public static class RadFilterExtensions
{
    public static void LoadLegacySettings(this Telerik.Web.UI.RadFilter filter, string state)
    {
        using (System.IO.MemoryStream memoryStream = new System.IO.MemoryStream())
        {
            var bytes = System.Convert.FromBase64String(state);
            memoryStream.Write(bytes, 0, bytes.Length);
            memoryStream.Seek(0, System.IO.SeekOrigin.Begin);

            var savedState = new System.Runtime.Serialization.Formatters.Binary.BinaryFormatter().Deserialize(memoryStream);
            filter.RootGroup.Expressions.Clear();
            ((System.Web.UI.IStateManager)filter.RootGroup).LoadViewState(savedState);
            filter.RecreateControl();
        }
    }

    // Converts a single legacy (BinaryFormatter ViewState) filter state string to the current format.
    // Use this during a one-time migration - the filter control is used as a temporary vehicle
    // to deserialize the old format and re-serialize it in the new format.
    public static string ConvertLegacySettings(this Telerik.Web.UI.RadFilter filter, string legacyState)
    {
        filter.LoadLegacySettings(legacyState);
        return filter.SaveSettings();
    }

    // Converts a collection of legacy filter states keyed by any ID type (e.g. int database PK).
    // Returns a dictionary mapping each key to its converted state string, ready to be saved back.
    public static Dictionary<TKey, string> ConvertMultipleLegacySettings<TKey>(this Telerik.Web.UI.RadFilter filter, IDictionary<TKey, string> legacyStates)
    {
        var results = new Dictionary<TKey, string>(legacyStates.Count);
        foreach (var kvp in legacyStates)
        {
            results[kvp.Key] = filter.ConvertLegacySettings(kvp.Value);
        }
        return results;
    }
}

protected void btnConvertFilter_Click(object sender, EventArgs e)
{
    // Example: load legacy states from the database, keyed by their record ID.
    // Replace this with your actual data access logic.
    var legacyStatesFromDb = new Dictionary<int, string>
    {
        // { recordId, legacyFilterState },
        // { 1, "AAEAAAD..." },
        // { 2, "AAEAAAD..." },
    };

    // Convert all states from the old BinaryFormatter ViewState format to the new format.
    Dictionary<int, string> convertedStates = RadFilter1.ConvertMultipleLegacySettings(legacyStatesFromDb);

    // Persist the converted states back to the database.
    foreach (var kvp in convertedStates)
    {
        // Example: UpdateFilterStateInDb(kvp.Key, kvp.Value);
    }
}

public static class RadFilterExtensions
{
    public static void LoadLegacySettings(this Telerik.Web.UI.RadFilter filter, string state)
    {
        using (System.IO.MemoryStream memoryStream = new System.IO.MemoryStream())
        {
            var bytes = System.Convert.FromBase64String(state);
            memoryStream.Write(bytes, 0, bytes.Length);
            memoryStream.Seek(0, System.IO.SeekOrigin.Begin);

            var savedState = new System.Runtime.Serialization.Formatters.Binary.BinaryFormatter().Deserialize(memoryStream);
            filter.RootGroup.Expressions.Clear();
            ((System.Web.UI.IStateManager)filter.RootGroup).LoadViewState(savedState);
            filter.RecreateControl();
        }
    }
}

protected void btnLoadSettings_Click(object sender, EventArgs e)
{
    string savedState = "......";

    try
    {
        // Try the new secure function, will work if it's the correct base64
        RadFilter1.LoadSettings(savedState);
    }
    catch (Exception)
    {
        // Fall back to Legacy Converter
        RadFilter1.LoadLegacySettings(savedState);
    }
}