I have a shared variable in a Form.
我有一个形式的共享变量。
Public Class FormHome
Public Shared db_config As Dictionary(Of String, String)
When I try to access this variable from another form,
当我尝试从另一种形式访问这个变量时，
FormHome.db_config.ContainsKey("m")
It works fine in my system (Windows 7). But I get object reference error (System.NullReferenceException) in Windows XP (client system) in exactly that line.
它在我的系统中运行良好(Windows 7)，但是在Windows XP(客户端系统)中，我得到了对象引用错误(system . nullreferenceexception)。
In both places, .NET 4.0 framework (Version - 4.0.30319.18034) is used by the application. Seems weird. What causes this error?
在这两个地方，应用程序都使用。net 4.0框架(版本- 4.0.30319.18034)。似乎是奇怪的。导致这个错误的原因是什么?
1 个解决方案
#1
0
This code
这段代码
Public Shared db_config As Dictionary(Of String, String)
only declares a variable (db_config) of the requested type. It does not create an object of that type.
只声明请求类型的变量(db_config)。它不会创建该类型的对象。
Before you create the object, db_config is still null, and thus attempting to access it will result in a NullReferenceException.
在创建对象之前，db_config仍然为空，因此尝试访问它将导致NullReferenceException。
To create an object, you need to use the New keyword. For example:
要创建对象，需要使用新的关键字。例如:
db_config = New Dictionary(Of String, String)()
You need to make sure that this object creation happens before the first attempt is made to access the variable's value. Because you're using a static (Shared) variable, doing it in the class's instance constructor may be too late. The regular instance constructor only gets executed when you create an instance of the object.
您需要确保在第一次尝试访问变量的值之前创建对象。因为您使用的是静态(共享)变量，所以在类的实例构造函数中执行它可能已经太晚了。只有当您创建对象的实例时，常规的实例构造函数才会被执行。
Therefore, I recommend doing it in the object's static constructor. As the language specification explains in more detail, the static constructor of a class is guaranteed to have been executed by the runtime by the time that any of the static members of that class is accessed.
因此，我建议在对象的静态构造函数中执行它。由于语言规范更详细地解释了，类的静态构造函数保证在访问该类的任何静态成员时被运行时执行。
Add the following code to your FormHome class:
将以下代码添加到FormHome类中:
Shared Sub New()
db_config = New Dictionary(Of String, String)()
End Sub
#1
0
This code
这段代码
Public Shared db_config As Dictionary(Of String, String)
only declares a variable (db_config) of the requested type. It does not create an object of that type.
只声明请求类型的变量(db_config)。它不会创建该类型的对象。
Before you create the object, db_config is still null, and thus attempting to access it will result in a NullReferenceException.
在创建对象之前，db_config仍然为空，因此尝试访问它将导致NullReferenceException。
To create an object, you need to use the New keyword. For example:
要创建对象，需要使用新的关键字。例如:
db_config = New Dictionary(Of String, String)()
You need to make sure that this object creation happens before the first attempt is made to access the variable's value. Because you're using a static (Shared) variable, doing it in the class's instance constructor may be too late. The regular instance constructor only gets executed when you create an instance of the object.
您需要确保在第一次尝试访问变量的值之前创建对象。因为您使用的是静态(共享)变量，所以在类的实例构造函数中执行它可能已经太晚了。只有当您创建对象的实例时，常规的实例构造函数才会被执行。
Therefore, I recommend doing it in the object's static constructor. As the language specification explains in more detail, the static constructor of a class is guaranteed to have been executed by the runtime by the time that any of the static members of that class is accessed.
因此，我建议在对象的静态构造函数中执行它。由于语言规范更详细地解释了，类的静态构造函数保证在访问该类的任何静态成员时被运行时执行。
Add the following code to your FormHome class:
将以下代码添加到FormHome类中:
Shared Sub New()
db_config = New Dictionary(Of String, String)()
End Sub