Form1 a1=new Form1();

.....

Formn an=new Formn();

private void button_Click(object sender, EventArgs e)
{
  if(a1==null||a1.Isdisposed)
{ 
 a1=new Form1();
}


}

//单例窗体，适当改动代码
 protected void ShowMdiChild<TForm>() where TForm : Form, new()
        {
            Form child = this.MdiChildren.FirstOrDefault(frm => frm is TForm);
            if (child == null)
            {
                child = new TForm();
                child.MdiParent = this;
                child.StartPosition = FormStartPosition.WindowsDefaultLocation;
                child.WindowState = FormWindowState.Maximized;
                child.Show();

            }
            else
            {
                child.StartPosition = FormStartPosition.WindowsDefaultLocation;
                child.WindowState = FormWindowState.Maximized;
                child.Select();
            }

        }
//调用方法
private void button_Click(object sender, EventArgs e)
{
   this.ShowMdiChild<a1>();
}

------解决方案--------------------
static Hashtable arr = new Hashtable();
public static T GetInstance<T>(string className){
if (arr[className] == null){
arr[className] = (T)System.Reflection.Assembly.Load("命名空间").CreateInstance("命名空间."+className);
}
return arr[className] as T;
}

------解决方案--------------------
C# code

static Hashtable arr = new Hashtable();
public static T GetInstance<T>() where T : class {
var t = typeof(T);
var k = t.FullName;
if (arr[k] == null){
arr[k] = (T)t.Assembly.CreateInstance(t.FullName);
}
return arr[k] as T;
}