The built-in dialog box derives from the C1Window class. To perform minor customizations to the dialog box, instantiate the C1SpellDialog, modify its properties, and then pass the customized dialog box to the CheckControlAsync method.

For example, the code below modifies the caption of the built-in dialog box:

var dlg = new C1SpellDialog();
dlg.Header = "Customized Caption!";
c1SpellChecker1.CheckControlAsync(textBox1, false, dlg);

Here is the customized version of the built-in dialog box: