How to make an OAuth 1 Twitter API call with C# (dotnet core 3.1)

I want to be able to search for twitter handles from a dotnet core API service. I’ve looked at the twitter documentation for users/search.json and begged, borrowed and stolen what code examples I can from the stackoverflow, etc. (see below), but all I get back is:

{"errors":[{"code":215,"message":"Bad Authentication data."}]}

when I execute the resulting curl command.

I’m sorry the code is a bit of a mess, but can anyone see what I’m doing wrong? Or better still, if there’s a library which will do this for me, I’ve been unable to find one, that would be even better.

using Xunit;
using System;
using System.Linq;
using System.Collections.Generic;
using OAuth; // OAuth.DotNetCore, 3.0.1
using System.IO;
using System.Net;

namespace TwitterLibTest
{
    public class BuildHeaderTest
    {  
        private static readonly string consumerKey = "...";

        private static readonly string consumerSecret = "...";

        private static readonly string method = "GET";

        private static readonly OAuthSignatureMethod oauthSignatureMethod = OAuthSignatureMethod.HmacSha1;

        private static readonly string oauthVersion = "1.0a";

        [Fact]
        public void Header()
        {
            var url = "https://api.twitter.com/1.1/users/search.json";

            var generatedNonce = RandomString(32);                        

            var generatedTimestamp = DateTimeOffset.Now.ToUnixTimeSeconds().ToString();

            var oauthToken = BuildAuthToken(consumerKey, consumerSecret);

            var generatedSignature = GetSignatureBaseString(method, url, generatedTimestamp, generatedNonce, consumerKey, oauthToken, oauthSignatureMethod.ToString(), oauthVersion, new SortedDictionary<string, string>());

            Console.WriteLine($"curl --request GET --url '{url}?q=soccer' --header 'authorization: OAuth oauth_consumer_key="{consumerKey}", oauth_nonce="{generatedNonce}", oauth_signature="{generatedSignature}", oauth_signature_method="{oauthSignatureMethod.ToString()}", oauth_timestamp="{generatedTimestamp}", oauth_token="{oauthToken}", oauth_version="{oauthVersion}"'");
        }

        private static Random random = new Random();

        private static string RandomString(int length)
        {
            const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
            return new string(Enumerable.Repeat(chars, length)
            .Select(s => s[random.Next(s.Length)]).ToArray());
        }

        // https://stackoverflow.com/questions/35677711/generate-oauth1-signature-in-c-sharp
        private static string GetSignatureBaseString(string method, string strUrl, string timeStamp,
            string nonce, string strConsumer, string strOauthToken, string oauthSignatureMethod,
            string oauthVersion, SortedDictionary<string, string> data)
        {
            //1.Convert the HTTP Method to uppercase and set the output string equal to this value.
            string Signature_Base_String = method.ToUpper();
            Signature_Base_String = Signature_Base_String.ToUpper();

            //2.Append the ‘&’ character to the output string.
            Signature_Base_String = Signature_Base_String + "&";

            //3.Percent encode the URL and append it to the output string.
            string PercentEncodedURL = Uri.EscapeDataString(strUrl);
            Signature_Base_String = Signature_Base_String + PercentEncodedURL;

            //4.Append the ‘&’ character to the output string.
            Signature_Base_String = Signature_Base_String + "&";

            //5.append OAuth parameter string to the output string.
            var parameters = new SortedDictionary<string, string>
            {
                {"oauth_consumer_key", strConsumer},
                {"oauth_token", strOauthToken },
                {"oauth_signature_method", oauthSignatureMethod},
                {"oauth_timestamp", timeStamp},
                {"oauth_nonce", nonce},
                {"oauth_version", oauthVersion}
            };            

            //6.append parameter string to the output string.
            foreach (KeyValuePair<string, string> elt in data)
            {
                parameters.Add(elt.Key, elt.Value);
            }

            bool first = true;
            foreach (KeyValuePair<string, string> elt in parameters)
            {
                if (first)
                {
                    Signature_Base_String = Signature_Base_String + Uri.EscapeDataString(elt.Key + "=" + elt.Value);
                    first = false;
                }
                else
                {
                    Signature_Base_String = Signature_Base_String + Uri.EscapeDataString("&" + elt.Key + "=" + elt.Value);
                }
            }

            return Signature_Base_String;
        }

        private string BuildAuthToken(string consumerKey, string consumerSecret)
        {
            var client = Client(consumerKey, consumerSecret);
            var response = Get(client);
            var tokenMap = Parse(response);

            return tokenMap["oauth_token"];
        }

        private static OAuthRequest Client(string consumerKey, string consumerSecret)
        {
            return new OAuthRequest
            {
                Method = method,
                Type = OAuthRequestType.RequestToken,
                SignatureMethod = OAuthSignatureMethod.HmacSha1,
                ConsumerKey = consumerKey,
                ConsumerSecret = consumerSecret,
                RequestUrl = "https://api.twitter.com/oauth/request_token",
                Version = oauthVersion,
            };
        }

        private static HttpWebResponse Get(OAuthRequest client)
        {
            string auth = client.GetAuthorizationHeader();
            var request = (HttpWebRequest) WebRequest.Create(client.RequestUrl);           

            request.Headers.Add("Authorization", auth);
            return (HttpWebResponse) request.GetResponse();
        }

        private static Dictionary<string, string> Parse(HttpWebResponse response)
        {
            using var stream = response.GetResponseStream() ;
            using var reader = new StreamReader( stream );
            var responseAsText = reader.ReadToEnd();

            var map = new Dictionary<string, string>();

            foreach( var token in responseAsText.Split("&"))
            {
                var tokens = token.Split("=");
                map.Add(tokens[0], tokens[1]);
            }

            return map;
        }
    }
}