Calling Web API Using HttpClient

static void Main(string[] args)  
{  
    CallWebAPIAsync()  
        .Wait();  
}  
static asyncTaskCallWebAPIAsync()  
{  
    using(var client = newHttpClient())  
    {  
        //Send HTTP requests from here.  
    }  
}

using(var client = newHttpClient())  
{  
    client.BaseAddress = newUri("http://localhost:55587/");  
    client.DefaultRequestHeaders.Accept.Clear();  
    client.DefaultRequestHeaders.Accept.Add(newMediaTypeWithQualityHeaderValue("application/json"));  
} 

using(var client = newHttpClient())  
{  
    client.BaseAddress = newUri("http://localhost:55587/");  
    client.DefaultRequestHeaders.Accept.Clear();  
    client.DefaultRequestHeaders.Accept.Add(newMediaTypeWithQualityHeaderValue("application/json"));  
    //GET Method  
    HttpResponseMessage response = awaitclient.GetAsync("api/Department/1");  
    if (response.IsSuccessStatusCode)  
    {  
        Departmentdepartment = awaitresponse.Content.ReadAsAsync < Department > ();  
        Console.WriteLine("Id:{0}\tName:{1}", department.DepartmentId, department.DepartmentName);  
        Console.WriteLine("No of Employee in Department: {0}", department.Employees.Count);  
    }  
    else  
    {  
        Console.WriteLine("Internal server Error");  
    }  
}

In the code given above, I have used GetAsync method to send HTTP GET request asynchronously. When the execution of this method is finished, it returns HttpResponseMessage, which contains HTTP response. If the response contains success code as response, it means the response body contains the data in the form of JSON. ReadAsAsync method is used to deserialize the JSON object.

HttpClient does not throw any error when HTTP response contains an error code, but it sets the IsSuccessStatusCode property to false. If we want to treat HTTP error codes as exceptions, we can use HttpResponseMessage.EnsureSuccessStatusCode method.

When we run the code, given above, it throws the exception "Internal Server error".

We have to configure the serializer to detect then handle self-referencing loops. The following code needs to be placed in Global.asax.cs in Web API:

GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings  
.PreserveReferencesHandling =Newtonsoft.Json.PreserveReferencesHandling.All;  

HTTP POST request

Following code is used to send a POST request for the department:

var department = newDepartment() { DepartmentName = "Test Department" };  
HttpResponseMessage response = awaitclient.PostAsJsonAsync("api/Department", department);  

if (response.IsSuccessStatusCode)  
{  
   // Get the URI of the created resource.  
   UrireturnUrl = response.Headers.Location;  
   Console.WriteLine(returnUrl);  
}

In this code, PostAsJsonAsync method serializes the object into JSON format and sends this JSON object in POST request. HttpClient has a built-in method "PostAsXmlAsync" to send XML in POST request. Also, we can use "PostAsync" for any other formatter.

HTTP PUT Request

Following code is used to send a PUT request for the department:

//PUT Method  
var department = newDepartment() { DepartmentId = 9, DepartmentName = "Updated Department" };  
HttpResponseMessage response = awaitclient.PutAsJsonAsync("api/Department", department);  
if (response.IsSuccessStatusCode)  
{  
   Console.WriteLine("Success");  
} 

Same as POST, HttpClient also supports all three methods: PutAsJsonAsync, PutAsXmlAsync and PutAsync.

HTTP DELETE Request

Following code is used to send a DELETE request for the department:

intdepartmentId = 9;  
HttpResponseMessage response = awaitclient.DeleteAsync("api/Department/" + departmentId);  
if (response.IsSuccessStatusCode)  
{  
   Console.WriteLine("Success");  
}

HttpClient only supports DeleteAsync method because Delete method does not have a request body.

Summary

HttpClient is used to send an HTTP request, using a URL. HttpClient can be used to make Web API requests from the console Application, Winform Application, Web form Application, Windows store Application, etc.