Dynamic Member List

I'd advise the TEXT8 property as you can incorporate this into your metadata management processes. As well, you'll have audit history on changes to the property.  That's how we manage the majority of our dependent lists. We use the full entity names rather than a shorthand/trimmed version, but I would think the trimmed version would work as well.

Your member filter should be:

U1#CU003900.Base.Where(Text8 Contains |CVEntity|)

or if using your custom function

U1#CU003900.Base.Where(Text8 Contains XFBR(...))

Your solution kinda works, but tbh I think you need to get acquainted with the wonderful world of Custom Member Lists. Once you understand how they work, they put you in total control. There is an introduction at the beginning of this blog post, although the post goes on to show an advanced usage; so here's a version that I believe covers your needs. It will have to be implemented in a Business Rule of type Finance; you don't need to attach it to the cube.

I added some comments that hopefully clarify what each call is doing.

Case Is = FinanceFunctionType.MemberListHeaders
	' technically this bit is optional but best practice. 
	' See the blog for explanation.
	Dim headers As New List(Of MemberListHeader)
	headers.Add(New MemberListHeader("ByEntityPrefix"))
	Return headers
	
Case Is = FinanceFunctionType.MemberList
	' Example usage: 
	'	U8#MyTopMember.CustomMemberList(BRName=MyBR, MemberListName=ByEntityPrefix, Entity=ES_123)
	If args.MemberListArgs.MemberListName.XFEqualsIgnoreCase("ByEntityPrefix") Then
		' get the dimension type we're working with
		Dim dimTypeCode As String = dimtype.GetItem(args.MemberListArgs.DimPk.DimTypeId).Abbrev
		' get the prefix from passed parameter
		Dim param As String = args.MemberListArgs.NameValuePairs.XFGetValue("Entity")
		Dim prefix As String = param.Substring(0,2).ToUpper
		' retrieve members
		' Note in the filter we use the member that "started" the list, i.e.
		' in U8#MyTop.CustomMemberList(...) it would be "MyTop".
		' Note also we use StartsWith, which should be faster than Contains
		Dim memberInfos As List(Of MemberInfo) = api.Members.GetMembersUsingFilter( _
			args.MemberListArgs.DimPk,  _
			$"{dimTypeCode}#{args.MemberListArgs.TopMember.Name}.Base.Where(Text8 StartsWith {prefix}) ",  _
			Nothing)
		' MemberList object needs a header containing the list name, 
		' so let's create it 
		Dim header As New MemberListHeader(args.MemberListArgs.MemberListName)
		' put it all together and return the list
		Dim theList As New MemberList(header, memberInfos)
		Return theList
	End If
					

You would then use it like this in CV rows or columns:

U8#YourTopMember.CustomMemberList(BRName=YourBRName, MemberListName=ByEntityPrefix, Entity=[|!YourEntityParam!|])

Note that, the way we've done it, this list can be applied to any dimension, not just U8. You could customize it further to make the prefix length dynamic, i.e. passing another parameter to the list and using that in the Substring call.

Let me know if there is any question, hopefully this opens a new world of possibilities 😊

JackLacava  Thanks for this solution. But still i have got the same problem :

I know how to use this functionality in Rows or Columns. I was using more primitive way: 

U1#CU003900.Base.Where(Name Contains XFBR(CKA_ParamHelper, returnLeft2, Ent=|!ParamFormPromptEntity!|))

CKA_ParamHelper: 
If args.FunctionName.XFEqualsIgnoreCase("returnLeft2") Then
Dim EntName As String = args.NameValuePairs.XFGetValue ("Ent")
Return EntName.Substring(0,2)
End If

But my problem is different. Both dimensions Entity and U1# i am using in Parameters. if i am using in U1# parameter of entity then i am getting this error as attached above. And this is my problem. If there is a way where i can use a parameter for U1# which will be looking at entity which were selected.