i have implemented "Expand All and Collapse All options in a drill down" in this manner

included a boolean parameter named "ExpandAllGroups"

and then modified the sub groups as shown below:

For each sub group (except the topmost)

Edit Group --> Grouping and Sorting Properties --> Visibility Tab --> Initial Visibilty --> Expression: =IIf(Parameters!ExpandAllGroups.Value, False, True)

This is working! But the problem is even when a group is expanded the symbol showed is "+" and similarly even if a group is collapsed the symbol "-" is showed.