It’s easy to spot the similarity between Serilog message templates:

// Serilog
var loggedInUserId = "nblumhardt";
Log.Information("Logged in {UserId}", loggedInUserId);

and C# 6 interpolated strings:

// C# 6
Log.Information($"Logged in {loggedInUserId}");

This is naturally because of their shared common ancestor, the .NET format string. Under the hood, interpolated strings in C# are converted by the compiler into a struct with the classic format string and argument values, logically along the lines of:

// C# 6 desugared
Log.Information(new FormattableString
    Format = "Logged in {0}",
    Args = new object[] { loggedInUserId }

The similarity has led a few of us, since seeing the new C# feature, to wonder whether a few keystrokes might be shaved off of the Serilog API if only the C# compiler would preserve the names of the “holes” in the interpolated string, i.e. loggedInUserId in this example.

There was initially mention of Serilog on the design thread for the feature, and more recently Andrey Shchekin most graciously raised a ticket on the Roslyn issue tracker as a prompt for some support in C# 7.

I’ve had some time to think about this feature now, and while the concision is appealing, I’m no longer convinced string interpolation (name preservation or not) is a good fit for Serilog.

1. A good variable name is not necessarily a good property name

Let’s take the example above; in our hypothetical C# 6 example the complete snippet would look like:

var loggedInUserId = "nblumhardt";
Log.Information($"Logged in {loggedInUserId}");

The variable name loggedInUserId obviously reflects what this value is being used for by the program. But, if that name was carried through to the log event, e.g.:

{ { "Properties": { "loggedInUserId": "nblumhardt", 

This would lead us to query the log with filters like:

loggedInUserId == "nblumhardt"

Fine if we only want to find events where the user was logged in, but in the context of a larger and more complex application, we’ll often want to find as many events as possible that are related to a specific user (who might be having some technical trouble!).

In that case, a more general property name is a better fit:

Log.Information("Logged in {UserId}", loggedInUserId);

// Elsewhere
Log.Information("Password for {UserId} successfully changed", loggedInUserId);

This small change allows us to query UserId == "nblumhardt" and find both matching events.

Log event property names are important enough for diagnostics that they deserve their own identity, separate from the variable names around them.

2. Holes don’t always have obvious names

What does the property name for this statement look like?

Log.Information($"Enabling categories {new[]{1, 2, 3}}");

Here the hole is filled with an expression that’s not a simple name, so some kind of mungeing would be necessary in order to turn it into a valid log event property name (a JSON identifier or similar).

Going down this path would definitely reduce the overall quality of log events, and elevating log events to first-class status is what Serilog’s all about.

So what’s the relationship between Serilog and C# 6, then?

String interpolation is a great feature, one I’ve looked forward to in C# for a long time. The idea of providing direct support for Serilog is a very interesting one and worth exploring, but I’m increasingly convinced it’s unnecessary.

Interpolation is nice when it keeps code DRY, cutting out the redundant clutter of {0} and {1} and preventing parameter mismatches.

In the case of Serilog, I think it’s incorrect to consider the property names like {UserId} as redundant; in a well-implemented logging strategy they’re an incredibly important part of the picture that deserve their own consideration. You wouldn’t let variable names determine the table and column names in a relational database – it’s exactly the same trade-off being considered here.

What do you think? I’d love to be convinced there’s more synergy to be found here, but I’m not seeing it at present.