Use Ctrl+Shift+P to open the command palette and search for the command "Fold to C# definitions"
Additional options:
foldToDefinitions.useSymbolsMethod: If true, uses executeDocumentSymbolProvider instead of instead of regExs to find classes/methods/properties and fold them (it is recommended to have this set to true)
foldToDefinitions.unfoldFirst: If true, executes an unfold recursively command before folding to definitions
foldToDefinitions.foldProperties: If true, also fold properties
foldToDefinitions.foldSummaries: If true, also folds summaries (ie: method comments with summary tags, see gif below)
foldToDefinitions.retainSelection: If true, the selection at the moment of executing the command is retained (causes some folded parts of code to be re-opened if you were selecting anything inside them)
