如果在数据库中找不到实体，如何防止出现错误

Question

我制作了一个抽认卡应用程序，用户可以在其中编辑每个抽认卡的难度。

Private Sub btnHard_Click(sender As Object, e As EventArgs) Handles btnHard.Click
Dim sqlstring As String = "select * from flashcards where Difficulty = 3" 'Select from flashcard table where difficulty = 3
dataadapter = New OleDb.OleDbDataAdapter(sqlstring, connection)
dt.Clear() 'Clears datatable
dataadapter.Fill(dt) 'Fills datatable
Dim index = rand.Next(dt.Rows.Count) ' generates index in the range 0 .. Count - 1
If txtBack.Visible = True Then
txtFront.Text = dt.Rows(index)(2).ToString()
txtBack.Visible = False
txtBack.Text = dt.Rows(index)(3).ToString()
Else
MsgBox("Please first reveal the back of the flashcard")
End If
End Sub

该按钮选择难度等于3的所有抽认卡，但如果没有记录，系统会产生错误。那么，如果没有这样困难的记录，我该如何获得它，以便系统生成消息呢？

Answer 1

Private Sub btnHard_Click(sender As Object, e As EventArgs) Handles btnHard.Click
Dim sqlstring As String = "select * from flashcards where Difficulty = 3" 'Select from flashcard table where difficulty = 3
dataadapter = New OleDb.OleDbDataAdapter(sqlstring, connection)
dt.Clear() 'Clears datatable
dataadapter.Fill(dt) 'Fills datatable
If dt.Rows.Count = 0 Then 'If record is not found in the database
MsgBox("There are no more flashcards inside this deck")
Exit Sub 'Code continus running if record is found
End If
Dim index = rand.Next(dt.Rows.Count) ' generates index in the range 0 .. Count - 1
If txtBack.Visible = True Then 'If the back of the flashcard is shown
txtFront.Text = dt.Rows(index)(2).ToString() 'Displays a random record in the third column (front of flashcard)
txtBack.Visible = False 'Does not display the back of the flashcard
txtBack.Text = dt.Rows(index)(3).ToString() 'Displays a random record in the fourth column ()
Else 'If the user has not pressed the reveal button before
MsgBox("Please first reveal the back of the flashcard")
End If
End Sub