In this article I will explain with an example, how to validate at-least one TextBox from Multiple TextBoxes in ASP.Net.
The at-least one TextBox validation will be performed using JavaScript and ASP.Net CustomValidator in ASP.Net.
 
 
HTML Markup
The HTML Markup consists of multiple ASP.Net TextBoxes, a CustomValidator and a Button. The ASP.Net CustomValidator has been assigned with ClientValidationFunction property in which the name of JavaScript function has been specified which will perform the validation.
<table>
    <tr>
        <th colspan="2">Address Proofs (At-least one)</th>
    </tr>
    <tr>
        <td>Passport Number</td>
        <td><asp:TextBox ID="txtPassport" runat="server" /></td>
    </tr>
    <tr>
        <td>Aadhar Number</td>
        <td><asp:TextBox ID="txtAadhar" runat="server" /></td>
    </tr>
    <tr>
        <td>PAN Number</td>
        <td><asp:TextBox ID="txtPAN" runat="server" /></td>
    </tr>
    <tr>
        <td colspan="2">
            <asp:CustomValidator ErrorMessage="Please enter at-least one Address proof." runat="server" ForeColor="Red" ClientValidationFunction="ValidateAddressProof" />
        </td>
    </tr>
    <tr>
        <td></td>
        <td><asp:Button Text="Submit" runat="server" /></td>
    </tr>
</table>
 
 
The ClientValidationFunction JavaScript function
Inside the ClientValidationFunction JavaScript function, first the TextBoxes are referenced and their values are fetched in respective labels.
Note: The ID of the ASP.Net controls, tend to change when Master Pages or UserControls are used and hence the ClientID property is used to fetch the IDs of the controls. For more details, please refer ASP.Net: JavaScript document.getElementById returns NULL when using Master Page.
 
Then, a check is performed one by one for each value of Passport number, Aadhar number and PAN number respectively. If at-least one of them has value then the validation is passed else the validation fails.
The validation is set OK with the help of args.IsValid property set to True and it is failed by setting the args.IsValid property to False.
<script type="text/javascript">
    function ValidateAddressProof(sender, args) {
        //Referencing and fetching the TextBox values.
        var passport = document.getElementById("<%=txtPassport.ClientID%>").value;
        var aadhar = document.getElementById("<%=txtAadhar.ClientID%>").value;
        var pan = document.getElementById("<%=txtPAN.ClientID%>").value;
 
        //Check if Passport Number is not blank.
        if (passport.trim() != "") {
            args.IsValid = true;
            return;
        }
 
        //Check if Aadhar Number is not blank.
        if (aadhar.trim() != "") {
            args.IsValid = true;
            return;
        }
 
        //Check if PAN Number is not blank.
        if (pan.trim() != "") {
            args.IsValid = true;
            return;
        }
 
        args.IsValid = false;
    }
</script>
 
 
Screenshot
Validate At-least one TextBox from Multiple TextBoxes in ASP.Net
 
 
Demo
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Downloads
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