REST client

Introduction

This tutorial teaches you a number of features in .NET Core and the C# language. You’ll learn:

  • The basics of the .NET Core Command Line Interface (CLI).
  • An overview of C# Language features.
  • Managing dependencies with NuGet
  • HTTP Communications
  • Processing JSON information
  • Managing configuration with Attributes.

You’ll build an application that issues HTTP Requests to a REST service on GitHub. You'll read information in JSON format, and convert that JSON packet into C# objects. Finally, you'll see how to work with C# objects.

There are a lot of features in this tutorial. Let’s build them one by one.

If you prefer to follow along with the final sample for this topic, you can download it. For download instructions, see Samples and Tutorials.

Prerequisites

You’ll need to set up your machine to run .NET core. You can find the installation instructions on the .NET Core page. You can run this application on Windows, Linux, macOS or in a Docker container. You’ll need to install your favorite code editor. The descriptions below use Visual Studio Code, which is an open source, cross platform editor. However, you can use whatever tools you are comfortable with.

Create the Application

The first step is to create a new application. Open a command prompt and create a new directory for your application. Make that the current directory. Type the command dotnet new console at the command prompt. This creates the starter files for a basic "Hello World" application.

Before you start making modifications, let’s go through the steps to run the simple Hello World application. After creating the application, type dotnet restore (see note) at the command prompt. This command runs the NuGet package restore process. NuGet is a .NET package manager. This command downloads any of the missing dependencies for your project. As this is a new project, none of the dependencies are in place, so the first run will download the .NET Core framework. After this initial step, you will only need to run dotnet restore (see note) when you add new dependent packages, or update the versions of any of your dependencies.

After restoring packages, you run dotnet build. This executes the build engine and creates your application. Finally, you execute dotnet run to run your application.

Adding New Dependencies

One of the key design goals for .NET Core is to minimize the size of the .NET installation. If an application needs additional libraries for some of its features, you add those dependencies into your C# project (*.csproj) file. For our example, you'll need to add the System.Runtime.Serialization.Json package so your application can process JSON responses.

Open your csproj project file. The first line of the file should appear as:

<Project Sdk="Microsoft.NET.Sdk">

Add the following immediately after this line:

   <ItemGroup>
      <PackageReference Include="System.Runtime.Serialization.Json" Version="4.3.0" />
   </ItemGroup> 

Most code editors will provide completion for different versions of these libraries. You'll usually want to use the latest version of any package that you add. However, it is important to make sure that the versions of all packages match, and that they also match the version of the .NET Core Application framework.

After you've made these changes, you should run dotnet restore (see note) again so that the package is installed on your system.

Making Web Requests

Now you're ready to start retrieving data from the web. In this application, you'll read information from the GitHub API. Let's read information about the projects under the .NET Foundation umbrella. You'll start by making the request to the GitHub API to retrieve information on the projects. The endpoint you'll use is: https://api.github.com/orgs/dotnet/repos. You want to retrieve all the information about these projects, so you'll use an HTTP GET request. Your browser also uses HTTP GET requests, so you can paste that URL into your browser to see what information you'll be receiving and processing.

You use the HttpClient class to make web requests. Like all modern .NET APIs, HttpClient supports only async methods for its long-running APIs. Start by making an async method. You'll fill in the implementation as you build the functionality of the application. Start by opening the program.cs file in your project directory and adding the following method to the Program class:

private static async Task ProcessRepositories()
{

}

You'll need to add a using statement at the top of your Main method so that the C# compiler recognizes the Task type:

using System.Threading.Tasks;

If you build your project at this point, you'll get a warning generated for this method, because it does not contain any await operators and will run synchronously. Ignore that for now; you'll add await operators as you fill in the method.

Next, rename the namespace defined in the namespace statement from its default of ConsoleApp to WebAPIClient. We'll later define a repo class in this namespace.

