How to update inventory lookup results grid

@albertobello83I gave it another try. My version is 21R2 21.205. Please see the attached clip (change the extension from pdf to mkv) also I made the below changes to your code.

// Replaced your DAC with:
using System;
using PX.Data;
using PX.Data.ReferentialIntegrity.Attributes;
using PX.Objects.CS;
using PX.Objects.IN;

namespace PX.Objects.SO
{
    public class USRCBItemClassEntityType : PX.Data.BQL.BqlString.Constant<USRCBItemClassEntityType>
    {
        public USRCBItemClassEntityType() : base("PX.Objects.IN.InventoryItem") { }
    }

    public class USRCBAttributeCategory : PX.Data.BQL.BqlString.Constant<USRCBAttributeCategory>
    {
        public USRCBAttributeCategory() : base("A") { }
    }


    [Serializable]
    [PXCacheName("Item Search Attributes")]
    [PXProjection(typeof(Select2<CSAttributeGroup,
                            InnerJoin<INItemClass, On<CSAttributeGroup.entityClassID, Equal<INItemClass.itemClassID>>>,
                            Where<CSAttributeGroup.entityType, Equal<USRCBItemClassEntityType>,
                                And<CSAttributeGroup.attributeCategory, Equal<USRCBAttributeCategory>,
                                    And<CSAttributeGroup.isActive, Equal<True>>>>>))]
    public partial class USRCBItemSearchAttributes : PX.Data.IBqlTable
    {
        #region Keys
        public class PK : PrimaryKeyOf<USRCBItemSearchAttributes>.By<attributeID, entityClassID, entityType>
        {
            public static USRCBItemSearchAttributes Find(PXGraph graph, string attributeID, string entityClassID, string entityType) => FindBy(graph, attributeID, entityClassID, entityType);
        }
        #endregion


        #region AttributeID
        public abstract class attributeID : PX.Data.BQL.BqlString.Field<attributeID> { }
        protected string _AttributeID;
        [PXDBString(10, IsKey = true, IsUnicode = true, InputMask = ">aaaaaaaaaa", BqlField = typeof(CSAttributeGroup.attributeID))]
        [PXUIField(DisplayName = "Attribute", Visibility = PXUIVisibility.SelectorVisible)]
        [CSAttributeSelector]
        public virtual string AttributeID
        {
            get { return this._AttributeID; }
            set { this._AttributeID = value; }
        }
        #endregion

        #region EntityClassID
        public abstract class entityClassID : PX.Data.BQL.BqlString.Field<entityClassID> { }
        protected string _EntityClassID;
        [PXDBString(30, IsKey = true, IsUnicode = true, BqlField = typeof(INItemClass.itemClassCD))]
        [PXUIField(DisplayName = "Entity Class", Visibility = PXUIVisibility.SelectorVisible)]
        [PXParent(typeof(Select<SOSiteStatusFilter,
            Where<SOSiteStatusFilter.itemClass, Equal<Current<USRCBItemSearchAttributes.entityClassID>>>>))]
        public virtual string EntityClassID
        {
            get { return this._EntityClassID; }
            set { this._EntityClassID = value; }
        }
        #endregion

        #region EntityType
        public abstract class entityType : PX.Data.BQL.BqlString.Field<entityType> { }
        protected string _EntityType;
        [PXDBString(200, IsKey = true, BqlField = typeof(CSAttributeGroup.entityType))]
        [PXUIField(DisplayName = "Type")]
        public virtual string EntityType
        {
            get { return this._EntityType; }
            set { this._EntityType = value; }
        }
        #endregion

        #region AttValue
        public abstract class attValue : PX.Data.BQL.BqlString.Field<attValue> { }
        protected string _AttValue;
        [PXDBString(255, IsUnicode = true, BqlField = typeof(CSAttributeGroup.defaultValue))]
        [PXUIField(DisplayName = "Value")]
        public virtual string AttValue
        {
            get { return this._AttValue; }
            set { this._AttValue = value; }
        }
        #endregion
    }
}


// Removed the below Event Handler
        protected virtual void _(Events.FieldUpdated<SOSiteStatusFilter, SOSiteStatusFilter.itemClass> e)



// Replaced your Attributes view with

        public PXSelect<USRCBItemSearchAttributes,
            Where<USRCBItemSearchAttributes.entityClassID, Equal<Current<SOSiteStatusFilter.itemClass>>>> Attributes;

I am not getting errors now but I guess you will need to look into your search method one more time. I would suggest to re-write the ItemResults view using a view delegate instead of using event handlers. I am not sure but you might know you can generate a value list and use it in the IN operator something like

Object[] values = new String[] { "Att1", "Att2" };
InventoryItem item = PXSelect<InventoryItem,
       Where<InventoryItem.itemStatus,
In<Required<InventoryItem.itemStatus>>>>.Select(Base, values);