Como enviar um certificado de cliente utilizando as classes HttpWebRequest e HttpWebResponse no Microsoft Visual C# .NET

//Uncomment the following code if you need a proxy. The boolean true is used to bypass the local address.
//WebProxy proxyObject = new WebProxy("Your Proxy value",true); 
//GlobalProxySelection.Select = proxyObject;

// Obtain the certificate. 
try
{
	//You must change the path to point to your .cer file location. 
	X509Certificate Cert = X509Certificate.CreateFromCertFile("C:\\mycert.cer");
	// Handle any certificate errors on the certificate from the server.
	ServicePointManager.CertificatePolicy = new CertPolicy();
	// You must change the URL to point to your Web server.
	HttpWebRequest Request = (HttpWebRequest)WebRequest.Create("https://YourServer/sample.asp");
	Request.ClientCertificates.Add(Cert);
	Request.UserAgent = "Client Cert Sample";
	Request.Method = "GET";
	HttpWebResponse Response = (HttpWebResponse)Request.GetResponse();
	// Print the repsonse headers.
	Console.WriteLine("{0}",Response.Headers);
	Console.WriteLine();
	// Get the certificate data.
	StreamReader sr = new StreamReader(Response.GetResponseStream(), Encoding.Default);
	int count;
	char [] ReadBuf = new char[1024];
	do
	{
		count = sr.Read(ReadBuf, 0, 1024);
		if (0 != count)
		{
			Console.WriteLine(new string(ReadBuf));
		}
						
	}while(count > 0);
}
catch(Exception e)
{
	Console.WriteLine(e.Message);
}
	

//Implement the ICertificatePolicy interface.
class CertPolicy: ICertificatePolicy
{
	public bool CheckValidationResult(ServicePoint srvPoint, 
X509Certificate certificate, WebRequest request, int certificateProblem)
	{
		// You can do your own certificate checking.
		// You can obtain the error values from WinError.h.

		// Return true so that any certificate will work with this sample.
		return true;
	}
}

using System;
using System.Net;
using System.IO;
using System.Text;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Runtime.InteropServices;

namespace SelectClientCert
{
	/// Sample that describes how how to select client cetificate and send it to the server.

	class MyCerts{

		private static int CERT_STORE_PROV_SYSTEM = 10;
		private static int CERT_SYSTEM_STORE_CURRENT_USER = (1 << 16);
		///private static int CERT_SYSTEM_STORE_LOCAL_MACHINE = (2 << 16);

		[DllImport("CRYPT32", EntryPoint="CertOpenStore", CharSet=CharSet.Unicode, SetLastError=true)]
		public static extern IntPtr CertOpenStore(
			int storeProvider, int encodingType,
			int hcryptProv, int flags, string pvPara);

		[DllImport("CRYPT32", EntryPoint="CertEnumCertificatesInStore", CharSet=CharSet.Unicode, SetLastError=true)]
		public static extern IntPtr CertEnumCertificatesInStore(
			IntPtr storeProvider,
			IntPtr prevCertContext);

		[DllImport("CRYPT32", EntryPoint="CertCloseStore", CharSet=CharSet.Unicode, SetLastError=true)]
		public static extern bool CertCloseStore(
			IntPtr storeProvider,
			int flags);
		
		X509CertificateCollection m_certs;

		public MyCerts(){
			m_certs = new X509CertificateCollection();
		}

		public int Init()
		{
			IntPtr storeHandle;
			storeHandle = CertOpenStore(CERT_STORE_PROV_SYSTEM, 0, 0, CERT_SYSTEM_STORE_CURRENT_USER, "MY");
			IntPtr currentCertContext;
			currentCertContext = CertEnumCertificatesInStore(storeHandle, (IntPtr)0);
			int i = 0;
			while (currentCertContext != (IntPtr)0) 
			{
				m_certs.Insert(i++, new X509Certificate(currentCertContext));
				currentCertContext = CertEnumCertificatesInStore(storeHandle, currentCertContext);
			}
			CertCloseStore(storeHandle, 0);

			return m_certs.Count;
		}
		
		public X509Certificate this [int index]
		{
			get 
			{
				// Check the index limits.
				if (index < 0 || index > m_certs.Count)
					return null;
				else
					return m_certs[index];
			}
		}
	};
	class MyHttpResource
	{
		String m_url;

		public MyHttpResource(string url){
			m_url = url;
		}

		public void GetFile(){

			HttpWebResponse  result = null;

			try{
			
				HttpWebRequest req = (HttpWebRequest)WebRequest.Create(m_url);
				req.Credentials  = CredentialCache.DefaultCredentials;

				///Method1
				//req.ClientCertificates.Add(X509Certificate.CreateFromCertFile("D:\\Temp\\cert\\c1.cer"));
		
				///Method2
				///Uses interop services
				MyCerts mycert = new MyCerts();
				if(mycert.Init() > 0)
					req.ClientCertificates.Add(mycert[0]);

				result = (HttpWebResponse)req.GetResponse();
				
				Stream ReceiveStream = result.GetResponseStream();
				Encoding encode = System.Text.Encoding.GetEncoding("utf-8");

				StreamReader sr = new StreamReader( ReceiveStream, encode );
				Console.WriteLine("\r\nResponse stream received");

				Char[] read = new Char[256];
				int count = sr.Read( read, 0, 256 );

				Console.WriteLine("HTTP Response...\r\n");
				while (count > 0) 
				{
					String str = new String(read, 0, count);
					Console.Write(str);
					count = sr.Read(read, 0, 256);
				}

			} 
			catch(WebException e) 
			{
            
				Console.WriteLine("\r\nError:");
				#if (DEBUG)
					Console.WriteLine(e.ToString());
				#else		
					Console.WriteLine(e.Message); 				
				#endif

			} 
			finally 
			{
				if ( result != null ) {
					result.Close();
				}
			}
				
		}
	
	}

	class CertSample
	{
		static void Main(string[] args)
		{
			try
			{
				if (args.Length < 1)
				{
					Console.WriteLine("No url is entered to download, returning.\n");
					Console.WriteLine("Usage: CertSample <urltoget>\n");
					Console.WriteLine("  e.g: CertSample https://servername \n"); 

					return;
				}

				MyHttpResource hr = new MyHttpResource(args[0]);
				hr.GetFile();
			}
			catch(Exception e)
			{
				Console.WriteLine(e.ToString());
			}
			return;
		}
	}
}