请大神帮我把下面代码转成C#代码

            Mystr = "dgfasksa245"
            TargetStr = ""
            For Item = 1 To Len(Mystr)
                If cell.Characters(Item, 1).Font.Size > 5 Then
                   TargetStr = TargetStr & Mid(Mystr, Item, 1)
                End If