Set Cell Formula in ExcelDNA UDF

I am working on a UDF(User defined function) for Excel user, the task seems easy with the help of ExcelDNA. But While I test it in a cell of Excel with following two formula, both show #VALUE!. Need help to get this solved, thanks.

=mySetCellFormula("Test", "")
=mySetCellFormula("Test", "A1")

Imports System.Net
Imports System.Drawing
Imports System.Windows.Forms
Imports Microsoft.Office.Core
Imports Excel = Microsoft.Office.Interop.Excel
Imports ExcelDna.Integration
Imports ExcelDna.Integration.XlCall

Public Module MyFunctions
    Public Function mySetCellFormula(ByVal sFormuaR1C1 As String, ByVal cellAddress As String) As String
        ' Get the correct application instance
        Dim xlApp As Excel.Application
        Dim xlSheet As Excel.Worksheet
        Dim xlCell As Excel.Range
        xlApp = CType(ExcelDnaUtil.Application, Excel.Application)
        xlSheet = CType(xlApp.ActiveSheet(), Excel.Worksheet)
        If cellAddress = "" Then
            xlCell = xlApp.ActiveCell()
        Else
            xlCell = xlSheet.Range(cellAddress)
        End If
        'xlCell.FormulaR1C1 = "=" & Chr(34) & sFormuaR1C1 & Chr(34)
        xlCell.FormulaR1C1 = "=" & sFormuaR1C1
        mySetCellFormula = ""
    End Function
End Module

Thank Govert for enlightening me on Excel Calculation Model and Macro concept, finally I work out a solution as follow:

Imports System.Net
Imports System.Drawing
Imports System.Windows.Forms
Imports Microsoft.Office.Core
Imports Excel = Microsoft.Office.Interop.Excel
Imports ExcelDna.Integration
Imports ExcelDna.Integration.XlCall

Public Module MyFunctions
    Public Function mySetCellFormula(ByVal sFormuaR1C1 As String, ByVal cellAddress As String) As String
        ' Get the correct application instance
        Dim xlApp As Excel.Application
        Dim xlSheet As Excel.Worksheet
        Dim xlCell As Excel.Range
        xlApp = CType(ExcelDnaUtil.Application, Excel.Application)
        xlSheet = CType(xlApp.ActiveSheet(), Excel.Worksheet)
        If cellAddress = "" Then
            xlCell = xlApp.ActiveCell()
        Else
            xlCell = xlSheet.Range(cellAddress)
        End If

        ExcelAsyncUtil.QueueAsMacro( _
            Sub()
                xlCell.FormulaR1C1 = "=" & sFormuaR1C1
            End Sub)

        mySetCellFormula = ""
    End Function
End Module