How to access WebAPI from a .Net 3.5 client in C#

So, like me, you may have come across some incompatibility within your WebAPI client that means you can’t use .Net 4/4.5 (which is required to use HttpClient and HttpResponseMessage etc.) – or you’re writing a web service to fill in due to this incompatibility. This solution should allow you to access your API (using POST) from .Net 3.5 (and above, maybe even below).

First of all lets assume you have the following model (taken from ASP.NET):

class Product {
    public string Name { get; set; }
    public double Price { get; set; }
    public string Category { get; set; }
}

Now usually you could access this using the following code (which will send a Product to be created):

HttpClient client = new HttpClient();
client.BaseAddress = new Uri("http://[address]/");

// Add an Accept header for JSON format.
client.DefaultRequestHeaders.Accept.Add(
new MediaTypeWithQualityHeaderValue("application/json"));

// Create a new product
var gizmo = new Product() { Name = "Gizmo", Price = 100, Category = "Widget" };
Uri gizmoUri = null;
            
HttpResponseMessage response = client.PostAsJsonAsync("api/products", gizmo).Result;
if (response.IsSuccessStatusCode)
{
    gizmoUri = response.Headers.Location;
}
else
{
    Console.WriteLine("{0} ({1})", (int)response.StatusCode, response.ReasonPhrase);
}

Lets say, however, that we simply want to post some request (which may have one or more request parameters of any type) that will return an object, in this case a Product. First of all we must create an object that will store this request (this class can be stored anywhere but personally I prefer to store in a file called JsonClasses.cs with all other request classes):

public class ProductRequest {
    public string Category { get; set; }
}

Next we need a generic (literally) method that will allow us to post any type of request and return with any type of object (we know in advance what type of object will be returned). This method will make use of the .Net WebClient and Json.Net:

public static object postWebApi(object data, Uri webApiUrl) {
    // Create a WebClient to POST the request
    WebClient client = new WebClient();

    // Set the header so it knows we are sending JSON
    client.Headers[HttpRequestHeader.ContentType] = "application/json";

    // Serialise the data we are sending in to JSON
    string serialisedData = JsonConvert.SerializeObject(data);

    // Make the request
    var response = client.UploadString(webApiUrl, serialisedData);

    // Deserialise the response into a GUID
    return JsonConvert.DeserializeObject(response);
}

Nothing needs to change within your web server application, other than including the ProductRequest class and adding code to deal with the request – but you probably already have that logic in place so it should just be a case of slightly modifying it. So next we just need to make the call:

// Create a ProductRequest for sending our request to our WebAPI
ProductRequest productRequest = new ProductRequest();
productRequest.Category = "test";

List products = postWebApi>(productRequest, new Uri("[url]"));

For completeness I have also included a GET request method:

public static object getWebApi(object data, Uri webApiUrl) {
    // Create a WebClient to GET the request
    WebClient client = new WebClient();

    // Set the header so it knows we are sending JSON
    client.Headers[HttpRequestHeader.ContentType] = "application/json";

    string queryString = "";

    if (data != null) {
        // Separate the KeyValuePairs in to a query string
        foreach (var pair in data) {
            if (queryString.Length != 0) {
                queryString += "&";
            }
    
            queryString += pair.Key + "=" + pair.Value;
        }
    }

    // Make the request
    var response = client.DownloadString(webApiUrl + "?" + queryString);

    // Deserialise the response into a GUID
    return JsonConvert.DeserializeObject(response);
}

which can be used as below:

Product product = getWebApi>(null, new Uri("[url]")); // With no value

I hope this comes in helpful to someone, it took me a little while to work out as there was no existing whole solution that I could find – as usual if anyone has any questions please comment below.