System.NullReferenceException：DataGridView操作报错及结账方案咨询

一、NullReferenceException 错误修复 #

错误原因 #

Compras.DataGridView3.get returned Nothing 由以下两种情况导致：

1. 直接通过窗体类名Compras访问控件，而非已实例化的运行窗体对象（比如窗体未打开，或未获取到正确实例）
2. DataGridView3控件名称拼写错误，或未在窗体设计器中正确初始化

修复步骤 #

1. 获取正确的窗体实例
   如果Compras是当前窗体，用Me替代Compras；如果是已打开的其他窗体，先获取运行实例：

   Dim comprasForm As Compras = Application.OpenForms.OfType(Of Compras)().FirstOrDefault()
   If comprasForm Is Nothing Then
       comprasForm = New Compras()
       comprasForm.Show()
   End If
   

   后续操作DataGridView3时统一使用comprasForm.DataGridView3。

2. 修正点击逻辑的计数器问题
   原代码依赖s1计数器的逻辑存在初始值错误（初始s1=0时，第一次点击会直接进入移除分支），改为直接查询商品是否存在，避免计数器与实际购物车状态不一致的问题。

3. 解决遍历删除行的集合异常
   直接在For Each循环中删除行会触发集合修改异常，改用倒序遍历或先收集待删除行再批量删除：

   For i As Integer = comprasForm.DataGridView3.Rows.Count - 1 To 0 Step -1
       Dim fila As DataGridViewRow = comprasForm.DataGridView3.Rows(i)
       If fila.Cells(1).Value IsNot Nothing AndAlso fila.Cells(1).Value.ToString().Trim() = "Paracetamol" Then
           comprasForm.DataGridView3.Rows.Remove(fila)
       End If
   Next
   

修复后的完整代码 #

Private Sub PictureBox5_Click(sender As Object, e As EventArgs) Handles PictureBox5.Click
    Dim comprasForm As Compras = Application.OpenForms.OfType(Of Compras)().FirstOrDefault()
    If comprasForm Is Nothing Then
        comprasForm = New Compras()
        comprasForm.Show()
    End If

    ' 检查商品是否已在购物车中
    Dim isExists As Boolean = False
    For Each fila As DataGridViewRow In comprasForm.DataGridView3.Rows
        If fila.Cells(1).Value IsNot Nothing AndAlso fila.Cells(1).Value.ToString().Trim() = "Paracetamol" Then
            isExists = True
            Exit For
        End If
    Next

    If Not isExists Then
        ' 添加商品
        comprasForm.DataGridView3.Rows.Add("1", "Paracetamol")
    Else
        ' 删除商品（倒序遍历避免集合修改异常）
        For i As Integer = comprasForm.DataGridView3.Rows.Count - 1 To 0 Step -1
            Dim fila As DataGridViewRow = comprasForm.DataGridView3.Rows(i)
            If fila.Cells(1).Value IsNot Nothing AndAlso fila.Cells(1).Value.ToString().Trim() = "Paracetamol" Then
                comprasForm.DataGridView3.Rows.Remove(fila)
            End If
        Next
    End If
End Sub

二、更简便的结账功能实现方案 #

直接操作DataGridView行的方式扩展性差，推荐采用数据源绑定的方式，用集合管理购物车数据，再绑定到DataGridView，便于后续扩展总价计算、数量修改等功能。

步骤1：定义购物车商品类 #

Public Class CartItem
    Public Property Id As String
    Public Property Name As String
    Public Property Quantity As Integer
    Public Property Price As Decimal ' 可选：用于价格计算
End Class

步骤2：初始化购物车集合并绑定到DataGridView #

在Compras窗体的Load事件中配置：

Private cartItems As New List(Of CartItem)()

Private Sub Compras_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    ' 绑定数据源到DataGridView
    DataGridView3.DataSource = cartItems
    ' 设置列显示名称
    DataGridView3.Columns("Id").HeaderText = "编号"
    DataGridView3.Columns("Name").HeaderText = "商品名称"
    DataGridView3.Columns("Quantity").HeaderText = "数量"
    DataGridView3.Columns("Price").HeaderText = "单价"
End Sub

步骤3：优化商品点击事件 #

Private Sub PictureBox5_Click(sender As Object, e As EventArgs) Handles PictureBox5.Click
    Dim comprasForm As Compras = Application.OpenForms.OfType(Of Compras)().FirstOrDefault()
    If comprasForm Is Nothing Then
        comprasForm = New Compras()
        comprasForm.Show()
    End If

    Dim targetItem = comprasForm.cartItems.FirstOrDefault(Function(item) item.Name = "Paracetamol")
    If targetItem Is Nothing Then
        ' 添加商品到购物车
        comprasForm.cartItems.Add(New CartItem With {
            .Id = "1",
            .Name = "Paracetamol",
            .Quantity = 1,
            .Price = 5.99 ' 示例单价
        })
    Else
        ' 移除商品（若需实现增减数量，可改为修改Quantity字段）
        comprasForm.cartItems.Remove(targetItem)
    End If

    ' 刷新DataGridView显示
    comprasForm.DataGridView3.DataSource = Nothing
    comprasForm.DataGridView3.DataSource = comprasForm.cartItems
End Sub

方案优势 #

• 数据与UI分离，逻辑更清晰，避免直接操作控件导致的异常
• 便于扩展结账功能（如计算总价、批量修改数量、清空购物车等）
• 代码维护性更强，后续新增商品只需复用相同逻辑

内容的提问来源于stack exchange，提问作者chivafight