Next, update the Main method to call this method. The ProcessRepositories method returns a Task, and you shouldn't exit the program before that task finishes. Therefore, you must use the Wait method to block and wait for the task to finish:

static void Main(string[] args)
{
    ProcessRepositories().Wait();
}

Now, you have a program that does nothing, but does it asynchronously. Let's improve it.

First you need an object that is capable to retrieve data from the web; you can use a HttpClient to do that. This object handles the request and the responses. Instantiate a single instance of that type in the Program class inside the Program.cs file.

namespace WebAPIClient
{
    class Program
    {
        private static readonly HttpClient client = new HttpClient();

        static void Main(string[] args)
        {
            //...
        }
    }
}

Let's go back to the ProcessRepositories method and fill in a first version of it:

private static async Task ProcessRepositories()
{
    client.DefaultRequestHeaders.Accept.Clear();
    client.DefaultRequestHeaders.Accept.Add(
        new MediaTypeWithQualityHeaderValue("application/vnd.github.v3+json"));
    client.DefaultRequestHeaders.Add("User-Agent", ".NET Foundation Repository Reporter");

    var stringTask = client.GetStringAsync("https://api.github.com/orgs/dotnet/repos");

    var msg = await stringTask;
    Console.Write(msg);
}

You'll need to also add two new using statements at the top of the file for this to compile:

using System.Net.Http;
using System.Net.Http.Headers;

This first version makes a web request to read the list of all repositories under the dotnet foundation organization. (The gitHub ID for the .NET Foundation is 'dotnet'). The first few lines set up the HttpClient for this request. First, it is configured to accept the GitHub JSON responses. This format is simply JSON. The next line adds a User Agent header to all requests from this object. These two headers are checked by the GitHub server code, and are necessary to retrieve information from GitHub.

After you've configured the HttpClient, you make a web request and retrieve the response. In this first version, you use the HttpClient.GetStringAsync(String) convenience method. This convenience method starts a task that makes the web request, and then when the request returns, it reads the response stream and extracts the content from the stream. The body of the response is returned as a String. The string is available when the task completes.

The final two lines of this method await that task, and then print the response to the console. Build the app, and run it. The build warning is gone now, because the ProcessRepositories now does contain an await operator. You'll see a long display of JSON formatted text.

Processing the JSON Result

At this point, you've written the code to retrieve a response from a web server, and display the text that is contained in that response. Next, let's convert that JSON response into C# objects.

The JSON Serializer converts JSON data into C# Objects. Your first task is to define a C# class type to contain the information you use from this response. Let's build this slowly, so start with a simple C# type that contains the name of the repository:

using System;

namespace WebAPIClient
{
    public class repo
    {
        public string name;
    }
}

Put the above code in a new file called 'repo.cs'. This version of the class represents the simplest path to process JSON data. The class name and the member name match the names used in the JSON packet, instead of following C# conventions. You'll fix that by providing some configuration attributes later. This class demonstrates another important feature of JSON serialization and deserialization: Not all the fields in the JSON packet are part of this class. The JSON serializer will ignore information that is not included in the class type being used. This feature makes it easier to create types that work with only a subset of the fields in the JSON packet.

Now that you've created the type, let's deserialize it. You'll need to create a DataContractJsonSerializer object. This object must know the CLR type expected for the JSON packet it retrieves. The packet from GitHub contains a sequence of repositories, so a List<repo> is the correct type. Add the following line to your ProcessRepositories method:

var serializer = new DataContractJsonSerializer(typeof(List<repo>));

You're using two new namespaces, so you'll need to add those as well:

using System.Collections.Generic;
using System.Runtime.Serialization.Json;

Next, you'll use the serializer to convert JSON into C# objects. Replace the call to GetStringAsync(String) in your ProcessRepositories method with the following two lines:

var streamTask = client.GetStreamAsync("https://api.github.com/orgs/dotnet/repos");
var repositories = serializer.ReadObject(await streamTask) as List<repo>;

Notice that you're now using GetStreamAsync(String) instead of GetStringAsync(String). The serializer uses a stream instead of a string as its source. Let's explain a couple features of the C# language that are being used in the second line above. The argument to ReadObject(Stream) is an await expression. Await expressions can appear almost anywhere in your code, even though up to now, you've only seen them as part of an assignment statement.

