C＃：如何在运行时添加下拉菜单项

Question

When creating a new project, i want this project to become available from a dropdown. The first project i create, works, and is placed as the menu item as well as a dropdown item, however when i try to add the next dropdown item it crashes and tells me "Object reference not set to an instance of an object."

This line creates the exception: 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;

Should have been declared outside the method. Sorry for the inconvinience.

/Thomas

Answer 2

Your ProjectToolStripMenuItem is set to null and it only gets assigned a value if the projects.Count is equal to zero. This is why you get the null reference exception for the second project. Since projects.Count is not zero, ProjectToolStripMenuItem never gets assigned.