使用Enum eration：

Dim aa As XlDirection
aa = xlDown

如果您特别使用String变量，那么一个选项是创建一个字典，将String表示Map到内置常量。

' Requires a reference to Microsoft Scripting Runtime
Static Function Directions() As Scripting.Dictionary
    Dim d As Scripting.Dictionary
    Set d = New Scripting.Dictionary
    
    d.Add "xlUp", xlUp
    d.Add "xlDown", xlDown
    d.Add "xlToLeft", xlToLeft
    d.Add "xlToRight", xlToRight
    
    Set Directions = d
End Function

Sub Whatever()
    Dim bb As String
    bb = "xlDown"
    
    Dim d As Range, f As Range
    Set d = Range("A1")
    
    Set f = Range(d, d.End(Directions(bb)))
End Sub

这里的Static关键字真的有点多余，但是它允许Directions中的d只被创建一次，并且只要程序在运行，它就可以保持它的值。它也可以是Static d As Scripting.Dictionary（这里是Static * 语句 *，而不是关键字）。更多信息请参见docs。
或者使用Function：

Public Function Direction(ByVal d As String) As XlDirection
    Select Case d
        Case "xlUp"
            Direction = xlUp
        Case "xlDown"
            Direction = xlDown
        Case "xlToLeft"
            Direction = xlToLeft
        Case "xlToRight"
            Direction = xlToRight
    End Select
End Function