I am building an application in one workbook that requires another
workbook be open. Here is the code I am using:

Private Sub OB1_Click()
Workbooks.Open Filename:="C:\Data1.xlsx"
UF1.Hide
End Sub

This does the job, but what is the code that determines if this
workbook "C:\Data1.xlsx" is already open so it doesn't try and re-open
it?

Thank you,

Rob