test.cs

private void cbPreview_CheckedChanged(object sender, EventArgs e)
{
    // preview the mail merge document
    if (cbPreview.Checked)
    {
        // save the template 
        gbMergeElements.Enabled = false;
        textControl1.EditMode = TXTextControl.EditMode.ReadAndSelect;
        textControl1.Save(out previewTemplate, 
            TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        // merge the template using the DataSourceManager
        IList<byte[]> documents = 
            dsManager.Merge(previewTemplate, textControl1);

        // load the merged document into TextControl
        textControl1.Load(documents[0], 
            TXTextControl.BinaryStreamType.InternalUnicodeFormat);
    }
    else
    {
        // load back the stored template
        gbMergeElements.Enabled = true;
        textControl1.EditMode = TXTextControl.EditMode.Edit;
        textControl1.Load(previewTemplate, 
            TXTextControl.BinaryStreamType.InternalUnicodeFormat);
    }
}