Windows Forms: Send an email using gmail API in C#

To send an email via gmail api, you need to enable your gmail api by accessing https://console.developers.google.com/ website, then create a new windows forms project and install the Google.Apis.Gmail.v1 from the Nuget Manage Packages.

Drag the TextBox, Label and Button from your visual studio toolbox to your winform, then design a simple UI that allows you to send an email in c# as shown below.

To play the demo, you should download the credentials.json file from the google console developer, then enable the Gmail API allow you to send an email using gmail api in c#.

Create a Base64UrlEncode method to encode your url as the following code.

string[] Scopes = { GmailService.Scope.GmailSend };
string ApplicationName = "SendMail";

string Base64UrlEncode(string input)
{
    var data = Encoding.UTF8.GetBytes(input);
    return Convert.ToBase64String(data).Replace("+", "-").Replace("/", "_").Replace("=", "");
}

Finally, Add code to handle the btnSend_Click event as shown below.

private void btnSend_Click(object sender, EventArgs e)
{
    UserCredential credential;
    //read your credentials file
    using (FileStream stream = new FileStream(Application.StartupPath + @"/credentials.json", FileMode.Open, FileAccess.Read))
    {
        string path = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
        path = Path.Combine(path, ".credentials/gmail-dotnet-quickstart.json");
        credential = GoogleWebAuthorizationBroker.AuthorizeAsync(GoogleClientSecrets.Load(stream).Secrets, Scopes, "user", CancellationToken.None, new FileDataStore(path, true)).Result;
    }
    string message = $"To: {txtTo.Text}\r\nSubject: {txtSubject.Text}\r\nContent-Type: text/html;charset=utf-8\r\n\r\n<h1>{txtMessage.Text}</h1>";
    //call your gmail service
    var service = new GmailService(new BaseClientService.Initializer() { HttpClientInitializer = credential, ApplicationName = ApplicationName });
    var msg = new Google.Apis.Gmail.v1.Data.Message();
    msg.Raw = Base64UrlEncode(message.ToString());
    service.Users.Messages.Send(msg, "me").Execute();
    MessageBox.Show("Your email has been successfully sent !", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
}

And don't forget to import the namespaces below to your form.

using Google.Apis.Auth.OAuth2;
using Google.Apis.Gmail.v1;
using Google.Apis.Services;
using Google.Apis.Util.Store;

To send an email using gmail api in c# you should copy the credentials.json file that you downloaded into your project, the set the Copy to Output Directory properties to copy always.