修改了Winform集合;枚举操作可能无法执行

Question

我的代码是：

    private void studentToolStripMenuItem_Click(object sender, EventArgs e)
    {
        foreach (Form frm in Application.OpenForms)
        {
            if (frm.Text == "student")
            {
                frm.Activate();
            }
            else
            {
                studetn obj = new studetn();
                obj.MdiParent = this;
                obj.Dock = DockStyle.Fill;
                obj.Show();
            }
        }                                                                     
    }

Answer 1

Show（） 命令修改您正在枚举的 Application.OpenForms 集合。这是不允许的。您应该在 foreach 循环之外创建并显示新表单：

private void studentToolStripMenuItem_Click(object sender, EventArgs e)
{
    // Find the student form
    Form frmStudent = null;
    foreach (Form frm in Application.OpenForms)
    {
        if (frm.Text == "student")
        {
            frmStudent = frm;
            break;
        }
    }                                                                     
    // if found - activate, if not found - create
    if(frmStudent!=null)
    {
        frmStudent.Activate();
    }
    else
    {
        studetn obj = new studetn();
        obj.MdiParent = this;
        obj.Dock = DockStyle.Fill;
        obj.Show();
    }
}