VB.NET调用Purchase类时遇“Purchase1未赋值即使用”错误求解决

问题根源 #

你只声明了Purchase类型的变量purchase1，但没有用New关键字创建类的实例——相当于只告诉程序“我要一个Purchase类型的变量”，但没给这个变量分配实际的内存空间，直接操作它的属性自然会触发未赋值使用的错误。

修复方案 #

1. 先实例化对象再操作属性 #

在使用purchase1之前，必须通过New关键字创建类的实例：

Dim purchase1 As New Purchase()

2. 修正后的完整代码 #

记录按钮点击事件

Private Sub btnRecord_Click(sender As Object, e As EventArgs) Handles btnRecord.Click
    ' 实例化Purchase对象
    Dim purchase1 As New Purchase()

    purchase1.description = txtitem.Text
    purchase1.price = txtprice.Text
    purchase1.quantity = txtquantity.Text
End Sub

读取按钮点击事件

注意属性访问不需要加括号（除非你的Getter是带参数的方法，通常属性直接访问即可），同时也要先实例化对象：

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    ' 实例化Purchase对象
    Dim purchase1 As New Purchase()

    txtitem.Text = purchase1.description
    txtprice.Text = purchase1.price
    txtquantity.Text = purchase1.quantity
End Sub

进阶优化：跨事件共享对象 #

如果需要在两个按钮之间共享同一个Purchase实例（比如记录数据后，点击另一个按钮读取刚才的记录），不能在每个事件里声明局部变量，要把purchase1声明为窗体级变量：

' 在窗体类的顶部声明，所有事件均可访问
Private purchase1 As Purchase

Private Sub btnRecord_Click(sender As Object, e As EventArgs) Handles btnRecord.Click
    ' 首次使用前实例化，避免重复创建
    If purchase1 Is Nothing Then
        purchase1 = New Purchase()
    End If

    purchase1.description = txtitem.Text
    purchase1.price = txtprice.Text
    purchase1.quantity = txtquantity.Text
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    ' 确保对象已实例化再访问，避免空引用错误
    If purchase1 IsNot Nothing Then
        txtitem.Text = purchase1.description
        txtprice.Text = purchase1.price
        txtquantity.Text = purchase1.quantity
    End If
End Sub

内容的提问来源于stack exchange，提问作者user14488435