未填写一个文本框时解决方案崩溃

Question

目前，我正在尝试编写代码以将新学生添加到数据库表中。 但是，当按提交时未填充其中一个文本框时，解决方案将崩溃。 请告诉我一种编写代码的方法：

Imports System.Data.OleDb
Public Class AddNewStudent
Dim cnnOLEDB As New OleDbConnection
Dim cmdInsert As New OleDbCommand
Dim con = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:ASSIGNMENTvbnassignmentfinal.accdb;"
Dim cmdDelete As New OleDbCommand
Dim cmdUpdate As New OleDbCommand
Dim cmdSearch As New OleDbCommand
Private Sub AddNewStudentSD_Load(sender As Object, e As EventArgs) Handles MyBase.Load
cnnOLEDB.ConnectionString = con
cnnOLEDB.Open()
End Sub
Private Sub btnClear_Click(sender As Object, e As EventArgs) Handles btnClear.Click
txtsem.ResetText()
txtsfn.ResetText()
txtsid.ResetText()
txtsln.ResetText()
txtsph.ResetText()
txtint.ResetText()
cmbgen.ResetText()

End Sub
Private Sub btnHome_Click(sender As Object, e As EventArgs) Handles btnhome.Click
UserHomepage.Show()
Me.Hide()
End Sub
Private Sub btnSubmit_Click(sender As Object, e As EventArgs) Handles btnsubmit.Click
cmdInsert.CommandText = "Insert into student Values(" + txtsid.Text + ",'" + txtint.Text + "','" + txtsfn.Text + "','" + txtsln.Text + "', '" + cmbgen.Text + "', " + txtsph.Text + ", '" + txtsem.Text + "');"
cmdInsert.CommandType = CommandType.Text
cmdInsert.Connection = cnnOLEDB
cmdInsert.ExecuteNonQuery()
MessageBox.Show("Are you sure you want to proceed?", "Proceed", MessageBoxButtons.YesNo, MessageBoxIcon.Question)
If DialogResult.Yes Then
UserHomepage.Show()
Me.Hide()
ElseIf DialogResult.No Then
Me.Show()
UserHomepage.Hide()
End If
End Sub
End Class

Answer 1

Private Sub btnSubmit_Click(sender As Object, e As EventArgs) Handles btnsubmit.Click
cnnOLEDB.Open()
cmdInsert.CommandText = "Insert into student Values(" + txtsid.Text + ",'" + txtint.Text + "','" + txtsfn.Text + "','" + txtsln.Text + "', '" + cmbgen.Text + "', '" + txtsph.Text + "', '" + txtsem.Text + "');"
cmdInsert.CommandType = CommandType.Text
cmdInsert.Connection = cnnOLEDB
cmdInsert.ExecuteNonQuery()
cnnOLEDB.Close()
MessageBox.Show("Are you sure you want to proceed?", "Proceed", MessageBoxButtons.YesNo, MessageBoxIcon.Question)
If DialogResult.Yes Then
UserHomepage.Show()
Me.Hide()
ElseIf DialogResult.No Then
Me.Show()
UserHomepage.Hide()
End If
End Sub

Answer 2

我不认为Visual Studio具有像MS-Access在其VBA环境中那样NZ function。

您可以创建自己的函数来模拟它：

Public Shared Function Nz(ByVal Value As Object, Optional ByVal Default as Object = String.Empty) As Object
If Value Is Nothing OrElse IsDBNull(Value) Then
Return Default
Else
Return Value
End If
End Function

然后修改动态 SQL 代码行，将任何 Null 文本字段转换为空字符串。

cmdInsert.CommandText = "Insert into student Values(" + NZ(txtsid.Text,"") + _
",'" + NZ(txtint.Text,"") + "','" + NZ(txtsfn.Text,"") + "','" + _
NZ(txtsln.Text,"") + "', '" + NZ(cmbgen.Text,"") + "', '" + NZ(txtsph.Text,"") _
+ "', '" + NZ(txtsem.Text,"") + "');"