Secondly, the as operator converts from the compile time type of object to List<repo>. The declaration of ReadObject(Stream) declares that it returns an object of type System.Object. ReadObject(Stream) will return the type you specified when you constructed it (List<repo> in this tutorial). If the conversion does not succeed, the as operator evaluates to null, instead of throwing an exception.

You're almost done with this section. Now that you've converted the JSON to C# objects, let's display the name of each repository. Replace the lines that read:

var msg = await stringTask;   //**Deleted this
Console.Write(msg);

with the following:

foreach (var repo in repositories)
    Console.WriteLine(repo.name);

Compile and run the application. It will print out the names of the repositories that are part of the .NET Foundation.

Controlling Serialization

Before you add more features, let's address the repo type and make it follow more standard C# conventions. You'll do this by annotating the repo type with attributes that control how the JSON Serializer works. In your case, you'll use these attributes to define a mapping between the JSON key names and the C# class and member names. The two attributes used are the DataContract attribute and the DataMember attribute. By convention, all Attribute classes end in the suffix Attribute. However, you do not need to use that suffix when you apply an attribute.

The DataContract and DataMember attributes are in a different library, so you'll need to add that library to your C# project file as a dependency. Add the following line to the <ItemGroup> section of your project file:

<PackageReference Include="System.Runtime.Serialization.Primitives" Version="4.3.0" />

After you save the file, run dotnet restore (see note) to retrieve this package.

Next, open the repo.cs file. Let's change the name to use Pascal Case, and fully spell out the name Repository. We still want to map JSON 'repo' nodes to this type, so you'll need to add the DataContract attribute to the class declaration. You'll set the Name property of the attribute to the name of the JSON nodes that map to this type:

[DataContract(Name="repo")]
public class Repository

The DataContractAttribute is a member of the System.Runtime.Serialization namespace, so you'll need to add the appropriate using statement at the top of the file:

using System.Runtime.Serialization;

You changed the name of the repo class to Repository, so you'll need to make the same name change in Program.cs (some editors may support a rename refactoring that will make this change automatically:)

var serializer = new DataContractJsonSerializer(typeof(List<Repository>));

// ...

var repositories = serializer.ReadObject(await streamTask) as List<Repository>;

Next, let's make the same change with the name field by using the DataMemberAttribute class. Make the following changes to the declaration of the name field in repo.cs:

[DataMember(Name="name")]
public string Name;

This change means you need to change the code that writes the name of each repository in program.cs:

Console.WriteLine(repo.Name);

Do a dotnet build followed by a dotnet run to make sure you've got the mappings correct. You should see the same output as before. Before we process more properties from the web server, let's make one more change to the Repository class. The Name member is a publicly accessible field. That's not a good object-oriented practice, so let's change it to a property. For our purposes, we don't need any specific code to run when getting or setting the property, but changing to a property makes it easier to add those changes later without breaking any code that uses the Repository class.

Remove the field definition, and replace it with an auto-implemented property:

public string Name { get; set; }

The compiler generates the body of the get and set accessors, as well as a private field to store the name. It would be similar to the following code that you could type by hand:

public string Name 
{ 
    get { return this._name; }
    set { this._name = value; }
}
private string _name;

Let's make one more change before adding new features. The ProcessRepositories method can do the async work and return a collection of the repositories. Let's return the List<Repository> from that method, and move the code that writes the information into the Main method.

