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Cory Robinson's Blog

Validating Custom Composite Web Controls

Have you ever wondered how you go about validating a custom composite web control?  Normally, with ASP.NET server controls, you have:

<asp:TextBox ID="PhoneTextBox" runat="server" />
<asp:RequiredFieldValidator ID="PhoneRequiredFieldValidator" runat="server" ControlToValidate="PhoneTextBox" Display="Dynamic"
ErrorMessage="Please enter a phone number" />

But with a custom composite web control, say one which includes a label, a text box, and a required field validator, any validators that refer to your control will not know how to validate the data.

Say we have the following custom composite web control:

public class LabelledTextBox : CompositeControl
    private Label _nameLabel = new Label();
    private TextBox _nameTextBox = new TextBox();
    private RequiredFieldValidator _nameRequiredFieldValidator = new RequiredFieldValidator();

    public LabelledTextBox()
        _nameLabel.ID = "NameLabel";
        _nameLabel.Text = "My Label: ";

        _nameTextBox.ID = "NameTextBox";

        _nameRequiredFieldValidator.ID = "NameRequiredFieldValidator";
        _nameRequiredFieldValidator.ControlToValidate = _nameTextBox.ID;

    protected override void CreateChildControls()


    public string Text
        get { return _nameTextBox.Text; }
        set { _nameTextBox.Text = value; }

The RequiredFieldValidator inside the composite control can validate the text box just fine, as it knows about it. However, if you want to validate that the text box has, say, an integer between 1 and 100, by adding a RangeValidator to the markup, the validation will not work. You will get an error stating that the LabelledTextBox control cannot be validated. So, after some failed google attempts to find out how to solve this problem, my coworker Bryan Smith recalled that in Fritz Onion's book, Essential ASP.NET With Examples in C#, Fritz mentioned the ValidationProperty attribute that could be used to specify which property of the composite control should be validated.

So if we just add that attribute to the custom composite control class, we can then validate it:

public class LabelledTextBox : CompositeControl

So now when we have the following in our markup, it will validate the text box just fine:

<asp:LabelledTextBox ID="AgeLabelledTextBox" runat="server"/>
<asp:RangeValidator ID="AgeValidator" runat="server" ControlToValidate="AgeLabelledTextBox" Display="Dynamic"
ErrorMessage="Please enter an age between 1 and 100" Type="Integer"MinimumValue="1" MaximumValue="100" />

Thanks to the ValidationProperty, the framebwork now knows that the Text property of the custom composite control should be checked to make sure the value is between 1 and 100.

But there is a gotcha here.  The client side validation will appear to work just fine with the above scenario.  But if you have two text boxes within the same composite control,only the first one will be validated. The reason for this, as Bryan and I discovered, is that the validation javascript actually iterates through the child nodes of the composite control(rendered as a span) until it sees a control that has a value property.  It then validates that control and that control only.

So again, always validate your input at both client AND server sides.

Read the complete post at

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