vb.net - 对象的类型初始值设定项引发异常

Question

我收到运行时错误“‘SensionVB.CommonCall’的类型初始值设定项引发异常。”当尝试实例化对象调用时:

Dim general As New SensionVB.CommonCall

当单步执行代码时，一旦遇到代码中的上述行并失败，它就会失败。

微软的建议是检查 null，或者添加新的，在这种情况下都不是 true

SensionVB = 项目 CommonCall = 具有各种功能的公共(public)类

Public Class CommonCall
Protected WithEvents litOne As System.Web.UI.WebControls.Literal
Public Shared sConnection As String = ConfigurationManager.ConnectionStrings("SerialStamperConnectionString").ConnectionString
Public Shared oConnection As New MySqlClient.MySqlConnection(sConnection)
Public Shared sMaxSerialNumber As String = "S000001"
Public Shared bLoggedin As Boolean = False
Public Shared iPageWidth As Integer = 1024

Public Shared isValidPassword As Boolean = False
Public Shared strCompanyID As String = ""
Public Shared strSecurityID As String = ""

Dim Response As New HttpResponse(HttpWriter.Null)
Const TimeOutInMinutes As Integer = 60
Const SiteTitle As String = "SensiOn"
Public Const PageWidth As Integer = 1250
Public SSOenabled As Boolean
Const SuperUserEmails As String = "<a href="https://stackoverflow.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="daa9afaabfa89abeb5b7bbb3b4f4b9b5b7" rel="noreferrer noopener nofollow">[email protected]</a>"
Public Sub Initialize()
    oConnection.Open()
End Sub

Answer 1

我看到三行可能是罪魁祸首:

---

Public Shared sConnection As String = ConfigurationManager.ConnectionStrings("SerialStamperConnectionString").ConnectionString

这里有几个潜在的问题。一，您的连接字符串名称(“SerialStamperConnectionString”)不正确。如果此名称与 web.config 中的键不匹配，则它将返回 Nothing，从而在您尝试获取 ConnectionString 属性的值时导致 NullReferenceException。

---

Public Shared oConnection As New MySqlClient.MySqlConnection(sConnection)

上面的连接字符串可能没有在 web.config 中正确配置，因此返回 Nothing 作为 ConnectionString。如果 sConnection 已设置为 Nothing，则 MysqlConnection 构造函数可能会抛出 NullReferenceException。

---

Dim Response As New HttpResponse(HttpWriter.Null)

如果 HttpWriter.Null 为 Nothing，那么这也可能是问题的根源(出于与上述相同的原因)。

---

如果这些建议似乎都没有引起问题，那么我建议发布有关您的问题的更多信息(包括堆栈跟踪)。提供一百万个声誉点不会向我们提供有关您的代码的任何其他信息。

---

附录
根据有关共享构造函数(以及扩展的成员)的 VB 文档:

Exactly when shared constructors are run is mostly implementation dependent, though several guarantees are provided if a shared constructor is explicitly defined:

• Shared constructors are run before any instance of a class type is created.
• Shared constructors are run before any instance members of a structure type are accessed, or before any constructor of a structure type is explicitly called. Calling the implicit parameterless constructor created for structures will not cause the shared constructor to run.
• Shared constructors are run before any of the type's shared members are referenced.
• Shared constructors are run before any types that derive from the type are loaded.
• A shared constructor will not be run more than once during a single execution of a program.

9.3.2 Shared Constructors

您可能会发现创建一个共享构造函数来初始化共享字段很有用:

Public Shared sConnection As String
Public Shared oConnection As MySqlClient.MySqlConnection

Shared Sub New()
    Dim l_connectionString = ConfigurationManager.ConnectionStrings("SerialStamperConnectionString")
    If l_connectionString IsNot Nothing Then
        sConnection = l_connectionString.ConnectionString
        oConnection = New MySqlClient.MySqlConnection(sConnection)
    Else
        ' What to do here?
    End If
End Sub

这很可能不会解决您的问题，但它可能会使查找和处理(空字符串等)变得更容易