dynamic"
ControlToValidate="txtChecknum"
SetFocusOnError="true"
ValidationExpression="^\d+$">
</asp:RegularExpressionValidator>
<asp:RequiredFieldValidator ID="rfvcandidate"
runat="server" ControlToValidate ="txtChecknum"
ErrorMessage="Required field. Numbers only." Display="Dynamic"
InitialValue="Please input only numbers">
</asp:RequiredFieldValidator>

ASKER CERTIFIED SOLUTION

Pawan Kumar

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