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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 method of the HtmlTextBox processing instance. Three possible approaches are listed below:

Validate Xhtml Using Event And IsValidXhtml

Use the IsValidXhtml inside the HtmlTextBox ItemDataBinding handler:

CopyC#
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 (procTxt.IsValidXhtml(validXhtml))
        {
            procTxt.Value = validXhtml;
        }
        else
        {
            procTxt.Value = systemXhtml;
        }
    };
CopyVB.NET
    Dim txt As New Telerik.Reporting.HtmlTextBox()
    AddHandler txt.ItemDataBinding, AddressOf MyEventHandler


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

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 procTxt.IsValidXhtml(validXhtml) Then
        procTxt.Value = validXhtml
    Else
        procTxt.Value = systemXhtml
    End If
End Sub

Validate Xhtml Using IsValidXhtml in Expression

Use the IsValidXhtml inside the HtmlTextBox Expression:

CopyC#
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(ReportItem.IsValidXhtml('{0}'), '{0}', '{1}')", validXhtml, systemXhtml);
CopyVB.NET
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(ReportItem.IsValidXhtml('{0}'), '{0}', '{1}')", validXhtml, systemXhtml)

Validate Xhtml Using Event And ValueError

Use a try-catch block to handle the exception:

CopyC#
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;
        }
    };
CopyVB.NET
    Dim txt As New Telerik.Reporting.HtmlTextBox()
    AddHandler txt.ItemDataBinding, AddressOf ValueErrorHandler


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

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