Twitter new DM API, legacy method of authorization not working – Twitter API

Just happened to stumble on your query. I am posting a code(C#) (though it is a bit late) which worked for me to send DM to Twitter using the new API. Hope this helps. Thanks to Danny Tuppeny’s blog

namespace TweetApp.Droid
{
class TweetDM
{
    const string TwitterApiBaseUrl = "https://api.twitter.com/1.1/";
    readonly string consumerKey, consumerKeySecret, accessToken, accessTokenSecret;
    readonly HMACSHA1 sigHasher;
    readonly DateTime epochUtc = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

    public TweetDM(string consumerKey, string consumerKeySecret, string accessToken, string accessTokenSecret)
    {
        this.consumerKey = consumerKey;
        this.consumerKeySecret = consumerKeySecret;
        this.accessToken = accessToken;
        this.accessTokenSecret = accessTokenSecret;

        sigHasher = new HMACSHA1(new ASCIIEncoding().GetBytes(string.Format("{0}&{1}", consumerKeySecret, accessTokenSecret)));
    }

    public Task<string> Tweet(string text, string recipientID)
    {
        JSONObject jasonobject = new JSONObject
        {
            @event = new TwitterEvent
            {
                type = "message_create",
                message_create = new msg_create
                {
                    target = new tgt
                    {
                        recipient_id = recipientID
                    },
                    message_data = new msg_data
                    {
                        text = text
                    }
                },
            }
        };

        var JsonString =JsonConvert.SerializeObject(jasonobject);


        var data4Auth = new Dictionary<string, string>   {
        };
        return PrepareAuth("direct_messages/events/new.json", data4Auth, JsonString);
      }

    Task<string> PrepareAuth(string url, Dictionary<string, string> data4Auth, string JsonString)
    {
        var fullUrl = TwitterApiBaseUrl + url;

        var timestamp = (int)((DateTime.UtcNow - epochUtc).TotalSeconds);

        data4Auth.Add("oauth_consumer_key", consumerKey);
        data4Auth.Add("oauth_signature_method", "HMAC-SHA1");
        data4Auth.Add("oauth_timestamp", timestamp.ToString());
        data4Auth.Add("oauth_nonce", "a"); // Required, but Twitter doesn't appear to use it, so "a" will do.
        data4Auth.Add("oauth_token", accessToken);
        data4Auth.Add("oauth_version", "1.0");

        // Generate the OAuth signature and add it to our payload.
        data4Auth.Add("oauth_signature", GenerateSignature(fullUrl, data4Auth));

        // Build the OAuth HTTP Header from the data.
        string oAuthHeader = GenerateOAuthHeader(data4Auth);
        
        // Setting Content details
        var JsonData = new StringContent(JsonString, Encoding.UTF8, "application/json");
        return SendRequest(fullUrl, oAuthHeader, JsonData);
    }

    string GenerateSignature(string url, Dictionary<string, string> data)
    {
        var sigString = string.Join(
            "&",
            data
                .Union(data)
                .Select(kvp => string.Format("{0}={1}", Uri.EscapeDataString(kvp.Key), Uri.EscapeDataString(kvp.Value)))
                .OrderBy(s => s)
        );

        var fullSigData = string.Format(
            "{0}&{1}&{2}",
            "POST",
            Uri.EscapeDataString(url),
            Uri.EscapeDataString(sigString.ToString())
        );

        return Convert.ToBase64String(sigHasher.ComputeHash(new ASCIIEncoding().GetBytes(fullSigData.ToString())));
    }

    string GenerateOAuthHeader(Dictionary<string, string> data)
    {
        return "OAuth " + string.Join(
            ", ",
            data
                .Where(kvp => kvp.Key.StartsWith("oauth_"))
                .Select(kvp => string.Format("{0}="{1}"", Uri.EscapeDataString(kvp.Key), Uri.EscapeDataString(kvp.Value)))
                .OrderBy(s => s)
        );
    }

    async Task<string> SendRequest(string fullUrl, string oAuthHeader, StringContent jsondata)
    {
        using (var http = new HttpClient())
        {
            http.DefaultRequestHeaders.Add("Authorization", oAuthHeader);
            var httpResp = await http.PostAsync(fullUrl, jsondata);
            var respBody = await httpResp.Content.ReadAsStringAsync();
            return respBody;
        }
    }
}
// Classes for creating JSON body
public class JSONObject
{
    public TwitterEvent @event;
}
    public class TwitterEvent
{
    public string type;
    public msg_create message_create;
   }
public class msg_create
{
    public tgt target;
    public msg_data message_data;
}
public class tgt
{
    public string recipient_id;
}
public class msg_data
{
    public string text;
}
}

To call:

 var twitter = new TweetDM(consumerKey, consumerKeySecret, accessToken, accessTokenSecret);
 await twitter.Tweet(textBox1.Text, textBox2.Text);