(.NET Core C#) Get a Xero OAuth2 Access Token via PKCE

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Gets a Xero OAuth2 access token via PKCE.

For more information, see https://github.com/XeroAPI/Xero-Postman-Tutorial-PKCE-Edition

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// This example requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

Chilkat.OAuth2 oauth2 = new Chilkat.OAuth2();
bool success;

// For Xero OAuth2, set the listen port equal to the port used
// in the Authorized Redirect URL for the Client ID.
// For example, in this case the Authorized Redirect URL would be http://localhost:55568/
// (The final "/" char is important to include in your Authorized Redirect URL in the Xero application settings in the Xero developer portal.)
// Your app should choose a port not likely not used by any other application.
// See Xero OAuth2 for Desktop Apps for more information.
oauth2.ListenPort = 55568;

oauth2.AuthorizationEndpoint = "https://login.xero.com/identity/connect/authorize";
oauth2.TokenEndpoint = "https://identity.xero.com/connect/token";

// Replace this with your actual Client ID.
oauth2.ClientId = "XERO_APP_OAUTH2_CLIENT_ID";

// To use PKCE, enable code challenge and set the code challenge method to "S256".
oauth2.CodeChallenge = true;
oauth2.CodeChallengeMethod = "S256";

// See https://developer.xero.com/documentation/oauth2/scopes for a complete list of scopes.
// You will need to change the scopes depending on what access is needed..
oauth2.Scope = "openid profile email accounting.transactions accounting.attachments accounting.settings payroll.employees offline_access";

// Begin the OAuth2 three-legged flow.  This returns a URL that should be loaded in a browser.
string url = oauth2.StartAuth();
if (oauth2.LastMethodSuccess != true) {
    Debug.WriteLine(oauth2.LastErrorText);
    return;
}

Debug.WriteLine("URL: " + url);

// At this point, your application should load the URL in a browser.
// For example, 
// in C#: System.Diagnostics.Process.Start(url);
// in Java: Desktop.getDesktop().browse(new URI(url));
// in VBScript: Set wsh=WScript.CreateObject("WScript.Shell")
//              wsh.Run url
// in Xojo: ShowURL(url)  (see http://docs.xojo.com/index.php/ShowURL)
// in Dataflex: Runprogram Background "c:\Program Files\Internet Explorer\iexplore.exe" sUrl        
// in dBASE: run(false, 'rundll32 url.dll,FileProtocolHandler "' + lcUrl + '"')
// The Google account owner would interactively accept or deny the authorization request.

// Add the code to load the url in a web browser here...
// Add the code to load the url in a web browser here...
// Add the code to load the url in a web browser here...
// System.Diagnostics.Process.Start(url);

// Now wait for the authorization.
// We'll wait for a max of 60 seconds.
int numMsWaited = 0;
while ((numMsWaited < 60000) && (oauth2.AuthFlowState < 3)) {
    oauth2.SleepMs(100);
    numMsWaited = numMsWaited + 100;
}

// If there was no response from the browser within 30 seconds, then 
// the AuthFlowState will be equal to 1 or 2.
// 1: Waiting for Redirect. The OAuth2 background thread is waiting to receive the redirect HTTP request from the browser.
// 2: Waiting for Final Response. The OAuth2 background thread is waiting for the final access token response.
// In that case, cancel the background task started in the call to StartAuth.
if (oauth2.AuthFlowState < 3) {
    oauth2.Cancel();
    Debug.WriteLine("No response from the browser!");
    return;
}

// Check the AuthFlowState to see if authorization was granted, denied, or if some error occurred
// The possible AuthFlowState values are:
// 3: Completed with Success. The OAuth2 flow has completed, the background thread exited, and the successful JSON response is available in AccessTokenResponse property.
// 4: Completed with Access Denied. The OAuth2 flow has completed, the background thread exited, and the error JSON is available in AccessTokenResponse property.
// 5: Failed Prior to Completion. The OAuth2 flow failed to complete, the background thread exited, and the error information is available in the FailureInfo property.
if (oauth2.AuthFlowState == 5) {
    Debug.WriteLine("OAuth2 failed to complete.");
    Debug.WriteLine(oauth2.FailureInfo);
    return;
}

if (oauth2.AuthFlowState == 4) {
    Debug.WriteLine("OAuth2 authorization was denied.");
    Debug.WriteLine(oauth2.AccessTokenResponse);
    return;
}

if (oauth2.AuthFlowState != 3) {
    Debug.WriteLine("Unexpected AuthFlowState:" + Convert.ToString(oauth2.AuthFlowState));
    return;
}

// Save the full JSON access token response to a file.
Chilkat.StringBuilder sbJson = new Chilkat.StringBuilder();
sbJson.Append(oauth2.AccessTokenResponse);
sbJson.WriteFile("qa_data/tokens/xero-access-token.json","utf-8",false);

// The saved JSON response looks like this:

// 	{
// 	 "access_token": "...",
// 	 "token_type": "Bearer",
// 	 "expires_in": 3600,
// 	 "refresh_token": "..."
// 	}
// 
Debug.WriteLine("OAuth2 authorization granted!");
Debug.WriteLine("Access Token = " + oauth2.AccessToken);