void MakeMatch()
{
if (SelectedExternal == null || SelectedExternal == null)
{
return;
}
SelectedInternal.Status = MatchStatus.NewMatch;
SelectedExternal.Status = MatchStatus.NewMatch;
Matched.Insert(0, new ManualMatchedPair <TInternal, TExternal>(this, SelectedInternal, SelectedExternal));
UnmatchedInternals.Remove(SelectedInternal);
UnmatchedExternals.Remove(SelectedExternal);
}