Try-Catch 和“继续”——这可能吗？

Question

我的代码中有一个部分用于查询网络上的所有 SQL Server 数据库。我首先尝试使用 SQL 登录来访问 SQL Server 实例，但如果失败，那么我想尝试使用我的 Windows 凭据进行连接。之后，如果我仍然无法连接，那么我希望代码失败，然后通知用户。

所以我想我要问的是如何从 Try-Catch 块内部循环到 Try-Catch 块上方的行：

String conxString = @"Data Source=SQLInstance1;User ID=FOO;Password=BAR;";
bool secondTime = false;

using (SqlConnection sqlConx = new SqlConnection(conxString))
     {
         Try{
               sqlConx.Open();
               DataTable tblDatabases = sqlConx.GetSchema("Databases");
               sqlConx.Close();
               secondTime = false;
               Console.WriteLine("SQL Server found!");
         }
         Catch(System.Data.SqlClient.SqlException e){
                if (!secondTime){
                   secondTime = true;
                   conxString = @"Data Source=SQLInstance1; Integrated Security=True;";
                      //Loop back to the using statement to try again with Windows Creds
                {
                 else{
                   Console.WriteLine("SQL Server not found or credentials refused");
                 }
                   //Report Failure to connect to user

         }
         finally{
            //Reset Variable
            secondTime = false;
         }

      }

Answer 1

我可能会走这条路：

String conxString = @"Data Source=Instance1;User ID=FOO;Password=BAR;";
//in your main function
if(!TryConnect(conxString))
{
   Console.WriteLine("SQL Creditials failed.  Trying with windows credentials...");
   conxString = "new conn string";
   TryConnect(conxString);
}
..............
//new function outside of your main function
private bool TryConnect(string connString)
{
   using (SqlConnection sqlConx = new SqlConnection(conxString))
     {
         Try{
               sqlConx.Open();
               DataTable tblDatabases = sqlConx.GetSchema("Databases");
               sqlConx.Close();
         }
         Catch(System.Data.SqlClient.SqlException e){
                return false;
         }
         return true;    
      }
}

Answer 2

成功后，您可以使用for 循环和break：

for (int attempt = 1; attempt <= 2; attempt++)
{
    try
    {
        /* perform attempt */
        var success = TryToConnect();
        if (success)
            break;
    }
    catch (Exception e)
    {
        /* report error */
    }
}

您还可以记录是否成功等或增加尝试次数或使尝试次数可配置。

Answer 3

此blog post（尽管是从 2005 年开始）显示了您的问题的可能解决方案：

使用转到

TryLabel:
try
{
    downloadMgr.DownLoadFile("file:///server/file", "c:\\file");
    Console.WriteLine("File successfully downloaded");
}
catch (NetworkException ex)
{
    if (ex.OkToRetry)
        goto TryLabel;
}

写一个包装器

public static bool Wrapper(DownloadManager downloadMgr)
{
    try
    {
        downloadMgr.DownLoadFile("file:///server/file", "c:\\file");
        return true;
    }

    catch (NetworkException ex)
    {
        Console.WriteLine("Failed to download file: {0}", ex.Message);
        return (!ex.OkToRetry);
    } 
}

static void Main(string[] args)
{
    while (!Wrapper(downloadMgr)) ;
}