How to send curl -X POST JSON in C#

To send curl -X POST JSON in C#, pass the curl command with -H "Content-Type: application/json" and -d '<json>' to Curl.ExecuteAsync, or call CurlRequestBuilder.Post(url).WithJson(obj) to serialize a C# object for you.

dotnet add package CurlDotNet

The curl command

using CurlDotNet;

var result = await Curl.ExecuteAsync("""
    curl -X POST https://httpbin.org/post \
      -H "Content-Type: application/json" \
      -d '{"name":"Ada Lovelace","age":36}'
    """);

Console.WriteLine(result.StatusCode);  // 200
Console.WriteLine(result.Body);

--json is shorter: it sets the body and adds both Content-Type: application/json and Accept: application/json.

using CurlDotNet;

var result = await Curl.ExecuteAsync(
    """curl --json '{"name":"Ada"}' https://httpbin.org/post""");

Console.WriteLine(result.StatusCode);

As with the curl CLI, -d on its own sends Content-Type: application/x-www-form-urlencoded. Many JSON APIs reject that, so always add the header (or use --json).

The fluent builder

WithJson serializes any object (System.Text.Json on .NET Framework and .NET 8+, Newtonsoft.Json on the netstandard2.0 build) and sets the Content-Type header:

using CurlDotNet.Core;

var result = await CurlRequestBuilder
    .Post("https://httpbin.org/post")
    .WithJson(new { name = "Ada Lovelace", age = 36 })
    .WithBearerToken("my-token")
    .ExecuteAsync();

Console.WriteLine(result.StatusCode);

One-liner helper

using CurlDotNet;

var result = await Curl.PostJsonAsync("https://httpbin.org/post", new { name = "Ada" });
Console.WriteLine(result.Body);

Build the JSON from variables safely

When values come from variables, prefer WithJson over string concatenation. It escapes quotes, backslashes and newlines correctly, and it keeps user input from being read as curl flags. If you do build a curl string, a C# raw interpolated string keeps the JSON braces readable:

using CurlDotNet;

var name = "Ada";
var result = await Curl.ExecuteAsync($$"""
    curl --json '{"name":""}' https://httpbin.org/post
    """);
Console.WriteLine(result.StatusCode);

Read the JSON response

using System.Text.Json;
using CurlDotNet;

var result = await Curl.PostJsonAsync("https://httpbin.org/post", new { id = 7 });
var echoed = result.ParseJson<JsonElement>().GetProperty("json");
Console.WriteLine(echoed.GetProperty("id").GetInt32());  // 7