Make HTTP Requests to the Web API from .NET Applications

Sep 14, 2022
10 minutes to read

You can send requests to a Web API service from any .NET application with the HttpClient library. Use the OData syntax to build requests. See the following topics for more information on OData query options:

The examples below send requests to the Web API service available at the following address: https://localhost:44319/.

Get a JWT Authentication Token

To obtain the JWT Authentication token for further data requests, send a request to the following endpoint: api/Authentication/Authenticate. The following example uses “Sam” as a user name and an empty password:

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;

namespace ConsoleApp1 {
    class Program {
        static async Task Main(string[] args) {
            HttpClient httpClient = new HttpClient();

            // Obtain a JWT token.
            StringContent httpContent = new StringContent(@"{ ""userName"": ""Sam"", ""password"": """" }", Encoding.UTF8, "application/json");
            var response = await httpClient.PostAsync("https://localhost:44319/api/Authentication/Authenticate", httpContent);

            // Save the token for further requests.
            var token = await response.Content.ReadAsStringAsync();

            // Set the authentication header. 
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
        }
    }
}

Operate with Business Objects

Get Business Objects

The following code retrieves the LastName and Email fields of the Employee business object where FirstName equals “Mary”:

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;

namespace ConsoleApp1 {
    class Program {
        static async Task Main(string[] args) {
            HttpClient httpClient = new HttpClient();

            // Obtain a JWT token. This example uses "Sam" as a user name and an empty password.
            StringContent httpContent = new StringContent(@"{ ""userName"": ""Sam"", ""password"": """" }", Encoding.UTF8, "application/json");
            var response = await httpClient.PostAsync("https://localhost:44319/api/Authentication/Authenticate", httpContent);

            // Save the token for further requests.
            var token = await response.Content.ReadAsStringAsync();

            // Set the authentication header. 
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);

            // Send a request to fetch data.
            string requestAddress = "https://localhost:44319/api/odata/Employee";
            var employees = await httpClient.GetStringAsync($"{requestAddress}?$filter=FirstName eq 'Mary'&$select=LastName,Email");
            Console.WriteLine(employees);
        }
    }
}

Result

JSON

{"@odata.context":"https://localhost:44319/api/odata/$metadata#Employee(LastName,Email)",
"value":[{"LastName":"Tellitson",
            "Email":"Mary_Tellitson@example.com"}]}

Create a Business Object

The code below adds a new Employee instance with the FirstName field set to “Mary” and the LastName field set to “Gordon”:

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1 {
    class Program {
        static async Task Main(string[] args) {
            HttpClient httpClient = new HttpClient();
            // Obtain a JWT token. This example uses "Sam" as a user name and an empty password.
            StringContent httpContent = new StringContent(@"{ ""userName"": ""Sam"", ""password"": """" }", Encoding.UTF8, "application/json");
            var response = await httpClient.PostAsync("https://localhost:44319/api/Authentication/Authenticate", httpContent);

            // Save the token for further requests.
            var token = await response.Content.ReadAsStringAsync();

            // Set the authentication header. 
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);

            // Pass data to the Web API service. 
            StringContent dataHttpContent = new StringContent(@"{ ""FirstName"": ""Mary"", ""LastName"":""Gordon"" }", Encoding.UTF8, "application/json");
            var dataResponse = await httpClient.PostAsync($"{requestAddress}", dataHttpContent);
            Console.WriteLine(dataResponse.StatusCode);
        }
    }
}

Result (the dataResponse.StatusCode value): 201 Created

Get a Reference Object

You can use one of the following techniques:

Technique 1 (Expand Query Parameter): The $expand OData query parameter allows you to obtain a reference business object together with the main object.
Technique 2 (Ref Endpoint): The $ref endpoint allows you to obtain a reference business object without its main object.

Technique 1 (Expand Query Parameter)

The example below uses $expand to get an Employee object with its related Department object:

using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1 {
    class Program {
        static async Task Main(string[] args) {
            HttpClient httpClient = new HttpClient();

            // Obtain a JWT token. This example uses "Sam" as a user name and an empty password.
            StringContent httpContent = new StringContent(@"{ ""userName"": ""Sam"", ""password"": """" }", Encoding.UTF8, "application/json");
            var response = await httpClient.PostAsync("https://localhost:44319/api/Authentication/Authenticate", httpContent);

            // Save the token for further requests.
            var token = await response.Content.ReadAsStringAsync();

            // Set the authentication header. 
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);

