VBA Open workbook and assign it to variable error - vba

Question

I'm writing macro that goes through the file and marks lines that match some conditions as OK. Everything works fine when launched from the file. The problem is that when I try to launch it from another workbook I keep getting this error "Run time error 1004: Select method or Range class failed" at line
Range("C2").Select
I think that the problem lies in assigning opened file to the variable mainFile. (I can browse for the file, it opens and I can enter the year I want then it crashes)
Could you tell me what I'm doing wrong?
Sub sbVBA_To_Open_Workbook_FileDialog()
Dim strFileToOpen As Variant
Dim mainFile As Workbook
strFileToOpen = Application.GetOpenFilename("Excel Files (*.xls*), *.xls*", MultiSelect:=False)
If TypeName(strFileToOpen) = "String" Then
Set mainFile = Workbooks.Open(strFileToOpen)
Else
MsgBox "No file selected."
Exit Sub
End If
'sub data works fine
Call data
With mainFile
'Everything below works fine when launched in the mainfile
Dim myYear As Date
myYear = InputBox("Choose year", "Choose year", 2018)
Range("C2").Select
Do Until ActiveCell.Value = ""
If ActiveCell.Offset(0, 2) = "M" Then
If ActiveCell.Offset(1, 2) = "C" Then
If Day(ActiveCell.Value) = 1 Then
If Year(ActiveCell.Value) = myYear Then
If Month(ActiveCell.Value) & Year(ActiveCell.Value) = Month(ActiveCell.Offset(1, 0).Value) & Year(ActiveCell.Offset(1, 0).Value) Then
'^Comparing years and months
If DateSerial(Year(ActiveCell.Offset(1, 0).Value), Month(ActiveCell.Offset(1, 0).Value + 1), 0) = ActiveCell.Offset(1, 0).Value Then
' ^End of month?
ActiveCell.Offset(0, 10).Value = "OK"
Else
ActiveCell.Offset(0, 10).Value = "NOK"
End If
Else
ActiveCell.Offset(0, 10).Value = "NOK"
End If
Else
ActiveCell.Offset(0, 10).Value = "NOK"
End If
Else
ActiveCell.Offset(0, 10).Value = "NOK"
End If
Else
ActiveCell.Offset(0, 10).Value = "NOK"
End If
Else
ActiveCell.Offset(0, 10).Value = "NOK"
End If
ActiveCell.Offset(1, 0).Select
Loop
End With
End Sub

Answer 1

Just because you open a workbook, it won’t automatically assume it’s the active workbook.
Please try the following code after the “With mainFile” row:
mainFile.Sheets([your desired sheet]).Activate
Then, before the range.select part, add activesheet:
ActiveSheet.Range(“C2”).Select