Google Forms Data Submission in C#

Google Forms Submission Service

naming file

GoogleFormsSubmissionService.cs

/// <summary>Submit form data to a Google form</summary>
/// <remarks>Kudos: https://stackoverflow.com/questions/52398172/how-to-fill-google-form-with-c-sharp </remarks>
public class GoogleFormsSubmissionService
{
    private string _baseUrl;
    private Dictionary<string, string> _field;
    private Dictionary<string, string[]> _checkbox;
    private HttpClient _client;

    public GoogleFormsSubmissionService(string formUrl)
    {
        if (string.IsNullOrEmpty(formUrl)) throw new ArgumentNullException(nameof(formUrl));
        _baseUrl = formUrl;

        _field = new Dictionary<string, string>();
        _checkbox = new Dictionary<string, string[]>();
        _client = new HttpClient();
    }

    /// <summary>Set multiple fields with individual values. Fields with empty values will be ignored.</summary>
    /// <remarks>All keys must be valid or exceptions will be thrown.</remarks>
    public void SetFieldValues(Dictionary<string, string> data)
    {
        if (data == null) throw new ArgumentNullException(nameof(data));
        if (data.Keys.Any(value => string.IsNullOrWhiteSpace(value))) throw new ArgumentNullException(nameof(data), "Empty keys are invalid");

        var fieldsWithData = data.Where(kvp => !string.IsNullOrWhiteSpace(kvp.Value));
        foreach (var kvp in fieldsWithData)
            _field[kvp.Key] = kvp.Value;
    }

    /// <summary>Set one or more values for a single checkbox field.  Values must match the text on the form Checkboxes.  Empty values will be ignored.</summary>
    public void SetCheckboxValues(string key, params string[] values)
    {
        if (string.IsNullOrWhiteSpace(key)) throw new ArgumentNullException(nameof(key));

        var valuesWithData = values.Where(value => !string.IsNullOrWhiteSpace(value)).ToArray();
        _checkbox[key] = valuesWithData;
    }

    /// <summary>Submits the previously set data asynchronously and returns the response.</summary>
    /// <remarks>See https://stackoverflow.com/a/52404185/117797 for queryParams formatting details</remarks>
    public async Task<HttpResponseMessage> SubmitAsync()
    {
        if (!_field.Any() && !_checkbox.Any())
        {
            throw new InvalidOperationException("No data has been set to submit");
        }

        var content = new FormUrlEncodedContent(_field);
        var url = _baseUrl;
        if (_checkbox.Any())
        {
            var queryParams = string.Join("&", _checkbox.Keys.SelectMany(key => _checkbox[key].Select(value => $"{key}={value.Replace(' ', '+')}")));
            url += $"?{queryParams}";
        }

        var response = await _client.PostAsync(url, content);
        if (response.IsSuccessStatusCode)
        {
            Console.Write("Google form data was submitted sucessfully");
        }
        else
        {
            var fieldText = string.Join(Environment.NewLine, _field.Keys.Select(key => $"{key}={string.Join(",", _field[key])}"));
            Console.Write($"Failed to submit Google form\n{response.StatusCode}: {response.ReasonPhrase}\n{url}\n{fieldText}");
        }

        return response;
    }
}

Usage Example C#

var fields = new Dictionary<string, string>
{
    { "entry.11111", "My text" },
    { "entry.22222", "Other text"},
    { "entry.33333", "LINQPad" },
};

var formUrl = "https://docs.google.com/forms/d/e/{YourFormCodeHere}/formResponse";
var submissionService = new GoogleFormsSubmissionService(formUrl);
submissionService.SetFieldValues(fields);
submissionService.SetCheckboxValues("entry.44444", "One fish", "Two fish");		//Multiple checkbox values
submissionService.SetCheckboxValues("entry.55555", "There can be only one");	//Single checkbox value

var response = await submissionService.SubmitAsync();

It's been a long, long time since I worked on Google Forms. The solution I presented here was for my specific use case, and I have not worked on it since. As such I can't offer any further thoughts (re: FileUpload field).

As for Captcha, as I understand that field is specifically to prevent automated responses. I have no clue how to distinguish between an automated response and the legitimate response of a code handler.