(Mono C#) Refresh a Xero OAuth2 Access Token

Refreshes a Xero OAuth2 access token. When an access token expires (after 1 hour), you will received a 401 status code indicating failure. When that happens, your application can run this code to refresh the access token, and then retry the request using the new access token. Refreshing an access token does not need user interaction (i.e. does not need to display a browser to have the user interactive authorize access).

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

// It is assumed we previously obtained an OAuth2 access token.
// This example loads the JSON access token file 
// saved by this example: Get Xero OAuth2 Access Token

Chilkat.JsonObject jsonToken = new Chilkat.JsonObject();
bool success = jsonToken.LoadFile("qa_data/tokens/xero-access-token.json");
if (success != true) {
    Debug.WriteLine("Failed to load xero-access-token.json");
    return;
}

// The access token JSON looks like this:

// {
//   "id_token": "eyJhbGci...dqRs3MctS_g",
//   "access_token": "eyJhbGci...sUoAhoQ",
//   "expires_in": 1800,
//   "token_type": "Bearer",
//   "refresh_token": "2f77b...bfc2ee16f",
//   "scope": "openid profile email accounting.transactions accounting.settings payroll.employees offline_access"
// }

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

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

// Replace these with actual values.
oauth2.ClientId = "XERO_APP_OAUTH2_CLIENT_ID";
oauth2.ClientSecret = "XERO_APP_OAUTH2_CLIENT_SECRET";
oauth2.UseBasicAuth = true;

// Get the "refresh_token"
oauth2.RefreshToken = jsonToken.StringOf("refresh_token");

// Send the HTTP POST to refresh the access token..
success = oauth2.RefreshAccessToken();
if (success != true) {
    Debug.WriteLine(oauth2.LastErrorText);
    return;
}

Debug.WriteLine(oauth2.AccessTokenResponse);

// The response contains a new access token and refresh token.
// Update the JSON and re-save to the token file.
jsonToken.UpdateString("access_token",oauth2.AccessToken);
jsonToken.UpdateString("refresh_token",oauth2.RefreshToken);

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

Debug.WriteLine("OAuth2 authorization granted!");
Debug.WriteLine("New Access Token = " + oauth2.AccessToken);