你需要做的是得到应用程序级的get表单，可以做如下。

FormCollection openforms = Application.OpenForms;
foreach (Form form in openforms)
{
    if (form.Name == "frmsomething")
    {
        form.Close();
    }
    // Whatever you want do with individual forms in form object
}

问题似乎是ShowInTaskBar = false导致.NET忽略打开的表单列表中的表单。但是，您可以使用Windows API将表单设置为不显示在任务栏中，如int this SO answer所述。

Form2 form
private void btn1_Click(object sender, EventArgs e)
    {
        form = new Form2();
        form.Show();
    }

private void btn2_Click(object sender, EventArgs e)
    {
        if(form != null && !form.IsDisposed){
              form.Close();
        }
    }