如何从客户端.ascx使用控件ID并在服务器端ascx.cs（ASP.NET）中使用它

Question

I have a ASP.NET control defined in my .acsx and some code on the server side that checks it in my .cs, creating a new custom validator if the data input is invalid.

I am trying to make the error message display next to the control, but I am unable to set the ControlToValidate attribute to the ID of the control. Visual Studio highlights ContactTelephoneTextBox with the error:

cannot convert source type

This is the Custom Validator in the .cs

Page.Validators.Add(new CustomValidator()
{
    Display = ValidatorDisplay.Dynamic,
    ControlToValidate = ContactTelephoneTextBox, 
    ID = "phoneInvalid",
    IsValid = false,
    ErrorMessage = "Please enter a valid phone number.", 
    field",
});

In my .ascx

<div id="ContactTelephone">
    <asp:Label ID="ContactTelephoneLabel" AssociatedControlID="ContactTelephoneTextBox"
        CssClass="required formFont2015 formLabel2015" runat="server">Contact number</asp:Label>
    <asp:TextBox ID="ContactTelephoneTextBox" CssClass="boxStyle2015" runat="server"></asp:TextBox>
</div>

Answer 1

ControlToValidate expects a string not a control, so this should work:

Page.Validators.Add(new CustomValidator()
{
    Display = ValidatorDisplay.Dynamic,
    ControlToValidate = ContactTelephoneTextBox.ClientID, 
    ID = "phoneInvalid",
    IsValid = false,
    ErrorMessage = "Please enter a valid phone number.", 
    field",
});