从另一个对象访问一个Form组件会抛出"System.NullReferenceException未处理

本文关键字：quot 未处理 NullReferenceException System 组件 Form 访问 一个对象 一个 | 更新日期: 2023-09-27 17:50:06

我试图修改一个文本框，属于Form2从一个WCF对象。

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. html "Text = "Hello world!";" line抛出System。NullReferenceException was unhandled"…

有任何想法，谢谢!

从另一个对象访问一个Form组件会抛出"System.NullReferenceException未处理

myForm可能不是Form2类型，或者它可能不包含textBox1字段。

正如在最初的问题的评论中所讨论的，问题是你指的是ActiveForm，当你想要的表单是不活跃的。每当尝试使用as关键字强制转换时，如果强制转换无效，结果将为空。由于您抓取了一个不能转换为Form2的表单(因为它是一种不同类型的表单)，因此您正确地收到了一个空引用异常。

假设你已经在Form2上实施了单例规则，并且你没有使用表单的名称，你可以通过应用程序的方式访问它。OpenForms集合如下:

(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 );
}

也就是说，我不认为我会把修改表单控件的责任交给WCF服务。我可以更快地在表单中使用服务触发的事件。

相关文章：