NULL reference exception while reading user sessions (Reflection)

Question

I have implemented the code for reading the active sessions using the reference Reading All Users Session and Get a list of all active sessions in ASP.NET.

P         


        

Answer 1

Try using this, _cachesRefs if _caches is NULL. The function below will return all user sessions collections for all multiple versions of Windows and including Windows Server.

It works.

public List<SessionStateItemCollection> GetAllUserSessions() {

List<Hashtable> hTables = new List<Hashtable>();

PropertyInfo propInfo = typeof(HttpRuntime).GetProperty("CacheInternal", BindingFlags.NonPublic | BindingFlags.Static);

object CacheInternal = propInfo.GetValue(null, null);

dynamic fieldInfo = CacheInternal.GetType().GetField("_caches", BindingFlags.NonPublic | BindingFlags.Instance);

if (fieldInfo != null) {
    object[] _caches = (object[])fieldInfo.GetValue(CacheInternal);
    for (int i = 0; i <= _caches.Length - 1; i++) {
        Hashtable hTable = (Hashtable)_caches(i).GetType().GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(_caches(i));
        hTables.Add(hTable);
    }
} else {
    fieldInfo = CacheInternal.GetType().GetField("_cachesRefs", BindingFlags.NonPublic | BindingFlags.Instance);
    dynamic cacheRefs = fieldInfo.GetValue(CacheInternal);
    foreach (void cacheRef_loopVariable in cacheRefs) {
        cacheRef = cacheRef_loopVariable;
        dynamic target = cacheRef.Target;
        fieldInfo = target.GetType().GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance);
        Hashtable hTable = fieldInfo.GetValue(target);
        hTables.Add(hTable);
    }
}

List<SessionStateItemCollection> sessionlist = new List<SessionStateItemCollection>();

foreach (void hTable_loopVariable in hTables) {
    hTable = hTable_loopVariable;
    foreach (DictionaryEntry entry in hTable) {
        object value = entry.Value.GetType().GetProperty("Value", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(entry.Value, null);
        if (value.GetType().ToString() == "System.Web.SessionState.InProcSessionState") {
            SessionStateItemCollection sCollection = (SessionStateItemCollection)value.GetType().GetField("_sessionItems", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(value);
            if (sCollection != null)
                sessionlist.Add(sCollection);
        }
    }
}

return sessionlist;

}

Answer 2

Exactly for your business case there is Application state variable in asp.net. It is similar to session state, but visible for all users request.

Answer 3

It sounds like a different version (or update) of .NET is running on 2003 than you have on XP / Win7, although it could also just be a platform-specific difference. If it was permissions / trust, you would have seen an exception. Instead, it seems more likely that simply: _caches does not exist on whatever version is on the 2003 machine. If you use reflection to access private state: you should entirely expect it to explode between versions / updates / platforms / at-whim / etc.

To investigate:

• check whether obj is null
• check whether obj.GetType().GetField("_caches", BindingFlags.NonPublic | BindingFlags.Instance) is null

(either of those things could cause this exception on the line you cite)

Answer 4

I've tried Paully's solution, which didn't compile in some points and lead to runtime errors in others. Anyway, inspired on his suggestion (thanks a lot! My vote goes for that), I came to my own, which compiles and gets me the expected data.

Also, I'm returning a IEnumerable and I'm using "yield return", which makes it more performatic for big lists (kind of lazy loading of data). Here it goes:

public static System.Collections.Generic.IEnumerable<SessionStateItemCollection> GetAllUserSessions()
{
    List<Hashtable> hTables = new List<Hashtable>();
    object obj = typeof(HttpRuntime).GetProperty("CacheInternal", BindingFlags.NonPublic | BindingFlags.Static).GetValue(null, null);
    dynamic fieldInfo = obj.GetType().GetField("_caches", BindingFlags.NonPublic | BindingFlags.Instance);

    //If server uses "_caches" to store session info
    if (fieldInfo != null)
    {
        object[] _caches = (object[])fieldInfo.GetValue(obj);
        for (int i = 0; i <= _caches.Length - 1; i++)
        {
            Hashtable hTable = (Hashtable)_caches[i].GetType().GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(_caches[i]);
            hTables.Add(hTable);
        }
    }
    //If server uses "_cachesRefs" to store session info
    else
    {
        fieldInfo = obj.GetType().GetField("_cachesRefs", BindingFlags.NonPublic | BindingFlags.Instance);
        object[] cacheRefs = fieldInfo.GetValue(obj);
        for (int i = 0; i <= cacheRefs.Length - 1; i++)
        {
            var target = cacheRefs[i].GetType().GetProperty("Target").GetValue(cacheRefs[i], null);
            Hashtable hTable = (Hashtable)target.GetType().GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(target);
            hTables.Add(hTable);
        }
    }

    foreach (Hashtable hTable in hTables)
    {
        foreach (DictionaryEntry entry in hTable)
        {
            object o1 = entry.Value.GetType().GetProperty("Value", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(entry.Value, null);
            if (o1.GetType().ToString() == "System.Web.SessionState.InProcSessionState")
            {
                SessionStateItemCollection sess = (SessionStateItemCollection)o1.GetType().GetField("_sessionItems", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(o1);
                if (sess != null)
                     yield return sess;
            }
        }
    }
}