            // Send a request to fetch data.
            string requestAddress = "https://localhost:44319/api/odata/Employee";
            var employees = await httpClient.GetStringAsync($"{requestAddress}?$filter=FirstName eq 'Mary'&$select=LastName,Email&$expand=Department");
            Console.WriteLine(employees);
        }
    }
}

Result

JSON

{"@odata.context":"https://localhost:44319/api/odata/$metadata#Employee(LastName,Email,Department())",
 "value":[{"LastName":"Tellitson",
           "Email":"Mary_Tellitson@example.com",
           "Department":{
                "Oid":"6eff292f-f871-4237-a22c-8a50aa747ea3",
                "Title":"Development Department",
                "Description":"The Information Technology Department manages the company's information infrastructure and online assets.",
                "Location":"Building 2",
                "Office":"205"}
        }]
}

The $expand parameter can be applied to more than one level of related business objects. The following example retrieves a data chain that consists of two related business objects:

// ...
string requestAddress = "https://localhost:44319/api/odata/Department";
var departments = await httpClient.GetStringAsync($"{requestAddress}?$select=Title&$expand=Employees($select=FirstName,LastName;$expand=Tasks($select=Subject))");
// ...

Result

JSON

{"@odata.context": "https://localhost:44319/api/odata/$metadata#Department(Title,Employees(FirstName,LastName,Tasks(Subject)))",
 "value": [{ "Title": "Human Resources",
             "Employees": [{ "FirstName": "Angela",
                             "LastName": "Gross",
                             "Tasks": [{ "Subject": "Create 2022 R&D Plans"},
                                       { "Subject": "Submit D&B Number to ISP for Credit Approval"},
                                       { "Subject": "Deliver R&D Plans for 2022"}]
                           },
                           { "FirstName": "Barbara",
                             "LastName": "Faircloth",
                             "Tasks": [{ "Subject": "Subject": "Deliver R&D Plans for 2022"},
                                       { "Subject": "Submit D&B Number to ISP for Credit Approval"},
                                       { "Subject": "Create 2022 R&D Plans"}]
                            }]
           },
          { "Title": "Purchasing",
            "Employees": [{ "FirstName": "Ernest",
                            "LastName": "Webb",
                            "Tasks": [{ "Subject": "Submit D&B Number to ISP for Credit Approval"},
                                       { "Subject": "Deliver R&D Plans for 2022"}]
                           },
                           ... 
                         ]
          }]
}

The default max expansion depth equals two. You can change this parameter as described in the following topic: Change the Expansion Depth for Related Business Objects.

Technique 2 (Ref Endpoint)

The example below gets the Employee‘s Department reference object:

using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1 {
    class Program {
        static async Task Main(string[] args) {
            HttpClient httpClient = new HttpClient();

            // Obtain a JWT token. This example uses "Sam" as a user name and an empty password.
            StringContent httpContent = new StringContent(@"{ ""userName"": ""Sam"", ""password"": """" }", Encoding.UTF8, "application/json");
            var response = await httpClient.PostAsync("https://localhost:44319/api/Authentication/Authenticate", httpContent);

            // Save the token for further requests.
            var token = await response.Content.ReadAsStringAsync();

            // Set the authentication header. 
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);

            // Send a request to fetch data.
            string requestAddress = "https://localhost:44319/api/odata/Employee/1/Department/$ref";
            // or 
            // string requestAddress = "https://localhost:44319/api/odata/Employee(1)/Department/$ref";
            var department = await httpClient.GetStringAsync(requestAddress);
            Console.WriteLine(department);
        }
    }
}

Result

JSON

{"@odata.context":"https://localhost:44319/api/odata/$metadata#Department/$entity",
"ID":1,
"Title":"Development Department",
"Office":"205","Location":"Building 2",
"Description":"The Information Technology Department manages the company's information infrastructure and online assets."}

Get an Associated Collection

You can use one of the following techniques:

Technique 1 (Expand Query Parameter): The $expand OData query parameter allows you to obtain objects from an associated collection together with the main object.
Technique 2 (Ref Endpoint): The $ref endpoint allows you to obtain objects from an associated collection without its main object.

