Validation of another controls based on the dropdown choice - asp.net

Question

I am facing a problem for validating the dependency controls. How to validate the another controls which is based on the values of dropdownlist (combo box). Please see the below example. For Example : A dropdown has two values A and B. If user selects "A", then rest of the fields should become required fields and if user selects value “B” from the dropdown, then all other fields should become non-mandatory fields.
Note : i am using the devexpress gridview with default template.
<dx:ASPxGridView ID="ABC" runat="server" AutoGenerateColumns="False"
DataSourceID="ABCDataSource" EnableTheming="True"
Width="100%" ClientInstanceName="gridABC">
<Columns>
<dx:GridViewCommandColumn Caption="Actions" ShowInCustomizationForm="True"
VisibleIndex="0">
<EditButton Visible="True">
</EditButton>
</dx:GridViewCommandColumn>
<dx:GridViewDataComboBoxColumn Caption="List of Type" FieldName="ListType"
ShowInCustomizationForm="True" VisibleIndex="4" Width="100px">
<PropertiesComboBox DataSourceID="ListTypeDataSource"
TextField="ListTypeABC" ValueField="ListTypeABCId" Width="100px">
<ValidationSettings>
<RequiredField ErrorText="ListType required" IsRequired="True" />
</ValidationSettings>
</PropertiesComboBox>
<EditFormSettings ColumnSpan="1" VisibleIndex="1" />
</dx:GridViewDataComboBoxColumn>
<dx:GridViewDataTextColumn Caption="Name" FieldName="Name" ShowInCustomizationForm="True"
VisibleIndex="2" Width="100px">
<PropertiesTextEdit MaxLength="30" Width="100px">
<ValidationSettings>
<RequiredField ErrorText="Item Name required" IsRequired="False" />
</ValidationSettings>
</PropertiesTextEdit>
<EditFormSettings ColumnSpan="1" VisibleIndex="2" />
</dx:GridViewDataTextColumn>
<dx:GridViewDataTextColumn Caption="Contact" FieldName="Contact" ShowInCustomizationForm="True"
VisibleIndex="3" Width="100px">
<PropertiesTextEdit MaxLength="30" Width="100px">
<ValidationSettings>
<RequiredField ErrorText="Contact" IsRequired="False" />
</ValidationSettings>
</PropertiesTextEdit>
<EditFormSettings ColumnSpan="1" VisibleIndex="3" />
</dx:GridViewDataTextColumn>
<Templates>
<EditForm>
<table>
<tr>
<td>`<dx:ASPxGridViewTemplateReplacement ReplacementType="EditFormEditors" ID="ASPxGridViewTemplateReplacement1"
runat="server">
</dx:ASPxGridViewTemplateReplacement>
`

Answer 1

The simplest approach is to set the DropDownList's AutoPostBack property to true and handle it's SelectedIndexChanged event. Then you can Enable/Disable the validator there.
Another approach is to use a CustomValidator. This validator is not dependent on a single control. You must write the validation rules on your own. For example the ClientValidationFunction:
<script type="text/javascript" >
function ClientValidate(source, arguments)
{
var txt = document.getElementById('TextBox1');
var ddl = document.getElementById('DropDownList1');
var decision = ddl.options[ddl.selectedIndex].text;
if(decision=='Yes')
{
arguments.IsValid = txt.value.length > 0;
}else{
arguments.IsValid = true;
}
}
</script>
<asp:DropDownList id="DropDownList1" runat="server">
<asp:ListItem Selected="True">Yes</asp:ListItem>
<asp:ListItem Selected="False">No</asp:ListItem>
</asp:DropDownList>
<asp:TextBox id="TextBox1" runat="server" />
<asp:Button ID="BtnSubmit" runat="server" Text="Submit" />
<asp:CustomValidator id="CustomValidator1"
ValidateEmptyText="true"
ControlToValidate="TextBox1"
ClientValidationFunction="ClientValidate"
OnServerValidate="ServerValidation"
Display="Static"
ErrorMessage="Please enter text!"
runat="server"/>
Remember to always implement a OnServerValidate because you should not rely on javascript only(can be disabled). This is easy:
void ServerValidation(object source, ServerValidateEventArgs args)
{
args.IsValid = DropDownList1.SelectedIndex == 1 || TextBox1.Text.Length > 0;
}