Do you avoid casts and use the "as operator" instead?

Use casts only if: a. You know 100% that you get that type back b. You want to perform a user-defined conversion

private void AMControlMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
 var auc = (AMUserControl)sender; 
   
 var aucSessionId = auc.myUserControl.Tag;
 // snip snip snip
}

private void AMControlMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
 var auc = sender as AMUserControl; 
   
 if (auc != null)
 {
 var aucSessionId = auc.myUserControl.Tag;
 // snip snip snip
 } 
   
}

More info here: http://blog.gfader.com/2010/08/avoid-type-casts-use-operator-and-check.html