How to integrate MailChimp in C#/.Net

Try this
MailChimp API Discuss
http://groups.google.com/group/mailchimp-api-discuss/browse_thread/thread/8e50a780819fdfd4?pli=1[^]

Install the NuGet package from the package manager console:

Install-Package MailChimp.Net.V3

Using it in code

IMailChimpManager manager = new MailChimpManager(apiKey); //if you have it in code

<add key="MailChimpApiKey" value="apiKEY" />
IMailChimpManager manager = new MailChimpManager(); //if you have it in config
Examples

// Instantiate new manager
IMailChimpManager mailChimpManager = new MailChimpManager(apiKey);
Getting all lists:

var mailChimpListCollection = await this.mailChimpManager.Lists.GetAllAsync().ConfigureAwait(false);
Getting 50 Lists:

var mailChimpListCollection = await this.mailChimpManager.Lists.GetAllAsync(new ListRequest
                                                               {
                                                                   Limit = 50
                                                               }).ConfigureAwait(false);
Getting Users from List:

var listId = "TestListId";
await this.mailChimpManager.Members.GetAllAsync(listId).ConfigureAwait(false);
Adding User To List

var listId = "TestListId";
var member = new Member { EmailAddress = $"githubTestAccount@test.com", Status = Status.Subscribed };
member.MergeFields.Add("FNAME", "HOLY");
member.MergeFields.Add("LNAME", "COW");
await this.mailChimpManager.Members.AddOrUpdateAsync(listId, member);

I wrote an article on a simple way up adding subscribers to a list using:

Dim mailchimp As New ZmailChimp
      Dim ListId$ = "9b2e63f0b9"   'List Sage' List
      Dim email$ = "samsmith20@anymail.com" '"sam19@postcodelite.com"
      Dim fieldListOnAdd = "FNAME,Sam,LNAME,Smith,MTYPE,User,MID,631637"
      Dim fieldListOnUpdate = "FNAME,Sam,LNAME,Smith,MID,631637"  'Don't change MTYPE
      'Put on 'Sage One' and 'Sage 50' group
      Dim groupList = "407da9f47d,05086211ba"

      With mailchimp
         .API$ = "46cMailChimpAPIKeyd1de-us14" 'MailChimp API key
         .dataCenter$ = "us14"  'Last 4 letters of API key
         .password$ = "Password!"
         MsgBox(.addSubscriber(ListId$, email, fieldListOnAdd, fieldListOnUpdate, groupList))
      End With
      mailchimp = Nothing

see:http://www.codeproject.com/Tips/1140339/Mail-Chimp-Add-Update-e-mail-to-List-and-Subscribe
this may save someone some time