NLog: variable with cached layout yields different values if used multiple times

Question

I log to Azure Blob Storage and want to have one three logfiles (containing different things) per start and I use a cached timestamp for this. Ideally the timestamp should be the same in all three files, but they differ if I log to the specific targets at different times.

I set the variable like this

<variable name='blobprefix' value='${cached:cached=true:Inner=${date:format=yyyy-MM-dd\\THH.mm.ss}' />

And this is how I use it for every blob:

<target type="AzureBlobStorage"
    name="general-azureblob"
    connectionString="${blobConnection}"
    blobName="${blobprefix}.log"> 
</target>

How can I have the variable evaluate exactly once.

Answer 1

You can use the ${processinfo:StartTime} like this:

<variable name='blobprefix' value='${processinfo:StartTime:format=yyyy-MM-dd\\THH.mm.ss:cached=true}' />

If you want to control the StartTime, then you can use NLog GDC and do the following at application startup (Before creating the first NLog Logger-object):

NLog.GlobalDiagnosticsContext.Set("StartupTime", DateTime.UtcNow);

And have the following in NLog.config (whenEmpty is only there as fallback):

<variable name='blobprefix' value='${gdc:StartupTime:format=yyyy-MM-dd\\THH.mm.ss:cached=true:whenEmpty:${date:format=yyyy-MM-dd\\THH.mm.ss:cached=true}}' />