Change the signature of ProcessRepositories to return a task whose result is a list of Repository objects:

private static async Task<List<Repository>> ProcessRepositories()

Then, just return the repositories after processing the JSON response:

var repositories = serializer.ReadObject(await streamTask) as List<Repository>;
return repositories;

The compiler generates the Task<T> object for the return because you've marked this method as async. Then, let's modify the Main method so that it captures those results and writes each repository name to the console. Your Main method now looks like this:

public static void Main(string[] args)
{
    var repositories = ProcessRepositories().Result;

    foreach (var repo in repositories)
        Console.WriteLine(repo.Name);
}

Accessing the Result property of a Task blocks until the task has completed. Normally, you would prefer to await the completion of the task, as in the ProcessRepositories method, but that isn't allowed in the Main method.

Reading More Information

Let's finish this by processing a few more of the properties in the JSON packet that gets sent from the GitHub API. You won't want to grab everything, but adding a few properties will demonstrate a few more features of the C# language.

Let's start by adding a few more simple types to the Repository class definition. Add these properties to that class:

[DataMember(Name="description")]
public string Description { get; set; }

[DataMember(Name="html_url")]
public Uri GitHubHomeUrl { get; set; }

[DataMember(Name="homepage")]
public Uri Homepage { get; set; }

[DataMember(Name="watchers")]
public int Watchers { get; set; }

These properties have built-in conversions from the string type (which is what the JSON packets contain) to the target type. The Uri type may be new to you. It represents a URI, or in this case, a URL. In the case of the Uri and int types, if the JSON packet contains data that does not convert to the target type, the serialization action will throw an exception.

Once you've added these, update the Main method to display those elements:

foreach (var repo in repositories)
{
    Console.WriteLine(repo.Name);
    Console.WriteLine(repo.Description);
    Console.WriteLine(repo.GitHubHomeUrl);
    Console.WriteLine(repo.Homepage);
    Console.WriteLine(repo.Watchers);
    Console.WriteLine();
}

As a final step, let's add the information for the last push operation. This information is formatted in this fashion in the JSON response:

2016-02-08T21:27:00Z

That format does not follow any of the standard .NET DateTime formats. Because of that, you'll need to write a custom conversion method. You also probably don't want the raw string exposed to users of the Repository class. Attributes can help control that as well. First, define a private property that will hold the string representation of the date time in your Repository class:

[DataMember(Name="pushed_at")]
private string JsonDate { get; set; }

The DataMember attribute informs the serializer that this should be processed, even though it is not a public member. Next, you need to write a public read-only property that converts the string to a valid DateTime object, and returns that DateTime:

[IgnoreDataMember]
public DateTime LastPush
{
    get
    {
        return DateTime.ParseExact(JsonDate, "yyyy-MM-ddTHH:mm:ssZ", CultureInfo.InvariantCulture);
    }
}

Let's go over the new constructs above. The IgnoreDataMember attribute instructs the serializer that this type should not be read to or written from any JSON object. This property contains only a get accessor. There is no set accessor. That's how you define a read-only property in C#. (Yes, you can create write-only properties in C#, but their value is limited.) The ParseExact(String, String, IFormatProvider) method parses a string and creates a DateTime object using a provided date format, and adds additional metadata to the DateTime using a CultureInfo object. If the parse operation fails, the property accessor throws an exception.

To use InvariantCulture, you will need to add the System.Globalization namespace to the using statements in repo.cs:

using System.Globalization;

Finally, add one more output statement in the console, and you're ready to build and run this app again:

Console.WriteLine(repo.LastPush);

Your version should now match the finished sample.

Conclusion

This tutorial showed you how to make web requests, parse the result, and display properties of those results. You've also added new packages as dependencies in your project. You've seen some of the features of the C# language that support object-oriented techniques.

Note

Starting with .NET Core 2.0, you don't have to run dotnet restore because it's run implicitly by all commands that require a restore to occur, such as dotnet new, dotnet build and dotnet run. It's still a valid command in certain scenarios where doing an explicit restore makes sense, such as continuous integration builds in Azure DevOps Services or in build systems that need to explicitly control the time at which the restore occurs.