To clarify my question this is the library property helper class Library property helper when you look at the documentation for computed from documentation here Computed documentation

ClassMethod FullNameComputation(cols As %Library.PropertyHelper) As %String
{
    return cols.getfield("FirstName")_" "_cols.getfield("LastName")
}
// ...

But you can't seem to throw a parameter i.e.par= "Firstname" cols.getfield(par) in the code without it throwing errors

I think the setstream wipes out the originalFilename field but the other way around results in zero bytes

Class User.Utility.Tasks.PurgeCustomMessages Extends %SYS.Task.Definition
{

Parameter TaskName = "PurgeCustomMessages";

Property MaxDelete As %String [ InitialExpression = 1 ];

Property MinBodyID As %String [ InitialExpression = 1 ];

Method OnTask() As %Status
{
    //Get The Types of message classes
    Set sc = $$$OK
    s tMessagesToOrpahan=0
    //open up table of messages types to purge 
    try{
        Set rs = ##class(%SQL.Statement).%ExecDirect(,"select Keyname as MessageBodyClassName,Datavalue as ObjectSQLQryName from Ens_Util.LookupTable where tablename='OrpahnedMessagePurging'")
    }
    catch err {
       Set sc = err.AsStatus() 
       //quit st
    }
    
    While rs.%Next() 
    {
        set msgBodyName=rs.ObjectSQLQryName
        set className=rs.MessageBodyClassName
        if msgBodyName="Ens.MessageBody"{
            set tMinMsgId=..MinBodyID

        }else{
        set tMinMsgId=..GetMinimumIDForMessage(rs.MessageBodyClassName)
        }
        Set tSuccess=0
        Set tFailed=0  
        set dt=$translate($zdatetime($horolog,8,1), " :", "")
        if tMinMsgId '=-1
        {
            set msgQuery="SELECT top ? ID from "_msgBodyName_" where id <?"
            set tStatement = ##class(%SQL.Statement).%New()
            set qStatus = tStatement.%Prepare(.msgQuery)
            if qStatus '= 1 {
                s ^OrphanedMsgTask(dt,msgBodyName,"Error") ="Prepare Failed"
                s sc= $System.Status.DisplayError(qStatus) quit}
            
            set tResult = tStatement.%Execute(..MaxDelete,tMinMsgId)
           
           While tResult.%Next() 
           {
                s tMessagesToOrpahan=1

                            try{
                                SET tSC1=$CLASSMETHOD(className,"%DeleteId",tResult.ID)
                            
                            if tSC1=$$$OK{
                                set tSuccess=tSuccess+1

                            }
                            else{
                                set tFailed=tFailed+1
                                s ^OrphanedMsgTaskFailed(className,tResult.ID)=tSC1
                            }
                            }
                            catch 
                            {
                                set tFailed=tFailed+1
                                set sc = $$$ERROR($$$GeneralError,"DeleteID Failure"_tResult.ID)
                                s ^OrphanedMsgTask(dt,msgBodyName,"Error") ="DeleteID Failure"
                            }
           }
        }
        else
        {
            s ^OrphanedMsgTask(dt,msgBodyName,"Status")="No messages in header table for "_msgBodyName
        }
        
      s ^OrphanedMsgTask(dt,msgBodyName,"Status")="Purged for "_msgBodyName  _" "_tSuccess_" messages and failed to purge " _tFailed _ " messages"   
    }
   
    quit sc
}

/// Method to return the minimum ID for the message type you are purging 
ClassMethod GetMinimumIDForMessage(pMessageType As %String) As %String
{
    set MessageID=-1
    set minIDQuery="SELECT  TOP(1) MessageBodyID  FROM Ens.MessageHeader where MessageBodyClassName=?"
    set tStatement = ##class(%SQL.Statement).%New()
    set qStatus = tStatement.%Prepare(.minIDQuery)
    if qStatus '= 1 {write "%Prepare failed:" do $System.Status.DisplayError(qStatus) quit}
    set tResult = tStatement.%Execute(pMessageType)
    While tResult.%Next() 
    {
    set MessageID=tResult.MessageBodyID
    Write tResult.MessageBodyID, !
    }
    w MessageID
    return MessageID
}

ClassMethod StreamPurge() As %Status
{
    set i=1
    while (i<200000){
        k ^CacheStream(i)
        s i=i+1
    }
    q $$$OK
}

ClassMethod Test() As %String
{

    set msgQuery="SELECT top ? ID from EnsLib_HL7.Message where id <?"
            set tStatement = ##class(%SQL.Statement).%New()
            set qStatus = tStatement.%Prepare(.msgQuery)
            s ^OrphanedMsgTask = qStatus
            if qStatus '= 1 {write "%Prepare failed:" do $System.Status.DisplayError(qStatus) quit}
            s ^OrphanedMsgTask =tStatement
            set tResult = tStatement.%Execute(1,..GetMinimumIDForMessage("EnsLib.HL7.Message"))
            While tResult.%Next() 
            {
                w tResult.ID
            }
            quit qStatus
}

/// Classmethod to create a list of the message types 
ClassMethod MessageTypes() As %ListOfDataTypes
{
    Set sc = $$$OK
    Set rs = ##class(%SQL.Statement).%ExecDirect(,"SELECT DISTINCT (MessageBodyClassName) AS messageType FROM Ens.MessageHeader where MessageBodyClassName Like 'messages%'")
    set messageTypeList=##class(%ListOfDataTypes).%New()
    While rs.%Next() {
        do messageTypeList.Insert(rs.messageType)
    }
    w messageTypeList.Count()
    for i = 1:1:messageTypeList.Count() {
        write !, messageTypeList.GetAt(i)}
}

No idea how to upload a file so here is the class