Question

我现在疯了...... 我无法理解为什么如果我在事件button2_Click_2中创建变量“server”，当它按钮3_Click_1为空时尝试访问它时。

我应该怎么做才能在button3_Click_1中访问它？

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO.Ports;
using TCCWindows.Lib;
using System.Web.Http.SelfHost;
using System.Web.Http;


namespace TCCWindows
{
    public partial class FormPrincipal : Form
    {   
        HttpSelfHostServer server;
        HttpSelfHostConfiguration config;

        public FormPrincipal()
        {
            InitializeComponent();
        }

        private void button2_Click_2(object sender, EventArgs e)
        {
            var config = new HttpSelfHostConfiguration(textBox1.Text);

            config.Routes.MapHttpRoute(
                "API Default", "api/{controller}/{id}",
                new { id = RouteParameter.Optional });
            HttpSelfHostServer server = new HttpSelfHostServer(config);
            server.OpenAsync();              
            MessageBox.Show("Server is ready!");  
        }

        private void button3_Click_1(object sender, EventArgs e)
        {
            server.CloseAsync();    
        }
    }

    public class ProductsController : ApiController
    {

        public string GetProductsByCategory(string category)
        {
            return (category ?? "Vazio");
        }
    }
}

Answer 1

您正在Button2_Click_2方法中声明一个名为server的新变量。您需要将其分配给该字段，因此请更改

HttpSelfHostServer server = new HttpSelfHostServer(config);

到

server = new HttpSelfHostServer(config);

Answer 2

您在两个按钮点击中都指的是server，但实际上您没有实例化该对象。

在config中实例化button2_Click_2后，您还需要实例化server：

server = new HttpSelfHostServer(config);

但如果button3_Click_1事件在button2_Click_2之前运行，您仍会抛出异常，因为server仍然为空。

除非您有某种方法可以强制执行单击哪个按钮，否则您可能希望将实例化的对象移动到构造函数中，以便在引用它们时确定它们不为null。

Answer 3

请注意删除var中的局部变量声明符HttpSelfHostServer和HttpSelfHostServer：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO.Ports;
using TCCWindows.Lib;
using System.Web.Http.SelfHost;
using System.Web.Http;


namespace TCCWindows
{
    public partial class FormPrincipal : Form
    {   
        HttpSelfHostServer server;
        HttpSelfHostConfiguration config;

        public FormPrincipal()
        {
            InitializeComponent();
        }

        private void button2_Click_2(object sender, EventArgs e)
        {
            config = new HttpSelfHostConfiguration(textBox1.Text);

            config.Routes.MapHttpRoute(
                "API Default", "api/{controller}/{id}",
                new { id = RouteParameter.Optional });
            server = new HttpSelfHostServer(config);
            server.OpenAsync();              
            MessageBox.Show("Server is ready!");  
        }

        private void button3_Click_1(object sender, EventArgs e)
        {
            server.CloseAsync();    
        }
    }

    public class ProductsController : ApiController
    {

        public string GetProductsByCategory(string category)
        {
            return (category ?? "Vazio");
        }
    }
}

在其他上下文C＃Windows窗体中，变量为null

3 个答案: