Implement Foursquare Authentication in ASP.NET MVC

Introduction

Here, I will explain about how to implement Foursquare authentication in ASP.NET MVC. In order to use Foursquare API in our web application, first you need to create an application in Foursquare Developer Page. The following are the steps to follow to create an application in Foursquare.

Step 1: Create Your App to Obtain Its Foursquare API Credentials

Once your app is created, Client ID, the Client Secret API credentials will be generated so that we can use them for the authentication.

Step 2: Implementing Authentication

Create a login button with Foursquare logo, which will be found here. Now we are using the OAuth 2.0 flow for the user authentication. We will direct users to the following url for login:

https://foursquare.com/oauth2/authenticate?client_id=YOUR_CLIENT_ID
&response_type=code
&redirect_uri=YOUR_REGISTERED_REDIRECT_URI

If the user accepts, they will be redirected back to:

 https://REDIRECT_URI_APP/?code=CODE

Once we get the code, we will exchange the code for an access token. To get the access token, we should make a request for the following URL:

https://foursquare.com/oauth2/access_token?client_id=YOUR_CLIENT_ID
    &client_secret=YOUR_CLIENT_SECRET&grant_type=authorization_code
    &redirect_uri=REDIRECT_URI
    &code=CODE

Now, we will use the above flow in ASP.NET MVC.

<a href="https://foursquare.com/oauth2/authenticate?client_id=CLIENTID
&response_type=code
&redirect_uri=http://localhost:51361/home/index">
<img src="~/Content/foursquare.png" />
    
</a>

Once the user authenticated, use the following code to get the access token:

 public ActionResult Index(string code)
{
    if (!string.IsNullOrEmpty(code))
    {
        //Make a request for access token
        WebClient client = new WebClient();
        string reply =
            client.DownloadString(
                "https://foursquare.com/oauth2/access_token?
		client_id=ZWZW4WMV13ZIXF1QODU0QNBU2PWNCCTNRA4NQSLREBE41BIY&
		client_secret=IWQI2YGXNZX21NXAUZRDOLOHZTQVXPH3FHTIWXBETMXAEBFE&
		grant_type=authorization_code&redirect_uri=http://localhost:51361/home/index&code=" +
                code);
             
        //Deserialize the JSON to get the access token
        var jss = new JavaScriptSerializer();
        var dict = jss.Deserialize<Dictionary<string, string>>(reply);
        //Save the in a session for api endpoints calls
        Session["access_token"] = dict["access_token"];
    }

    return View();
}

Save the access token in a session. Once you have an access token, it is easy use any of the endpoints, by just adding oauth_token=ACCESS_TOKEN to your GET or POST request.

Get the user profile using access token:

//Read access_token from the session 
var acessToken = Session["access_token"];
                WebClient client = new WebClient();
                var reply =
                    client.DownloadString(
                        "https://api.foursquare.com/v2/users/self?oauth_token=" + 
			acessToken + "&v=20141112");

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