Question

创建新项目时，我希望此项目可从下拉列表中获得。第一个项目我创建，工作，并作为菜单项和下拉项放置，但是当我尝试添加下一个下拉项时，它崩溃并告诉我“对象引用未设置为对象的实例”。

此行创建例外： ProjectToolStripMenuItem.DropDownItems.Add（newproject）;

ArrayList projects = new ArrayList();

private void toolStripMenuItem1_Click(object sender, EventArgs e)
{    
   Project moldProject = new Project();    
   moldProject.ProjectName = "unnamed";

   ToolStripMenuItem ProjectToolStripMenuItem = null;

   if (projects.Count == 0)
   {
       ProjectToolStripMenuItem = new System.Windows.Forms.ToolStripMenuItem();

       ProjectToolStripMenuItem.Name = "projectToolStripMenuItem";
       ProjectToolStripMenuItem.Size = new System.Drawing.Size(44, 20);
       ProjectToolStripMenuItem.Text = "unnamed*";

       this.menuStrip1.Items.Add(ProjectToolStripMenuItem);
   }

   ToolStripMenuItem newproject = new System.Windows.Forms.ToolStripMenuItem();

   string name = "aboutToolStripMenuItem" + (projects.Count + 1).ToString();
   newproject.Name = name;

   newproject.Size = new System.Drawing.Size(211, 22);
   newproject.Text = "unnamed*";

   ProjectToolStripMenuItem.DropDownItems.Add(newproject);

   projects.Add(moldProject);
}

Answer 1

ToolStripMenuItem ProjectToolStripMenuItem = null;

应该在方法之外声明。很抱歉给您带来不便。

/托马斯

Answer 2

ProjectToolStripMenuItem设置为null，如果projects.Count等于零，则只会为其分配值。这就是为什么你得到第二个项目的空引用异常。由于projects.Count不为零，因此永远不会分配ProjectToolStripMenuItem。

c＃：如何在运行时添加下拉菜单项

2 个答案: