Questions How to add an attribute column to SO Line Grid.

I am attempting to add the SQFTINBOX stock item attribute to the Sales Order Grid.  

I have declared a class PXAddAttributeColumns as below: 

  public class PXAddAtttributeColumns : CRAttributesFieldAttribute
{
    string[] _names;
    bool _IsForSelector;

    public PXAddAtttributeColumns(string[] names, Type entityType, Type entityIDField, Type classIDField, bool IsForSelector = true)
        : base(entityType, entityIDField, classIDField)
    {
        _names = names;
        _IsForSelector = IsForSelector;
    }

    public override void CacheAttached(PXCache sender)
    {
        this._IsActive = true;
        base.CacheAttached(sender);
    }

    protected override void AttributeFieldSelecting(PXCache sender, PXFieldSelectingEventArgs e, PXFieldState state, string attributeName, int idx)
    {
        if (_names.Any(attributeName.Equals))
        {
            //Out-of-box DisplayName is prefixed with "$Attributes$-" - if you need to take that off.
            state.DisplayName = (!String.IsNullOrEmpty(state.DisplayName)) ? (state.DisplayName.Replace("$Attributes$-", "")) : attributeName;
            state.Visible = true;
            //Requires AutoGenerateColumns="AppendDynamic" for PXGrid Control for dynamic Attribute columns creation
            state.Visibility = (_IsForSelector) ? PXUIVisibility.SelectorVisible : PXUIVisibility.Dynamic;
        }
        base.AttributeFieldSelecting(sender, e, state, attributeName, idx);
    }

    public override void CommandPreparing(PXCache sender, PXCommandPreparingEventArgs e)
    {
        base.CommandPreparing(sender, e);

        if (e.BqlTable == null && aggregateAttributes && sender.GetItemType().IsDefined(typeof(PXProjectionAttribute), true))
        {
            e.BqlTable = _BqlTable;
        }
    }
}

Add Extended SO Line as below:

      public class SOLineExtension : PXCacheExtension<SOLine>
{
    public abstract class itemAttributes : IBqlField { }

    [PXAddAtttributeColumns(new[] {"SQFTINBOX"},
            typeof(CSAnswerType.inventoryAnswerType),
            typeof(SOLine.inventoryID),
            typeof(InventoryItem.itemClassID), false)]
    public virtual string[] ItemAttributes { get; set; }
}

I received an error message during validation as shown below: 

Validation started.
Copying the website c:\deployment\sites\#####\Customization\#####\siterootValidation\siterootWebsite
Patching the file c:\deployment\sites\#####\Customization\#####\siterootValidation\siterootWebsite\App_RuntimeCode\PXAddAttributeColumns.cs
Patching the file c:\deployment\sites\#####\Customization\#####\siterootValidation\siterootWebsite\App_RuntimeCode\SOLine.cs
Done
Validating Binary Files
Validating Sql Scripts
Validating the website c:\deployment\sites\#####\Customization\#####\siterootValidation\siterootWebsite
IIS APPPOOL\#####
Building directory '\WebSiteValidationDomain\App_RuntimeCode\'.
\App_RuntimeCode\PXAddAttributeColumns.cs(6): error CS0246: The type or namespace name 'CRAttributesFieldAttribute' could not be found (are you missing a using directive or an assembly reference?)
\App_RuntimeCode\PXAddAttributeColumns.cs(18): error CS0115: 'PXAddAtttributeColumns.CacheAttached(PXCache)': no suitable method found to override
\App_RuntimeCode\PXAddAttributeColumns.cs(24): error CS0115: 'PXAddAtttributeColumns.AttributeFieldSelecting(PXCache, PXFieldSelectingEventArgs, PXFieldState, string, int)': no suitable method found to override
\App_RuntimeCode\PXAddAttributeColumns.cs(37): error CS0115: 'PXAddAtttributeColumns.CommandPreparing(PXCache, PXCommandPreparingEventArgs)': no suitable method found to override
\App_RuntimeCode\SOLine.cs(30): error CS0246: The type or namespace name 'PXAddAtttributeColumnsAttribute' could not be found (are you missing a using directive or an assembly reference?)
\App_RuntimeCode\SOLine.cs(30): error CS0246: The type or namespace name 'PXAddAtttributeColumns' could not be found (are you missing a using directive or an assembly reference?)
\App_RuntimeCode\SOLine.cs(31): error CS0246: The type or namespace name 'CSAnswerType' could not be found (are you missing a using directive or an assembly reference?)
\App_RuntimeCode\PXAddAttributeColumns.cs(6): error CS0246: The type or namespace name 'CRAttributesFieldAttribute' could not be found (are you missing a using directive or an assembly reference?)
Compiler time, in seconds: 3.4108918
Validation failed.

Does anyone know why this might be failing validation or where I am going wrong?  

Thank you