如果图层存在，则：

Private Sub addlayer()
' This example creates a new layer called "New_Layer"
    Dim layerObj As AcadLayer
    
    ' Add the layer to the layers collection
    Set layerObj = ThisDrawing.Layers.Add("New_Layer")
    
    ' Make the new layer the active layer for the drawing
    ThisDrawing.ActiveLayer = layerObj
    
    ' Display the status of the new layer
     MsgBox layerObj.Name & " has been added." & vbCrLf & _
            "LayerOn Status: " & layerObj.LayerOn & vbCrLf & _
            "Freeze Status: " & layerObj.Freeze & vbCrLf & _
            "Lock Status: " & layerObj.Lock & vbCrLf & _
            "Color: " & layerObj.color, , "Add Example"
End Sub

Option Explicit
Public Sub test()
    Dim bResult        As Boolean
    bResult = DoesLayerExist("XXX_001")
    If bResult Then
        MsgBox "Layer XXX_001 exists"
    Else
        MsgBox "Not Today Mark"
    End If
End Sub
Public Function DoesLayerExist(sLayer As String) As Boolean
    Dim oLayers        As AcadLayers
    Dim oLayer         As AcadLayer
    On Error GoTo Skippy
    Set oLayers = ThisDrawing.Layers
    Set oLayer = oLayers.Item(sLayer)
    DoesLayerExist = True
    Exit Function
Skippy:
    
End Function

Public Sub test_01()
    Dim bResult        As Boolean
    Dim svLayerList(0 To 9) As String
    Dim cnt            As Integer
    Dim sLayName       As String
    svLayerList(0) = "Layer0"
    svLayerList(1) = "0"
    svLayerList(2) = "Layer2"
    svLayerList(3) = "NoWay"
    svLayerList(4) = "Layer4"
    svLayerList(5) = "Layer5"
    svLayerList(6) = "Layer6"
    svLayerList(7) = "Layer7"
    svLayerList(8) = "Layer8"
    svLayerList(9) = "Bogus"
    For cnt = 0 To UBound(svLayerList)
        sLayName = svLayerList(cnt)
        bResult = DoesLayerExist(sLayName)
        
        If bResult Then
            MsgBox "Layer : " & sLayName & vbCrLf & " exists"
        Else
            MsgBox "Layer : " & sLayName & vbCrLf & " does NOT exist"
        End If
    Next
End Sub
Public Function DoesLayerExist(sLayer As String) As Boolean
    Dim oLayers        As AcadLayers
    Dim oLayer         As AcadLayer
    
    On Error Resume Next
    Set oLayers = ThisDrawing.Layers
    Set oLayer = oLayers.Item(sLayer)
    DoesLayerExist = (Err.Number = 0)
    Err.Clear
End Function