C-m, C-o // collapse all
C-m, C-l // expand all
C-m, C-m // collapse, expand one
Where C is short for Ctrl. You can find more shortcuts at the msdn site
You can find the commands by going to Keyboard Options under the Tools menu, they are mapped to these commands
- Edit.CollapseToDefinitions (
C-m, C-o), - Edit.ToggleOutliningExpansion (
C-m, C-m) and - Edit.ToggleAllOutlining (
C-m, C-l)

If you are using Visual Studio Code the commands are similar but work differently. The commands are roughly these
C-k 2– Fold Level 2C-k C-j– Unfold allC-k [– Unfold recursively (unfold where the caret is)
You can search for Fold or Unfold commands (using C-S-p) to find all of them.