Using MailChimp.NET MergeVars in ASP.NET WebForms. This is primarily to illustrate the use of the MergeVar class for sending custom merge variables via the subscribe method.

CustomMergeVars.cs

using MailChimp.Lists;
using System.Runtime.Serialization;

namespace MyNamespace {
    
    [DataContract]
    public class CustomMergeVars: MergeVar
    {
      [DataMember(Name="FNAME")]
      public string FirstName { get; set; }
      
      [DataMember(Name="LNAME")]
      public string LastName { get; set; }
    } 
    
}

default.aspx.cs

using System;
using MailChimp;
using MailChimp.Helper;
using MailChimp.Lists;

namespace MyNamespace {
    
    public partial class _default : System.Web.UI.Page
    {
        protected void SubmitFormLink_Click(object sender, EventArgs e)
        {
            CommsValidator.IsValid = true;

            Validate();
            if (IsValid)
            {
                var mc = new MailChimpManager(AppGlobals.MailChimpAPIKey);
                try
                {
                    var email = new EmailParameter()
                    {
                        Email = txtEmailAddress.Text
                    };

                    var mergeVars = new CustomMergeVars
                    {
                        FirstName = txtFirstName.Text,
                        LastName = txtLastName.Text
                    };                   

                    // see here for full documentation on the lists/subscribe api call
                    // http://apidocs.mailchimp.com/api/2.0/lists/subscribe.php
                    mc.Subscribe(listId: AppGlobals.MailChimpMailingListID, 
                                 emailParam: email, 
                                 mergeVars: mergeVars, 
                                 updateExisting: true, 
                                 sendWelcome: true);

                    Response.Clear();
                    Response.Redirect("follow_success.aspx");
                    Response.End();
                }
                catch
                {
                    CommsValidator.IsValid = false;
                }
            }
        }

    }
}