Technique 1 (Expand Query Parameter)

The following example gets LastName, Email, and the related Tasks collection of the Employee business object where FirstName equals “Mary”:

using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1 {
    class Program {
        static async Task Main(string[] args) {
            HttpClient httpClient = new HttpClient();

            // Obtain a JWT token. This example uses "Sam" as a user name and an empty password.
            StringContent httpContent = new StringContent(@"{ ""userName"": ""Sam"", ""password"": """" }", Encoding.UTF8, "application/json");
            var response = await httpClient.PostAsync("https://localhost:44319/api/Authentication/Authenticate", httpContent);

            // Save the token for further requests.
            var token = await response.Content.ReadAsStringAsync();

            // Set the authentication header. 
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);

            // Send a request to fetch data.
            string requestAddress = "https://localhost:44319/api/odata/Employee";
            var employees = await httpClient.GetStringAsync($"{requestAddress}?$filter=FirstName eq 'Mary'&$select=LastName,Email&$expand=Tasks");
            Console.WriteLine(employees);
        }
    }
}

Result

JSON

{"@odata.context":"https://localhost:44319/api/odata/$metadata#Employee(LastName,Email,Tasks())",
 "value":[{"LastName":"Tellitson",
           "Email":"Mary_Tellitson@example.com",
           "Tasks":[{"Oid":"b958f20a-118d-4af0-b249-94445608549d",
                     "Subject":"2022 Brochure Designs",
                     "DueDate":"2022-01-15T00:00:00+04:00",
                     "StartDate":"0001-01-01T00:00:00Z",
                     "Status":"Deferred",
                     "PercentCompleted":0,
                     "Priority":"Normal"},
                    {"Oid":"7de87fc8-4dc0-4b76-82b3-18dffdc61ba4",
                     "Subject":"Review Benefits",
                     "DueDate":"2021-10-02T00:00:00+04:00",
                     "StartDate":"2021-09-12T00:00:00+04:00",
                     "Status":"Completed",
                     "PercentCompleted":100,
                     "Priority":"Normal"},
                    {"Oid":"67d36cda-a261-489f-afa9-c8ac43e1c2ea",
                     "Subject":"Lunch Potluck",
                     "DueDate":"2021-10-03T00:00:00+04:00",
                     "StartDate":"0001-01-01T00:00:00Z",
                     "Status":"Deferred",
                     "PercentCompleted":0,
                     "Priority":"Low"}
                    ]
        }]
}

Technique 2 (Ref Endpoint)

The example below gets the Employee‘s Tasks collection:

using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1 {
    class Program {
        static async Task Main(string[] args) {
            HttpClient httpClient = new HttpClient();

            // Obtain a JWT token. This example uses "Sam" as a user name and an empty password.
            StringContent httpContent = new StringContent(@"{ ""userName"": ""Sam"", ""password"": """" }", Encoding.UTF8, "application/json");
            var response = await httpClient.PostAsync("https://localhost:44319/api/Authentication/Authenticate", httpContent);

            // Save the token for further requests.
            var token = await response.Content.ReadAsStringAsync();

            // Set the authentication header. 
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);

            // Send a request to fetch data.
            string requestAddress = "https://localhost:44319/api/odata/Employee(1)/Tasks/$ref";
            // or 
            // string requestAddress = "https://localhost:44319/api/odata/Employee/1/Tasks/$ref";
            var tasks = await httpClient.GetStringAsync(requestAddress);
            Console.WriteLine(tasks);
        }
    }
}

Result

JSON

{"@odata.context":"https://localhost:44319/api/odata/$metadata#DemoTask",
"value":[
    {
        "ID":8,
        "Subject":"Approve Overtime Pay",
        "Description":"Brett, the overtime I submitted was not paid and I'm being told it was not approved. I thought you approved this. What is the problem?\r\nBrett Wade: I did approve it. It was error in payroll. Trying to figure it out.",
        "DueDate":"2022-04-22T00:00:00+04:00",
        "StartDate":"2022-03-28T00:00:00+04:00",
        "PercentCompleted":0,
        "Status":"Completed",
        "Priority":"Normal",
        "ActualWorkHours":15,
        "EstimatedWorkHours":19
    },
    {
        "ID":9,
        "Subject":"Move Inventory to New Warehouse",
        "Description":"Robin, you are point person to get all inventory moved to the new warehouse location. You can hire temp workers if needed.",
        "DueDate":"2022-04-24T00:00:00+04:00",
        "StartDate":null,
        "PercentCompleted":0,
        "Status":"NotStarted",
        "Priority":"Low",
        "ActualWorkHours":0,
        "EstimatedWorkHours":10
    },
    {
        "ID":10,
        "Subject":"Shipping Label Artwork",
        "Description":"Kevin wants new shipping labels and I cannot print them without the artwork from your team. Can you please hurry and send it to me.\r\nMorgan Kennedy: Send me the specs and I will work on it when I can.",
        "DueDate":"2022-04-24T00:00:00+04:00",
        "StartDate":"2022-04-19T00:00:00+04:00",
        "PercentCompleted":0,
        "Status":"InProgress",
        "Priority":"High",
        "ActualWorkHours":19,
        "EstimatedWorkHours":12
    }
]}