如何在运行时更改NLog目标的连接字符串？

Question

在我的WebAPI应用程序中，我试图在Application_BeginRequest基于我的自定义头值将登录记录到多个独立的数据库中。

为了实现这一点，我尝试修改Nlog.config中的目标，如下所示：

<target name="database" 
        xsi:type="Database" 
        connectionString="${gdc:item=myConnectionstring}" 
        commandText="..sql command..">..params..</target>

在Application_BeginRequest中，我尝试这样设置myConnectionstring:

GlobalDiagnosticsContext.Set("myConnectionString", connectionString);

但是这并不起作用，在NLog的internalLogFile中出现了错误：

2015-02-19 12:12:48.8776 Error Error when writing to database System.InvalidOperationException: The ConnectionString property has not been initialized.
   at System.Data.SqlClient.SqlConnection.PermissionDemand()
   at System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
   at System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource`1 retry)
   at System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry)
   at System.Data.SqlClient.SqlConnection.Open()
   at NLog.Targets.DatabaseTarget.OpenConnection(String connectionString)
   at NLog.Targets.DatabaseTarget.WriteEventToDatabase(LogEventInfo logEvent)
   at NLog.Targets.DatabaseTarget.Write(LogEventInfo logEvent)

我尝试使用恒定的连接字符串配置将以下参数写入数据库：

<parameter name="@cn" layout="${gdc:myConnectionstring}"/>
<parameter name="@cn1" layout="${gdc:item=myConnectionstring}"/>

但它们似乎是空的，然而当我调试我的WebApi操作时，抛出了我期望写入数据库的错误，GlobalDiagnosticsContext.Get("myConnectionString") 返回了期望的值。 我做错了什么？为什么我可以在GlobalDiagnosticsContext.Get("myConnectionString")看到期望的值，但${gdc:item=myConnectionstring}解析为空字符串？

- Vladimirs

Answer 1

找到了另一种实现的方法：

var databaseTarget = (DatabaseTarget)LogManager.Configuration.FindTargetByName("database");
databaseTarget.ConnectionString = connectionString;
LogManager.ReconfigExistingLoggers();

然而，仍然不清楚为什么 GlobalDiagnosticsContext 的东西不起作用。

- Vladimirs

Answer 2

感谢您的解决方案！

如果您想使用异步数据库目标，您需要先获取包装目标：

DatabaseTarget databaseTarget;
var target = LogManager.Configuration.FindTargetByName("database");
if (target is AsyncTargetWrapper)
    databaseTarget = (target as AsyncTargetWrapper).WrappedTarget as DatabaseTarget;
else
    databaseTarget = target as DatabaseTarget;
if (databaseTarget == null)
    throw new NullReferenceException("Can't find DatabaseTarget with name 'database'");

databaseTarget.ConnectionString = connectionStringSelected.ConnectionString;
LogManager.ReconfigExistingLoggers();

- bonzaster