Fluent HTTP

NOTE: Everything beyond URL building and parsing requires installing Flurl.Http rather than the base Flurl package.

Basic Usage

A pretty common way to think about interacting with an HTTP service is "I want to build a URL and then call it." Flurl.Http allows you to express that pretty concisely:

using Flurl;
using Flurl.Http;

var result = await "https://some-api.com"

Get things other than strings:

T poco = await "http://api.foo.com".GetJsonAsync<T>();
byte[] bytes = await "http://site.com/image.jpg".GetBytesAsync();
Stream stream = await "http://site.com/music.mp3".GetStreamAsync();

The above examples all send an HTTP GET request. All other common verbs are also supported:

var result = await "http://api.foo.com".PostJsonAsync(requestObj).ReceiveJson<T>();
var resultStr = await "http://api.foo.com/1".PatchJsonAsync(requestObj).ReceiveString();
var resultStr2 = await "http://api.foo.com/2".PutStringAsync("hello").ReceiveString();
var resp = await "http://api.foo.com".OptionsAsync();
await "http://api.foo.com".HeadAsync();

Responses and Error Handling

Most of the examples above demonstrate getting some representation of the response body only. So how do you handle a failure response, such as a 400, whose body may take a very different shape? Flurl differs from HttpClient in that it throws on any 4xx or greater response status by default, and the FlurlHttpException thrown allows you to inspect the body separately:

try {
    var result = await url.PostJsonAsync(requestObj).ReceiveJson<T>();
catch (FlurlHttpException ex) {
    var err = await ex.GetResponseJsonAsync<TError>(); // or GetResponseStringAsync(), etc.
    logger.Write($"Error returned from {ex.Call.Request.Url}: {err.SomeDetails}");

Of course, you may want to inspect the response status independently of error-handling, or inspect other response properties such as headers. Using GetAsync, or just excluding the ReceiveXXX, from non-GET calls, returns an IFlurlResponse:

var resp1 = await "http://api.foo.com".GetAsync();
var resp2 = await "http://api.foo.com".PostJsonAsync(requestObj);

From which you can read other information before consuming the body:

int status = resp.StatusCode;
string headerVal = resp.Headers.FirstOrDefault("my-header");
T body = await resp.GetJsonAsync<T>();

If you prefer inspecting status codes this way exclusively, you can disable the exception-throwing behavior for specific statuses or all of them:

var resp2 = await "http://api.foo.com".AllowHttpStatus(400, 401).GetAsync();
var resp3 = await "http://api.foo.com".AllowHttpStatus("400-403,5xx").GetAsync();
var resp1 = await "http://api.foo.com".AllowAnyHttpStatus().GetAsync();

In the last example, x, X, and * are all valid wildcards. Note that you do not need to include 2xx or 3xx codes explicitly; those are considered "success" statuses and never throw.

Simulating a Browser

Flurl includes first-class support for things like form posts and cookies, which are more typical with web browsers than REST APIs.

Simulate an HTML form post:

await "http://site.com/login".PostUrlEncodedAsync(new { 
    user = "user", 
    pass = "pass"

Or a multipart form POST (typically associated with file uploads):

var resp = await "http://api.com".PostMultipartAsync(mp => mp
    .AddString("name", "hello!")                // individual string
    .AddStringParts(new {a = 1, b = 2})         // multiple strings
    .AddFile("file1", path1)                    // local file path
    .AddFile("file2", stream, "foo.txt")        // file stream
    .AddJson("json", new { foo = "x" })         // json
    .AddUrlEncoded("urlEnc", new { bar = "y" }) // URL-encoded                      
    .Add(content));                             // any HttpContent

Download a file:

// filename is optional here; it will default to the remote file name
var path = await "http://files.foo.com/image.jpg"
    .DownloadFileAsync("c:\\downloads", filename);

Send some cookies with a request:

var resp = await "https://cookies.com"
    .WithCookie("name", "value")
    .WithCookies(new { cookie1 = "foo", cookie2 = "bar" })

Better yet, grab response cookies from the first request and let Flurl determine when to send them back (per RFC 6265):

await "https://cookies.com/login".WithCookies(out var jar).PostUrlEncodedAsync(credentials);
await "https://cookies.com/a".WithCookies(jar).GetAsync();
await "https://cookies.com/b".WithCookies(jar).GetAsync();

Or avoid all those WithCookies calls and use a CookieSession:

using var session = new CookieSession("https://cookies.com");
// set any initial cookies on session.Cookies
await session.Request("a").GetAsync();
await session.Request("b").GetAsync();
// read cookies at any point using session.Cookies

In the above examples, jar and session.Cookies are instances of CookieJar. This is Flurl's equivalent of CookieContainer from the HttpClient stack, but with one major advantage: it is not bound to an HttpMessageHandler, hence you can simulate multiple cookie "sessions" on a single HttClient/Handler instance. It can also be easily persisted and reloaded between sessions:

// string-based persistence:
var saved = jar.ToString();
var jar2 = CookieJar.LoadFromString(saved);

// file-based persistence:
using var writer = new StreamWriter("path/to/file");

using var reader = new StreamReader("path/to/file");
var jar2 = CookieJar.LoadFrom(reader);

Additional Use Cases

Set request headers:

// one:
await url.WithHeader("Accept", "text/plain").GetJsonAsync();
// multiple:
await url.WithHeaders(new { Accept = "text/plain", User_Agent = "Flurl" }).GetJsonAsync();

In the second example above, User_Agent will automatically render as User-Agent in the header name. Hyphens are very common in header names but not allowed in C# identifiers; underscores, just the opposite.

Authenticate using Basic authentication:

await url.WithBasicAuth("username", "password").GetJsonAsync();

Or an OAuth 2.0 bearer token:

await url.WithOAuthBearerToken("mytoken").GetJsonAsync();

Specify a timeout:

await url.WithTimeout(10).GetAsync(); // 10 seconds
await url.WithTimeout(TimeSpan.FromMinutes(2)).GetAsync();

Handle a timeout error:

try {
    var result = await url.GetStringAsync();
catch (FlurlHttpTimeoutException) {
    // handle timeouts
catch (FlurlHttpException) {
    // handle error responses

FlurlHttpTimeoutException inherits from FlurlHttpException and hence could be handled from the same catch block, but the above example demonstrates handling it as a special case.

Weird verbs or content? Use one of the lower-level methods:

await url.PostAsync(content); // content is a System.Net.Http.HttpContent
await url.SendJsonAsync(HttpMethod.Trace, data);
await url.SendAsync(
    new HttpMethod("CONNECT"),
    httpContent, // optional
    cancellationToken,  // optional
    HttpCompletionOption.ResponseHeaderRead);  // optional

Cancel a request:

var cts = new CancellationTokenSource();
var task = url.GetAsync(cts.Token);

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