Completed course 2 adn 3, working on 4

Assets/Scripts/UnitSelection.cs

@@ -28,6 +28,15 @@ public class UnitSelection : MonoBehaviour
     // Update is called once per frame
     void Update()
     {
+        //mouse down
+        if(Input.GetMouseButtonDown(0))
+        {
+            startPos = Input.mousePosition;
+            ToggleSelectionVisual(false);
+            selectedUnits = new List<Unit>();
+            TrySelect(Input.mousePosition);
+            startPos = Input.mousePosition;
+        }
         //mouse up
         if(Input.GetMouseButtonUp(0))
         {
@@ -38,14 +47,6 @@ public class UnitSelection : MonoBehaviour
         {
             UpdateSelectionBox(Input.mousePosition);
         }
-        //mouse down
-        if(Input.GetMouseButtonDown(0))
-        {
-            startPos = Input.mousePosition;
-            ToggleSelectionVisual(false);
-            selectedUnits = new List<Unit>();
-            TrySelect(Input.mousePosition);
-        }
     }
 
     //called when we release the selection box
@@ -110,4 +111,13 @@ public class UnitSelection : MonoBehaviour
             unit.ToggleSelectionVisual(selected);
         }
     }
+    //removes all destroyed or missing units from the selected list
+    public void RemoveNullUnitsFromSelection()
+    {
+        for(int x = 0; x < selectedUnits.Count; x++)
+        {
+            if(selectedUnits[x] == null)
+                selectedUnits.RemoveAt(x);
+        }
+    }
 }