How to display popup messages in the admin

If you need to interact with the sitecore interface with a custom field or a custom sitecore admin screen you will probably need some input from the user, alert, ...

This operation is possible with only a few lines of code:

Show a popup:
Sitecore.Context.ClientPage.ClientResponse.Alert("My message");

Download a file
Sitecore.Context.ClientPage.ClientResponse.Download(@"c:\MyPath\Myfile.xxx");

To create an input, use this function:
Input(string text, string defaultValue, string validation, string validationText, int maxLength)

A complete example of code for the input:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Sitecore.Data.Items;
using Sitecore.Data.Fields;
using Sitecore;
using Sitecore.Diagnostics;

namespace MyNamespace
{
    class MyClass : Sitecore.Shell.Framework.Commands.Command
    {
        public override void Execute(Sitecore.Shell.Framework.Commands.CommandContext context)
        {
            if (context.Items.Length == 1)
            {
                Sitecore.Data.Items.Item item = context.Items[0];
                System.Collections.Specialized.NameValueCollection parameters = new System.Collections.Specialized.NameValueCollection();
                parameters["id"] = item.ID.ToString();
                parameters["language"] = item.Language.ToString();
                parameters["database"] = item.Database.Name;
                Sitecore.Context.ClientPage.Start(this, "Run", parameters);
            }
        }

        protected void Run(Sitecore.Web.UI.Sheer.ClientPipelineArgs args)
        {
            if (args.IsPostBack)
            {
                if ((!String.IsNullOrEmpty(args.Result)) && args.Result != "undefined" && args.Result != "null")
                {
                    Sitecore.Data.Database db = Sitecore.Configuration.Factory.GetDatabase(args.Parameters["database"]);
                    Sitecore.Data.Items.Item myItem = db.GetItem(new Sitecore.Data.ID(args.Parameters["id"]));                        
                }
            }
            else
            {

                Sitecore.Context.ClientPage.ClientResponse.Input("My Message:"
                    , "My Default Value"
                    , Sitecore.Configuration.Settings.ItemNameValidation
                    , "'$Input' is not a valid name.", MaxLenght);
                args.WaitForPostBack();
            }
        }
    }
}