无法编辑我的DataGrid WPF Framework 4.5的单元格

My ObservableCollection has three items, and the rows are consistently shown in the datagrid. I cannot turn into edit mode a single cell of my DataGrid. I tried click, click-click, double-click, F2,…, but the whole row stay selected. How can I let the user edit the datagrid. I found similar datagrid edit-questions in other posts but no-one solved my problem. Here is the code (WPF NetFramework 4.5). Pursposely, only the first column is non-editable (readonly).My ObservableCollection has three items, and th