Introduction

You can easily learn how to share session states in ASP.NET from different websites on different domains.

Using the Code

The first thing you need to do is to configure your Aspstate database. Open cmd.exe as administrator and write this command (modify it if necessary) and execute it:

%SystemRoot%\Microsoft.NET\Framework\v4.0.30319\aspnet_regsql.exe 
-S [Server] -E -sstype c -d [Database] -ssadd

After that, modify TempGetAppID Stored Procedure as below:

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER OFF
GO

ALTER PROCEDURE [dbo].[TempGetAppID]
@appName tAppName,
@appId int OUTPUT
AS
SET @appName = LOWER('King Session State')
SET @appId = 1

SELECT @appId = AppId
FROM dbo.ASPStateTempApplications
WHERE AppId = 1

IF @appId IS NULL BEGIN
        INSERT dbo.ASPStateTempApplications
        VALUES (@appId, @appName)
END

RETURN 0

Execute it to make sure all websites have the same AppId in which sessions will be shared.

Now modify web.config and add these lines to it:

<system.web>
    <sessionState mode="SQLServer" allowCustomSqlDatabase="true" 
    sqlConnectionString="Data source=.;Initial Catalog=SessionDB;User ID=???;
    Password=***" timeout="60" />
</system.web>

From now the ApplicationId of your sites will be same & your session states will be shared in all websites on the current domain. (in sub domains). To share sessionState on any website you need to set the same sessionId on them. You need to pass sessionId from source website to destination either using GET of POST methods. The codes below do it for you by using GET method (using Request.QueryString) :

using System.Web.SessionState;
protected void Page_Load(object sender, EventArgs e)
{
    if (Session["UserId"] == null) // Is Shared SessionState Available?
    {
        string sessionId = Request.QueryString["sid"];
        // The SessionId From Source Website.

        SessionIDManager manager = new SessionIDManager();
        bool isRedirected; // Get true if cookieless=true and you redirected to the current page.
        bool isNewSession; // Get true if a new "ASP.NET_SessionId" cookie will be created.
        manager.SaveSessionID(Context, sessionId, out isRedirected, out isNewSession);
        // Change sessionId in httpResponse of the current Context

        Session["Temp"] = "Some useless data.";
        // this operation is neccesary because sessionId read from Request and write to Response

        Response.Redirect(Request.Path, true);
        // Now redirect to current page to check if shared SessionState is available
    }
    else
    {
        // Do something after sharing the sessionStates
    }
}

History

Version 1.0: Share session states in sub domains of current domain.
Version 1.1: Share session states in different domains.

Sharing ASP.NET Web Application's Session State