try
{
    ////retrieve current user profile details
    Uri targetWeb = new Uri(Request.QueryString["SPHostUrl"]);
    string targetRealm = TokenHelper.GetRealmFromTargetUrl(targetWeb);
    var responseToken = TokenHelper.GetAppOnlyAccessToken(TokenHelper.SharePointPrincipal, targetWeb.Authority, targetRealm);

    HttpWebRequest request = HttpWebRequest.CreateHttp(String.Format("{0}/_api/SP.UserProfiles.PeopleManager/GetMyProperties", Request["SPHostUrl"].Substring(0, 8 + Request["SPHostUrl"].Substring(8).IndexOf("/"))));
    request.Accept = "application/json;odata=verbose";
    request.Headers.Add("Authorization", String.Format("{0} {1}", responseToken.TokenType, responseToken.AccessToken));
    Stream s = request.GetResponse().GetResponseStream();

    using (StreamReader sr = new StreamReader(s))
    using (Newtonsoft.Json.JsonReader reader = new JsonTextReader(sr))
    {
        JsonSerializer serializer = new JsonSerializer();

        // read the json from a stream
        // json size doesn't matter because only a small piece is read at a time from the HTTP request
        var p = serializer.Deserialize(reader);

    }
}
catch (Exception ex){}

//To get the request digest value.
static async Task<string> GetDigestAsync(string endPoint, HttpClient client)
{
    string digest = null;
    HttpResponseMessage response = await client.PostAsync(endPoint, new StringContent(""));
    string jsonReturned = await response.Content.ReadAsStringAsync();
    JObject objReturned = JObject.Parse(jsonReturned);
    if (response.IsSuccessStatusCode)
    {
        digest = await response.Content.ReadAsStringAsync();
    }
    JObject rss = JObject.Parse(digest);

    return rss["d"]["GetContextWebInformation"]["FormDigestValue"].ToString();
}

//Make a post request to get the request digest value
var digest = await GetDigestAsync(siteUrl + "/_api/contextinfo", client);

//Now include this digest value as a header in your actual POST requests calls
client.DefaultRequestHeaders.Add("X-HTTP-Method", "POST");
client.DefaultRequestHeaders.Add("X-RequestDigest", digest);