Problem Description

I want to read and write Workspace or global data to a durable storage, that is accessible only to my extension.

This was not always as straight-forward as it is now. In the past you would need to implement local storage using custom files written to temporary storage such as %APP_DATA%. There are three problems with this approach.

  1. Using %APP_DATA% is often locked down.
  2. Files stored in %APP_DATA% can be deleted at any time (sometimes nightly) by IT policy.
  3. There is no guarantee that you have, or will continue to have, permissions to the filesystem.

This can cause inconsistency issues in how extensions behave.

Solution

The VS Code API contains a Memento object for this.

According to the documentation, A memento represents a storage utility. It can store and retrieve values. We can use this object as a guaranteed, durable storage for persisting state during and between VSCode Sessions.

When you activate an extension, VSCode passes an ExtensionContext object to your activate function. This object contains two Memento objects - a workspaceState property, which stores objects at the Workspace scope, and the globalState property, which stores objects at the Extension scope.

Example: Creating a Generic Class to Read and Write Data to the Local Store

You can create a simple class for storing and retrieving data from the ExtensionContext by wrapping either of the State objects in a class. By wrapping the State object in a class, you can implement extension-specific custom formatters, or encrypting/hashing data.

Once you have created this class, you can use it to write data to and from the local storage object, scoped to either the Workspace or the Extension.

Initialize localStorageService with Workspace Scope

*in file: extension.ts

export function activate(context: vscode.ExtensionContext): any {

  //Create your objects - Needs to be a well-formed JSON object.
  let someObject : ISomeObject = null;
  let someOtherObject : ISomeObject = new SomeObject();

  //Initialize the global application manager
  let storageManager = new LocalStorageService(context.workspaceState);

  //Write your objects from the Workspace Store
  storageManager.setValue<ISomeObject>("SomeObject", someObject);

  //Read your objects to the Workspace Store
  someOtherObject = storageManager.getValue<ISomeObject>("SomeObject");

}

Initialize localStorageService with Extension Scope

*in file: extension.ts

export function activate(context: vscode.ExtensionContext): any {

  //Create your objects - Needs to be a well-formed JSON object.
  let someObject : ISomeObject = null;
  let someOtherObject : ISomeObject = new SomeObject();

  //Initialize the global application manager
  let storageManager = new LocalStorageService(context.globalState);

  //Write your objects from the Workspace Store
  storageManager.setValue<ISomeObject>("SomeObject", someObject);

  //Read your objects to the Workspace Store
  someOtherObject = storageManager.getValue<ISomeObject>("SomeObject");
}

Take-aways

the Memento class in the Visual Studio Code Extension API provides a guaranteed, durable storage for persisting state during and between VSCode Sessions.

If you wrap it in a simple helper class, you can implement custom data or security formatters to fine tune how you store data.

References