A simple C# class to post messages to a Slack channel. You must have the Incoming WebHooks Integration enabled. This class uses the Newtonsoft Json.NET serializer available via NuGet. Scroll down for an example test method. Enjoy!

SlackClient.cs

using Newtonsoft.Json;
using System;
using System.Collections.Specialized;
using System.Net;
using System.Text;

//A simple C# class to post messages to a Slack channel
//Note: This class uses the Newtonsoft Json.NET serializer available via NuGet
public class SlackClient
{
	private readonly Uri _uri;
	private readonly Encoding _encoding = new UTF8Encoding();
	
	public SlackClient(string urlWithAccessToken)
	{
		_uri = new Uri(urlWithAccessToken);
	}
	
	//Post a message using simple strings
	public void PostMessage(string text, string username = null, string channel = null, string emoji = null, string icon = null)
	{
		Payload payload = new Payload()
		{
			Channel = channel,
			Username = username,
			Text = text,
			Emoji = emoji,
			Icon = icon
		};
		
		PostMessage(payload);
	}
	
	//Post a message using a Payload object
	public void PostMessage(Payload payload)
	{
		string payloadJson = JsonConvert.SerializeObject(payload);
		
		using (WebClient client = new WebClient())
		{
			NameValueCollection data = new NameValueCollection();
			data["payload"] = payloadJson;
	
			var response = client.UploadValues(_uri, "POST", data);
			
			//The response text is usually "ok"
			string responseText = _encoding.GetString(response);
		}
	}
}

//This class serializes into the Json payload required by Slack Incoming WebHooks
public class Payload
{
	[JsonProperty("channel")]
	public string Channel { get; set; }
	
	[JsonProperty("username")]
	public string Username { get; set; }
	
	[JsonProperty("text")]
	public string Text { get; set; }
		
	[JsonProperty("icon_emoji")]
	public string Emoji { get; set; }

	[JsonProperty("icon_url")]
	public string Icon { get; set; }

}

SlackClientTest.cs

void TestPostMessage()
{	
	string urlWithAccessToken = "https://hooks.slack.com/services/TXXXXXXXX/BXXXXXXXX/xxxxxxxxxxxxxxxxxxxxxxxx";
	// replace the URL above with the URL Slack provides on the configured Incoming Webhook page. Format included here for reference only.

	SlackClient client = new SlackClient(urlWithAccessToken);
	
	try {
	// post message with an icon from URL
	client.PostMessage(username: "Mr. Kenobi",
			   text: "This is not the message you are looking for !",
			   channel: "#general",
			   icon : "https://slack.com/img/icons/app-57.png");	// icon example from Slack API
	}
	catch (Exception){
		TaskDialog.Show("Could not send message to Slack");	// Autodesk implementation of MessageBox, replace as necessary
		throw;
	}
	try {
	// post message with an icon from an emoji
	client.PostMessage(username: "Mr. Kenobi",
			   text: "This is not the message you are looking for !",
			   channel: "#general",
			   emoji : ":ghost:");	// parsing of the emoji is handled by Slack
	}
	catch (Exception){
		TaskDialog.Show("Could not send message to Slack");	// Autodesk implementation of MessageBox, replace as necessary
		throw;
	}

}