OAuth authentication for offline application using .NET SDK

Use of a redirect URI is optional, and you can still request offline access to get a refresh token back without using a redirect URI. When not using a redirect URI, the user would just need to manually copy/paste the authorization code into the app.

You can do that like this:

First, get the authorization URL, requesting offline access, like:

var authorizeUri = DropboxOAuth2Helper.GetAuthorizeUri(oauthResponseType:OAuthResponseType.Code,
													   clientId: AppKey,
													   redirectUri: (String)null,
													   state: state,
													   tokenAccessType: TokenAccessType.Offline,
													   scopeList: null,
													   includeGrantedScopes: IncludeGrantedScopes.None);

 Open that in the user's browser, such as like in the example:

System.Diagnostics.Process.Start(authorizeUri.ToString());

Have the user copy/paste the code into your app. This can be via your UI, but for the sake of example, this is how a console app might do this:

var code = Console.ReadLine();

Finish the process by exchanging the authorization code:

var tokenResult = await DropboxOAuth2Helper.ProcessCodeFlowAsync(code: code, appKey: AppKey, appSecret: AppSecret, redirectUri: null);

(Note that this is the non-PKCE flow, for server-side apps. For the PKCE flow, for client-side apps, a code challenge/verifier is used instead of the app secret.)

Then you can make a client using the resulting credentials:

var client = new DropboxClient(appKey: AppKey,
                      appSecret: AppSecret,
                      oauth2AccessToken: tokenResult.AccessToken,
                      oauth2RefreshToken: tokenResult.RefreshToken,
                      oauth2AccessTokenExpiresAt: tokenResult.ExpiresAt.Value);

The SDK will automatically perform the refresh process for you when needed when using a client with the necessary credentials like this.