using System.DirectoryServices; // 添加引用 System.DirectoryServices

try
{
    DirectoryEntry appPool = new DirectoryEntry("IIS://localhost/W3SVC/AppPools");
    DirectoryEntry findPool = appPool.Children.Find("DefaultAppPool", "IIsApplicationPool");
    findPool.Invoke("Start", null);     // Start|Stop|Recycle [Recycle:应用程序池回收，如果状态为Stop会报错]
    appPool.CommitChanges();
    appPool.Close();
    Response.Write("DefaultAppPool 操作成功！");
}
catch (Exception ex)
{
    Response.Write("DefaultAppPool 未找到！");
}