Както каза АМД най-чистия начин е с Task и async/await..

using System.Threading.Tasks;



public static Task<string> GetUserSubscribeMethodAsync(User user)

{

    if (user == null)

        throw new ArgumentNullException("user");



    var tcs = new TaskCompletionSource<string>();



    string req_params = "token=" + user.Token.AccessToken;



    try

    {

        PostJsonRequest<UserSubscribeStatus>(ContractConstants.QelloService.BILLING_USER_STATUS, req_params, response =>

        {

            if ((response.Error != null) || (response.Data == null) || 

                (response.Data.ActiveRecurring == null) || 

                (response.Data.ActiveRecurring.Length <= 0))

            {

                tcs.SetException(new UnableToGetUserSubscribeMethodException());

            }

            else

            {

                string paymentMethod = response.Data.ActiveRecurring[0].Trim();

                tcs.SetResult(paymentMethod);

            }

        });

    }

    catch (Exception e)

    {

        tcs.SetException(e);

    }



    return tcs.Task;

}







// C# 5  



string paymentMethod = null;

try 

{

    paymentMethod = await GetUserSubscribeMethodAsync(user);

}

catch (Exception e)

{

    // handle any errors here (ArgumentNullException, UnableToGetUserSubscribeMethodException, etc.)

}