curl Cookie Errors in C# (-b, -c and Cookie Jars)

curl cookie errors in C# are almost always about how the cookie was supplied or where the jar is saved. CurlDotNet handles cookies the way the curl tool does: -b 'name=value' sends a cookie string, -b FILE loads a Netscape cookie file, -c JAR writes every cookie the server set to a jar file, and -j drops session cookies when loading. The one cookie error that throws is a jar that cannot be written: CurlWriteErrorException, exit code 23. This page shows each option and the mistakes that cause trouble.

Sending a Cookie String (-b name=value)

using CurlDotNet;

var result = await Curl.ExecuteAsync("curl -b 'session=abc123; theme=dark' https://httpbin.org/cookies");
Console.WriteLine(result.Body); // {"cookies": {"session": "abc123", "theme": "dark"}}

--cookie 'name=value' is the long form. Several -b strings are joined with ; , so -b 'a=1' -b 'b=2' sends Cookie: a=1; b=2.

Builder Equivalent

using CurlDotNet.Core;

var result = await CurlRequestBuilder
    .Get("https://httpbin.org/cookies")
    .WithCookie("session=abc123; theme=dark")
    .ExecuteAsync();

WithCookie follows the same rule as -b: a value without = is treated as a cookie file name. WithCookieJar(path) is the builder form of -c.

Sending the Header Directly

A cookie is just a request header, so -H works too; it bypasses the cookie engine:

var result = await Curl.ExecuteAsync("curl -H 'Cookie: session=abc123' https://httpbin.org/cookies");

Cookie Jars (-b FILE and -c JAR)

A value without = is a file name. -c JAR (--cookie-jar) writes the cookies after the transfer in curl’s Netscape format (#HttpOnly_ prefixes, expiry capped at 400 days like curl), and -b JAR sends them on a later command, so cookies persist across separate Curl.ExecuteAsync calls:

using CurlDotNet;

// 1. Log in and save the cookies the server sets
await Curl.ExecuteAsync("curl -c cookies.txt https://httpbin.org/cookies/set/session/abc123");

// 2. Send them back on the next call (and keep the jar up to date)
var result = await Curl.ExecuteAsync("curl -b cookies.txt -c cookies.txt https://httpbin.org/cookies");
Console.WriteLine(result.Body);   // {"cookies": {"session": "abc123"}}

Rules that match curl:

Errors

CurlWriteErrorException (23): the jar cannot be saved

Thrown after the transfer when the -c file cannot be written: its directory does not exist, the path is read-only, or permission is denied. FilePath is the jar path, and the message says what failed.

using CurlDotNet;
using CurlDotNet.Exceptions;

try
{
    await Curl.ExecuteAsync("curl -c missing-dir/cookies.txt https://httpbin.org/cookies/set/session/abc123");
}
catch (CurlWriteErrorException ex)
{
    Console.WriteLine($"Could not save the cookie jar {ex.FilePath}: {ex.Message}");
}

Fix: point -c at a writable file in a directory that exists (create it first; --create-dirs applies to -o output files only).

A missing -b file is not an error

-b cookies.txt with no such file silently starts with an empty jar, exactly like curl, so no Cookie header is sent. If cookies seem to vanish, check that the path is relative to the process working directory (Directory.GetCurrentDirectory()), not to your source file. No read error (exit code 26) is raised for cookie files.

CurlCookieException is reserved

CurlCookieException (no exit code, property CookieJarPath) exists in CurlDotNet.Exceptions, but nothing in the library throws it today. Catch CurlWriteErrorException for jar problems.

Common Mistakes

A -b value without = is a file name. -b session loads a file called session (and finds nothing); write -b 'session=abc123' to send a cookie.

Spaces Without Quotes

Multiple cookies are separated by ; , which includes a space, so quote the whole value:

var result = await Curl.ExecuteAsync("curl -b 'a=1; b=2' https://httpbin.org/cookies");

Expecting Cookies to Persist Without a Jar

Each Curl.ExecuteAsync call starts with no cookies. To carry cookies from one call to the next, write them with -c JAR and read them with -b JAR as shown above, or pass the value explicitly with -b 'name=value'.

Debugging Cookies

-v writes the outgoing request headers, including Cookie:, at the start of result.Body:

var result = await Curl.ExecuteAsync("curl -v -b 'session=abc123' https://httpbin.org/cookies");
Console.WriteLine(result.Body);

https://httpbin.org/cookies echoes back the cookies it received, which makes it a convenient test endpoint.