An exception occurs when C# operates IIS to create an application pool: Solution to invalid index

public static string CreateAppPool(string appPoolName, string frameworkVersion, string managedPipelineMode)
         {
             DirectoryEntry rootfolder = new DirectoryEntry("IIS://localhost/W3SVC/APPPOOLS");

             try
             {
                 DirectoryEntry appPool = rootfolder.Children.Add(appPoolName, "IIsApplicationPool");
                 if (appPool.Properties.Contains("AppPoolCommand"))
                 {
                     appPool.Properties["AppPoolCommand"][0] = "1";
                 }
                 if (appPool.Properties.Contains("ManagedRuntimeVersion"))
                 {
                     appPool.Properties["ManagedRuntimeVersion"][0] = "v4.0";
                 }
                 //以下代码在WindowsServer2008版本为6.0的系统上会出现异常：
                 //无效索引（Exception from HRESULT:0x80070585）
                 //if (appPool.Properties.Contains("AppPoolIdentityType"))
                 //{
                 //    appPool.Properties["AppPoolIdentityType"][0] = "4";
                 //}
                 if (appPool.Properties.Contains("Enable32BitAppOnWin64"))
                 {
                     appPool.Properties["Enable32BitAppOnWin64"][0] = true;
                 }
                 appPool.CommitChanges();
                 return null;
             }
             catch (System.Exception ex)
             {
                 LogToFile.Save(ex, "CreateAppPool");
                 return ex.Message;
             }
         }