每当我尝试填充文本框时，问题总是出现"将DBNull转换为字符串无效"并且不知道该怎么办，
我使用的数据库是Microsoft Access。 我是 vb.net 新手，只是想理解表格，但无法理解 DBNull 的东西
这是我用于填充和检索的代码。 我认为问题出在这里
'POPULATE LISTVIEW
Private Sub Populate(LRN As String, FullName As String, Address As String, Parent As String, DateofBirth As String, Gender As String, studguardian As String, studaddress As String,
sy1 As String, sy2 As String, sy3 As String, sy4 As String, sy5 As String, sy6 As String, sy7 As String, sy8 As String, sy9 As String, sy10 As String, sy11 As String, sy12 As String, sy13 As String, sy14 As String, sy15 As String, sy16 As String, sy17 As String, sy18 As String, sy19 As String, sy20 As String, sy21 As String, sy22 As String, sy23 As String, sy24 As String,
s1 As String, s2 As String, s3 As String, s4 As String, s5 As String, s6 As String, s7 As String, s8 As String, s9 As String, s10 As String, s11 As String, s12 As String, s13 As String, s14 As String, s15 As String, s16 As String, s17 As String, s18 As String, s19 As String, s20 As String, s21 As String, s22 As String, s23 As String, s24 As String,
g1 As String, g2 As String, g3 As String, g4 As String, g5 As String, g6 As String, g7 As String, g8 As String, g9 As String, g10 As String, g11 As String, g12 As String, g13 As String, g14 As String, g15 As String, g16 As String, g17 As String, g18 As String, g19 As String, g20 As String, g21 As String, g22 As String, g23 As String, g24 As String,
n1 As String, n2 As String, n3 As String, n4 As String, n5 As String, n6 As String, n7 As String, n8 As String, n9 As String, n10 As String, n11 As String, n12 As String, n13 As String, n14 As String, n15 As String, n16 As String, n17 As String, n18 As String, n19 As String, n20 As String, n21 As String, n22 As String, n23 As String, n24 As String,
ga1 As String, ga2 As String, ga3 As String, ga4 As String, ga5 As String, ga6 As String, ga7 As String, ga8 As String, ga9 As String, ga10 As String, ga11 As String, ga12 As String, ga13 As String, ga14 As String, ga15 As String, ga16 As String, ga17 As String, ga18 As String, ga19 As String, ga20 As String, ga21 As String, ga22 As String, ga23 As String, ga24 As String,
r1 As String, r2 As String, r3 As String, r4 As String, r5 As String, r6 As String, r7 As String, r8 As String, r9 As String, r10 As String, r11 As String, r12 As String, r13 As String, r14 As String, r15 As String, r16 As String, r17 As String, r18 As String, r19 As String, r20 As String, r21 As String, r22 As String, r23 As String, r24 As String)
'ROW ARRAY
Dim row As String() = New String() {LRN, FullName, Address, Parent, DateofBirth, Gender, studguardian, studaddress,
sy1, sy2, sy3, sy4, sy5, sy6, sy7, sy8, sy9, sy10, sy11, sy12, sy13, sy14, sy15, sy16, sy17, sy18, sy19, sy20, sy21, sy22, sy23, sy24,
s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14, s15, s16, s17, s18, s19, s20, s21, s22, s23, s24,
g1, g2, g3, g4, g5, g6, g7, g8, g9, g10, g11, g12, g13, g14, g15, g16, g17, g18, g19, g20, g21, g22, g23, g24,
n1, n2, n3, n4, n5, n6, n7, n8, n9, n10, n11, n12, n13, n14, n15, n16, n17, n18, n19, n20, n21, n22, n23, n24,
ga1, ga2, ga3, ga4, ga5, ga6, ga7, ga8, ga9, ga10, ga11, ga12, ga13, ga14, ga15, ga16, ga17, ga18, ga19, ga20, ga21, ga22, ga23, ga24,
r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, r13, r14, r15, r16, r17, r18, r19, r20, r21, r22, r23, r24
}
Dim item As ListViewItem = New ListViewItem(row)
'ADD TO ROWS COLLECTION
ListView1.Items.Add(item)
End Sub
'RETRIEVE FROM DB
Private Sub Retrieve()
ListView1.Items.Clear()
'SQL STM
Dim sql As String = "SELECT* FROM stutb "
cmd = New OleDbCommand(sql, con)
'OPEN CON,RETRIEVE,FILL LISTVIEW
Try
con.Open()
adapter = New OleDbDataAdapter(cmd)
adapter.Fill(dt)
'LOOP THRU DT
For Each row In dt.Rows
Populate(row(1), row(2), row(3), row(4), row(5), row(6), row(7), row(8), row(9), row(10), row(11), row(12), row(13), row(14), row(15), row(16), row(17), row(18), row(19), row(20), row(21), row(22), row(23), row(24), row(25), row(26), row(27), row(28), row(29), row(30), row(31), row(32),
row(33), row(34), row(35), row(36), row(37), row(38), row(39), row(40), row(41), row(42), row(43), row(44), row(45), row(46), row(47), row(48), row(49), row(50), row(51), row(52), row(53), row(54), row(55), row(56), row(57), row(58), row(59), row(60), row(61), row(62), row(63), row(64), row(65),
row(66), row(67), row(68), row(69), row(70), row(71), row(72), row(73), row(74), row(75), row(76), row(77), row(78), row(79), row(80), row(81), row(82), row(83), row(84), row(85), row(86), row(87), row(88), row(89), row(90), row(91), row(92), row(93), row(94), row(95), row(96), row(97), row(98),
row(99), row(100), row(101), row(102), row(103), row(104), row(105), row(106), row(107), row(108), row(109), row(110), row(111), row(112), row(113), row(114), row(115), row(116), row(117), row(118), row(119), row(120), row(121), row(122), row(123), row(124), row(125), row(126),
row(127), row(128), row(129), row(130), row(131), row(132), row(133), row(134), row(135), row(136), row(137), row(138), row(139), row(140), row(141), row(142), row(143), row(144), row(145), row(146), row(147), row(148), row(149), row(150), row(151), row(152))
Next
'CLEAR DATATABLE
dt.Rows.Clear()
con.Close()
Catch ex As Exception
MsgBox(ex.Message)
con.Close()
End Try
End Sub
这是从数据库中检索数据时的常见问题。 您需要在将值分配给控件之前或时检查 null。 例如(如果很多，我使用内联(：
Texbox_SomeTextBox.Text = If(isDBNull(MyDatabaseColomn), "", MyDatabaseColomn)
因此，当您为列表视图创建每一行时，您需要检查数据集中返回的每一列是否为 null。 这有点乏味，但很有必要。
如果您知道哪些字段允许空值并且是字符串，请尝试row(2) & ""。