Answer 5

As of Feb. 2010, testing on IIS 10.0 (Windows 10), the above solutions didn't quite work, so I hacked together the following which worked for me. It builds on awerdan's answer and incorporates Diogo Damiani's.

    // attempt to get Asp.Net internal cache
    // adapted from https://stackoverflow.com/a/46554310/1086134
    private static object getAspNetInternalCacheObj ()
    {
        object aspNetCacheInternal = null;

        PropertyInfo cacheInternalPropInfo = typeof(HttpRuntime).GetProperty("CacheInternal", BindingFlags.NonPublic | BindingFlags.Static);
        if (cacheInternalPropInfo != null)
        {
            aspNetCacheInternal = cacheInternalPropInfo.GetValue(null, null);
            return aspNetCacheInternal;
        }

        // At some point, after some .NET Framework's security update, that internal member disappeared.
        // https://stackoverflow.com/a/45045160
        // 
        // We need to look for internal cache otherwise.
        //
        var cacheInternalFieldInfo = HttpRuntime.Cache.GetType().GetField("_internalCache", BindingFlags.NonPublic | BindingFlags.Static);
        if (cacheInternalFieldInfo == null)
            return null;

        var httpRuntimeInternalCache = cacheInternalFieldInfo.GetValue(HttpRuntime.Cache);
        var httpRuntimeInternalCacheField = httpRuntimeInternalCache.GetType().GetField("_cacheInternal", BindingFlags.NonPublic | BindingFlags.Instance);
        if (httpRuntimeInternalCacheField == null)
            return null;

        aspNetCacheInternal = httpRuntimeInternalCacheField.GetValue(httpRuntimeInternalCache);
        return aspNetCacheInternal;
    }

    // adapted from https://stackoverflow.com/a/39422431/1086134
    private static IEnumerable<System.Web.SessionState.SessionStateItemCollection> getAllUserSessions()
    {
        List<Hashtable> hTables = new List<Hashtable>();
        object obj = getAspNetInternalCacheObj();
        if (obj == null)
            yield break;

        dynamic fieldInfo = obj.GetType().GetField("_caches", BindingFlags.NonPublic | BindingFlags.Instance);
        //If server uses "_caches" to store session info
        if (fieldInfo != null)
        {
            object[] _caches = (object[])fieldInfo.GetValue(obj);
            for (int i = 0; i <= _caches.Length - 1; i++)
            {
                Hashtable hTable = (Hashtable)_caches[i].GetType().GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(_caches[i]);
                hTables.Add(hTable);
            }
        }
        //If server uses "_cachesRefs" to store session info
        else
        {
            fieldInfo = obj.GetType().GetField("_cachesRefs", BindingFlags.NonPublic | BindingFlags.Instance);
            object[] cacheRefs = fieldInfo.GetValue(obj);
            for (int i = 0; i <= cacheRefs.Length - 1; i++)
            {
                var target = cacheRefs[i].GetType().GetProperty("Target").GetValue(cacheRefs[i], null);
                Hashtable hTable = (Hashtable)target.GetType().GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(target);
                hTables.Add(hTable);
            }
        }

        foreach (Hashtable hTable in hTables)
        {
            foreach (DictionaryEntry entry in hTable)
            {
                object o1 = entry.Value.GetType().GetProperty("Value", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(entry.Value, null);
                if (o1.GetType().ToString() == "System.Web.SessionState.InProcSessionState")
                {
                    System.Web.SessionState.SessionStateItemCollection sess = (System.Web.SessionState.SessionStateItemCollection)o1.GetType().GetField("_sessionItems", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(o1);
                    if (sess != null)
                        yield return sess;
                }
            }
        }
    }

Answer 6

I know this is an old thread, but this may save someone some time. Another thing to check is that obj is of type System.Web.Caching.CacheMultiple. I had this same problem and it was a platform-specific issue as Marc Gravell suggested. It turned out that on the Windows 2003 server, obj was type System.Web.Caching.CacheSingle and there was a null reference exception when trying to get the value for "_caches".

If that is the case, you can still get a list of active sessions with (Hashtable)obj.GetType().GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(obj);