为什么它显示System.NullReferenceException： 'Object reference not set to an instance of an object.'

Question

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
workbook = APP.Workbooks.Open("C:Usersact08Copy of Copy of test1.xlsx")
worksheet = workbook.Sheets.Item(1)
'each textbox input to excel
worksheet.Cells(1, 1).Value = TextBox1.Text
worksheet.Cells(1, 2).Value = TextBox2.Text
worksheet.Cells(2, 1).Value = TextBox3.Text
'save the input
workbook.Close(SaveChanges:=True)
APP.Quit()
End Sub

Answer 1

如果不进行强制转换，变量最终将成为Object，该Object没有Excel对象所需的属性和方法。

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Dim APP As New Excel.Application
Dim workbook = DirectCast(APP.Workbooks.Open("C:Usersact08Copy of Copy of test1.xlsx"), Excel.Workbook)
Dim worksheet = DirectCast(workbook.Sheets.Item(1), Excel.Worksheet)
'each textbox input to excel
Dim cell = DirectCast(worksheet.Cells(1, 1), Excel.Range)
cell.Value = TextBox1.Text
Dim cell2 = DirectCast(worksheet.Cells(1, 2), Excel.Range)
cell2.Value = TextBox2.Text
Dim cell3 = DirectCast(worksheet.Cells(2, 1), Excel.Range)
cell3.Value = TextBox3.Text
'save the input
workbook.Close(SaveChanges:=True)
APP.Quit()
End Sub