从另一个对象访问Form组件将引发“未处理System.NullReferenceException”

问题描述

| 我正在尝试从WCF对象中修改属于Form2的TextBox。

namespace server2
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        private ServiceHost duplex;

        private void Form2_Load(object sender,EventArgs e)     /// once the form loads,create and open a new ServiceEndpoint.
        {
            duplex = new ServiceHost(typeof(ServerClass));
            duplex.AddServiceEndpoint(typeof(IfaceClient2Server),new NetTcpBinding(),\"net.tcp://localhost:9080/service\");
            duplex.open();
            this.Text = \"SERVER *on-line*\";
        }
    }


    class ServerClass : IfaceClient2Server
    {


        IfaceServer2Client callback;

        public ServerClass()
        {
            callback = OperationContext.Current.GetCallbackChannel<IfaceServer2Client>();

        }

        public void StartConnection(string name)
        {
            var myForm = Form.ActiveForm as Form2;
            myForm.textBox1.Text = \"Hello World!\";  /// <- this one trows “System.NullReferenceException was unhandled”
                                                    /// unless Form2 is selected when this fires.

            callback.Message_Server2Client(\"Welcome,\" + name );
        }


        public void Message_Cleint2Server(string msg)
        {
        }

        public void Message2Client(string msg)
        {
        }

    }




    [ServiceContract(Namespace = \"server\",CallbackContract = typeof(IfaceServer2Client),SessionMode = SessionMode.required)]


    public interface IfaceClient2Server           ///// what comes from the client to the server.
    {
        [OperationContract(IsOneWay = true)]
        void StartConnection(string clientName);

        [OperationContract(IsOneWay = true)]
        void Message_Cleint2Server(string msg);
    }


    public interface IfaceServer2Client          ///// what goes from the sertver,to the client.
    {
        [OperationContract(IsOneWay = true)]
        void AcceptConnection();

        [OperationContract(IsOneWay = true)]
        void RejectConnection();

        [OperationContract(IsOneWay = true)]
        void Message_Server2Client(string msg);
    }

}

然而，\“ myForm.textBox1.Text = \” Hello world！\“; \”行引发了System.NullReferenceException未被处理\“ ... 任何想法，谢谢！     

解决方法

正如最初问题的注释中所讨论的那样，问题在于您要使用的ActiveForm是您想要的表单未处于活动状态。每当尝试使用

as

关键字进行强制转换时，如果强制转换无效，结果将为null。由于您获取了无法转换为Form2的表单（因为它是另一种表单），因此您正确地收到了null引用异常。 假设您在Form2上执行了单例规则，并且没有使用表单的名称，则可以通过Application.OpenForms集合来访问它，如下所示：

(Form2)Application.OpenForms[\"Form2\"];

在您的代码示例中，可能看起来像这样：

public void StartConnection(string name)
{
    //var myForm = Form.ActiveForm as Form2;
    var myForm = Application.OpenForms[\"Form2\"] as Form2;
    myForm.textBox1.Text = \"Hello world!\";  /// <- this one trows “System.NullReferenceException was unhandled”
                                            /// unless Form2 is selected when this fires.

    callback.Message_Server2Client(\"Welcome,\" + name );
}

就是说，我不认为我有责任为WCF服务修改表单控件。我会更快地消耗表单中服务触发的事件。     ,

myForm

可能不是

Form2

类型，或者可能不包含

textBox1

字段。对于这两种情况，请确保检查是否为空。