Why is my list of selected units cleared after the first time

I loose all objects after the first selection with alpha kes. The first time i can save my list with

 private void ComputeKeySave()
 {
     if (Input.GetKey(KeyCode.LeftControl))
     {
         _key1 = new List<GameObject>(_selectedUnits);
     }
 }

but after selecting the units with

 private void ComputeKeySelection()
 {
     ComputeKeySave();
     if (Input.GetKeyDown(KeyCode.Alpha1))
     {
         _selectedUnits = _key1;
         SetSelectorActivation(true);
     }
 }

i can select only one time, after deseelect my units, i lose all values in _key1 list