Visual Basic

Set xlBook = xl.Workbooks.Open(FileName:=ExcelFile, _
Password:=ExcelPassword)

Set xlBook = xl.Workbooks.Open(FileName:=ExcelFile, _
Password:=ExcelPassword)

Sub Button1_Click()

Dim xl As New Excel.Application
Dim xlsheet As Excel.Worksheet
Dim xlwbook As Excel.Workbook

Dim ExcelFile As String
Dim ExcelPassword As String
Dim i As Integer

'Set Filename
ExcelFile = "C:\Users\Dave Auld\Desktop\password_test.xlsx"

'Create Excel Application
Set xl = CreateObject("Excel.application")
xl.Visible = False

On Error Resume Next

For i = 1 To 6
 ExcelPassword = "ABC" + CStr(i)
 
 Set xlBook = xl.Workbooks.Open(Filename:=ExcelFile, Password:=ExcelPassword)

    If Err.Number > 0 Then
        MsgBox "Error: " + Err.Number + ": " + Err.Description, , "Error"
        'Reset the error for next loop
        Err = 0
    Else
        'No Error - File Opened okay
        MsgBox "File Opened with Password " + ExcelPassword
        'Stop Seraching
        Exit For
    End If
 Next i
 'Quit Excel
    xl.Quit

End Sub

Imports Excel = Microsoft.Office.Interop.Excel

Public Class Form1
    Dim xl As New Excel.Application
    Dim xlwbook As Excel.Workbook

    Dim ExcelFile As String
    Dim ExcelPassword As String
    Dim ExcelNword As String

    Private Sub btnStart_Click() Handles btnStart.Click

        xl.Visible = False
        ExcelFile = "D:\" + "text.xls"

        For i = 1 To 6
            Try
                ExcelNword = CStr(i)
                ExcelPassword = "ABC" + ExcelNword
                xlwbook = xl.Workbooks.Open(Filename:=ExcelFile, Password:=ExcelPassword)
                MsgBox(String.Format("Workbook could be opened with password {0}", ExcelPassword))

            Catch ex As Exception
                If Err.Number = 1004 Then
                    MsgBox(String.Format("'{0}' is the wrong password!", ExcelPassword), , "Error")
                Else
                    MsgBox(String.Format("Fehler {0}: {1}", Err.Number.ToString, Err.Description), , "Error")
                End If
            End Try
        Next i

        xl.Quit()
        xlwbook = Nothing
        xl = Nothing

    End Sub
End Class

ExcelFile = "D:\Users\Test\Desktop\test" & i &".xls"