AtlassianOAuthenticator.cs

using System;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.Extensions.Options;

namespace YourProject
{
	public interface IAtlassianOAuthenticator
	{
		HttpClient Client { get; }
		Task<AtlassianOAuthTokenResponse> AuthenticateAsync();
	}
	
	public interface IPasswordRetriever // you have to implement this class
	{
		string GetPassword(string key);
	}
	
	public class AtlassianOAuthenticator : IAtlassianOAuthenticator
	{
		private readonly YourProjectOptions _options;
		private readonly IPasswordRetriever _passwordRetriever;

		public HttpClient Client { get; }

		public AtlassianOAuthenticator(
			IOptions<YourProjectOptions> options,
			IPasswordRetriever passwordRetriever,
			HttpClient client
		)
		{
			_options = options.Value;
			_passwordRetriever = passwordRetriever;
			Client = client;

			client.BaseAddress = AtlassianConstants.AtlassianApiRootUri;
		}

		internal string GetPassword(string nameExt)
		{
			var name = _options.ApplicationName + nameExt;
			var password = _passwordRetriever.GetPassword(name); // you write this one
			return password;
		}

		public async Task<AtlassianOAuthTokenResponse> AuthenticateAsync()
		{
			var credentials = new AtlassianOAuthCredentials()
			{
				audience = AtlassianConstants.YourInstanceRootUrl,
				grant_type = "client_credentials",
				client_id = GetPassword(".JiraOAuth2ClientId"),
				client_secret = GetPassword(".JiraOAuth2ClientSecret")
			};

			var response = await Client.PostAsJsonAsync("oauth/token", credentials);

			response.EnsureSuccessStatusCode();

			var result = await response.DeserializeAsync<AtlassianOAuthTokenResponse>();
			return result;
		}
	}
}