XHTML Validation

The HtmlTextBox requires valid XHTML and you should make sure you provide such otherwise the HtmlTextBox would throw exception. To handle this exception or just check whether the HtmlTextBox would be able to handle the content you set as value, you should use the IsValidXhtml expressions function or Telerik.Reporting.Processing.XhtmlValidator.IsValidXhtml static method. Three possible approaches are listed below:

Validate Xhtml Using IsValidXhtml in Expression

Use the IsValidXhtml inside the HtmlTextBox Expression :

const string validXhtml = "<b>valid xhtml</b>.";
const string systemXhtml = "Provided html is not acceptable.";

Telerik.Reporting.HtmlTextBox txt = new Telerik.Reporting.HtmlTextBox();
txt.Value = string.Format("=Iif(IsValidXhtml('{0}'), '{0}', '{1}')", validXhtml, systemXhtml);

Const validXhtml As String = "<b>valid xhtml</b>."
Const systemXhtml As String = "Provided html is not acceptable."

Dim txt As New Telerik.Reporting.HtmlTextBox()
txt.Value = String.Format("=Iif(IsValidXhtml('{0}'), '{0}', '{1}')", validXhtml, systemXhtml)

Validate Xhtml Using Event And IsValidXhtml

Use the IsValidXhtml inside the HtmlTextBox ItemDataBinding handler:

const string validXhtml = "<b>valid xhtml</b>.";
const string systemXhtml = "Provided html is not acceptable.";

Telerik.Reporting.HtmlTextBox txt = new Telerik.Reporting.HtmlTextBox();
txt.ItemDataBinding += delegate(object sender, EventArgs args)
    {
        Telerik.Reporting.Processing.HtmlTextBox procTxt = (Telerik.Reporting.Processing.HtmlTextBox)sender;
        if (Telerik.Reporting.Processing.XhtmlValidator.IsValidXhtml(validXhtml))
        {
            procTxt.Value = validXhtml;
        }
        else
        {
            procTxt.Value = systemXhtml;
        }
    };

Const validXhtml As String = "<b>valid xhtml</b>."
Const systemXhtml As String = "Provided html is not acceptable."

Public Sub TestValidateXhtmlWithEventAndIsValidXhtml()
    Dim txt As New Telerik.Reporting.HtmlTextBox()
    AddHandler txt.ItemDataBinding, AddressOf MyEventHandler
End Sub

Private Sub MyEventHandler(ByVal sender As Object, ByVal e As EventArgs)
    Dim procTxt As Telerik.Reporting.Processing.HtmlTextBox = DirectCast(sender, Telerik.Reporting.Processing.HtmlTextBox)
    If Telerik.Reporting.Processing.XhtmlValidator.IsValidXhtml(validXhtml) Then
        procTxt.Value = validXhtml
    Else
        procTxt.Value = systemXhtml
    End If
End Sub

Validate Xhtml Using Event And ValueError

Use a try-catch block to handle the exception:

const string invalidXhtml = "<b>invalid xhtml.";
const string systemXhtml = "Provided html is not acceptable.";

Telerik.Reporting.HtmlTextBox txt = new Telerik.Reporting.HtmlTextBox();
txt.ItemDataBinding += delegate(object sender, EventArgs args)
    {
        Telerik.Reporting.Processing.HtmlTextBox procTxt = (Telerik.Reporting.Processing.HtmlTextBox)sender;
        try
        {
            procTxt.Value = invalidXhtml;
            // html that will not be accepted
        }
        catch (Exception)
        {
            procTxt.Value = systemXhtml;
        }
    };

Const invalidXhtml As String = "<b>invalid xhtml."
Const defaultXhtml As String = "Provided html is not acceptable."

Public Sub TestValidateXhtmlWithEventAndValueError()
    Dim txt As New Telerik.Reporting.HtmlTextBox()
    AddHandler txt.ItemDataBinding, AddressOf ValueErrorHandler
End Sub

Private Sub ValueErrorHandler(ByVal sender As Object, ByVal e As EventArgs)
    Dim procTxt As Telerik.Reporting.Processing.HtmlTextBox = DirectCast(sender, Telerik.Reporting.Processing.HtmlTextBox)
    Try
        ' html that will not be accepted
        procTxt.Value = invalidXhtml
    Catch generatedExceptionName As Exception
        procTxt.Value = defaultXhtml
    End Try
End Sub