.NET HttpClient。如何POST字符串值？

How can I create using C# and HttpClient the following POST request:

我的 WEB API 服务需要这样的请求：

[ActionName("exist")]
[HttpPost]
public bool CheckIfUserExist([FromBody] string login)
{           
    return _membershipProvider.CheckIfExist(login);
}

在 ASP.NET 4.5 中（对于 4.0，您需要安装Microsoft.AspNet.WebApi.Client http://nuget.org/packages/Microsoft.AspNet.WebApi.Client/NuGet 包）：

using System;
using System.Collections.Generic;
using System.Net.Http;

class Program
{
    static void Main(string[] args)
    {
        Task.Run(() => MainAsync());
        Console.ReadLine();
    }

    static async Task MainAsync()
    {
        var client = new HttpClient();
        client.BaseAddress = new Uri("http://localhost:6740");
        var content = new FormUrlEncodedContent(new[]
        {
            new KeyValuePair<string, string>("", "login")
        });
        var result = await client.PostAsync("/api/Membership/exists", content);
        string resultContent = await result.Content.ReadAsStringAsync();
        Console.WriteLine(resultContent);
    }
}