How do I wait until Task is finished in C#?

Your Print method likely needs to wait for the continuation to finish (ContinueWith returns a task which you can wait on). Otherwise the second ReadAsStringAsync finishes, the method returns (before result is assigned in the continuation). Same problem exists in your send method. Both need to wait on the continuation to consistently get the results you want. Similar to below

private static string Send(int id)
{
    Task<HttpResponseMessage> responseTask = client.GetAsync("aaaaa");
    string result = string.Empty;
    Task continuation = responseTask.ContinueWith(x => result = Print(x));
    continuation.Wait();
    return result;
}

private static string Print(Task<HttpResponseMessage> httpTask)
{
    Task<string> task = httpTask.Result.Content.ReadAsStringAsync();
    string result = string.Empty;
    Task continuation = task.ContinueWith(t =>
    {
        Console.WriteLine("Result: " + t.Result);
        result = t.Result;
    });
    continuation.Wait();  
    return result;
}