Serilog Expressions

An embeddable mini-language for filtering, enriching, and formatting Serilog events, ideal for use with JSON or XML configuration.

Getting started

Install the package from NuGet:

dotnet add package Serilog.Expressions

The package adds extension methods to Serilog's Filter, WriteTo, and Enrich configuration objects, along with an ExpressionTemplate type that's compatible with Serilog sinks accepting an ITextFormatter.

Filtering example

Serilog.Expressions adds ByExcluding() and ByIncludingOnly() overloads to the Filter configuration object that accept filter expressions:

Log.Logger = new LoggerConfiguration()
    .Filter.ByExcluding("RequestPath like '/health%'")
    .CreateLogger();

Events with a RequestPath property that matches the expression will be excluded by the filter.

Note that if the expression syntax is invalid, an ArgumentException will be thrown from the ByExcluding() method, and by similar methods elsewhere in the package. To check expression syntax without throwing, see the Try*() methods in the SerilogExpression class.

An appSettings.json JSON configuration example

In appSettings.json configuration this is written as:

{
  "Serilog": {
    "Using": ["Serilog.Expressions"],
    "Filter": [
      {
        "Name": "ByExcluding",
        "Args": {
          "expression": "RequestPath like '/health%'"
        }
      }
    ]
  }
}

An <appSettings> XML configuration example

In XML configuration files, this is written as:

  <appSettings>
    <add key="serilog:using:Expressions" value="Serilog.Expressions" />
    <add key="serilog:filter:ByExcluding.expression" value="RequestPath like '/health%'" />
  </appSettings>

Supported configuration APIs

Serilog.Expressions adds a number of expression-based overloads and helper methods to the Serilog configuration syntax:

• Filter.ByExcluding(), Filter.ByIncludingOnly() - use an expression to filter events passing through the Serilog pipeline
• WriteTo.Conditional() - use an expression to select the events passed to a particular sink
• Enrich.When() - conditionally enable an enricher when events match an expression
• Enrich.WithComputed() - add or modify event properties using an expression

Formatting with ExpressionTemplate

Serilog.Expressions includes the ExpressionTemplate class for text formatting. ExpressionTemplate implements ITextFormatter, so it works with any text-based Serilog sink, including Console, File, Debug, and Email:

// using Serilog.Templates;

Log.Logger = new LoggerConfiguration()
    .WriteTo.Console(new ExpressionTemplate(
        "[{@t:HH:mm:ss} {@l:u3} ({SourceContext})] {@m} (first item is {Cart[0]})\n{@x}"))
    .CreateLogger();

// Produces log events like:
// [21:21:40 INF (Sample.Program)] Cart contains ["Tea","Coffee"] (first item is Tea)

Templates are based on .NET format strings, and support standard padding, alignment, and format specifiers.

Along with standard properties for the event timestamp (@t), level (@l) and so on, "holes" in expression templates can include complex expressions over the first-class properties of the event, like {SourceContext} and {Cart[0]} in the example..

Templates support customizable color themes when used with the Console sink:

    .WriteTo.Console(new ExpressionTemplate(
        "[{@t:HH:mm:ss} {@l:u3}] {@m}\n{@x}", theme: TemplateTheme.Code))

Newline-delimited JSON (for example, replicating the CLEF format) can be generated using object literals:

    .WriteTo.Console(new ExpressionTemplate(
        "{ {@t, @mt, @r, @l: if @l = 'Information' then undefined() else @l, @x, ..@p} }\n"))

Language reference

Properties

The following properties are available in expressions:

• All first-class properties of the event - no special syntax: SourceContext and Cart are used in the formatting examples above
• @t - the event's timestamp, as a DateTimeOffset
• @m - the rendered message
• @mt - the raw message template
• @l - the event's level, as a LogEventLevel
• @x - the exception associated with the event, if any, as an Exception
• @p - a dictionary containing all first-class properties; this supports properties with non-identifier names, for example @p['snake-case-name']
• @i - event id; a 32-bit numeric hash of the event's message template
• @r - renderings; if any tokens in the message template include .NET-specific formatting, an array of rendered values for each such token

The built-in properties mirror those available in the CLEF format.

Literals

Array and object literals support the spread operator: [1, 2, ..others], {a: 1, ..others}. Specifying an undefined property in an object literal will remove it from the result: {..User, Email: Undefined()}

Operators and conditionals

A typical set of operators is supported:

• Equality = and inequality <>, including for arrays and objects
• Boolean and, or, not
• Arithmetic +, -, *, /, ^, %
• Numeric comparison <, <=, >, >=
• Existence is null and is not null
• SQL-style like and not like, with % and _ wildcards (double wildcards to escape them)
• Array membership with in and not in
• Accessors a.b
• Indexers a['b'] and a[0]
• Wildcard indexing - a[?] any, and a[*] all
• Conditional if a then b else c (all branches required; see also the section below on conditional blocks)

Comparision operators that act on text all accept an optional postfix ci modifier to select case-insensitive comparisons:

User.Name like 'n%' ci

Functions

Functions are called using typical Identifier(args) syntax.

Except for the IsDefined() function, the result of calling a function will be undefined if:

• any argument is undefined, or
• any argument is of an incompatible type.

Functions that compare text accept an optional postfix ci modifier to select case-insensitive comparisons:

StartsWith(User.Name, 'n') ci

Template directives

Conditional blocks

Within an ExpressionTemplate, a portion of the template can be conditionally evaluated using #if.

Log.Logger = new LoggerConfiguration()
    .WriteTo.Console(new ExpressionTemplate(
        "[{@t:HH:mm:ss} {@l:u3}{#if SourceContext is not null} ({SourceContext}){#end}] {@m}\n{@x}"))
    .CreateLogger();

// Produces log events like:
// [21:21:45 INF] Starting up
// [21:21:46 INF (Sample.Program)] Firing engines

The block between the {#if <expr>} and {#end} directives will only appear in the output if <expr> is true - in the example, events with a SourceContext include this in parentheses, while those without, don't.

It's important to notice that the directive requires a Boolean true before the conditional block will be evaluated. It wouldn't be sufficient in this case to write {#if SourceContext}, since no values other than true are considered "truthy".

The syntax supports {#if <expr>}, chained {#else if <expr>}, {#else}, and {#end}, with arbitrary nesting.

Repetition

If a log event includes structured data in arrays or objects, a template block can be repeated for each element or member using #each/in (newlines, double quotes and construction of the ExpressionTemplate omitted for clarity):

{@l:w4}: {SourceContext}
      {#each s in Scope}=> {s}{#delimit} {#end}
      {@m}
{@x}

This example uses the optional #delimit to add a space between each element, producing output like:

info: Sample.Program
      => Main => TextFormattingExample
      Hello, world!

When using {#each <name> in <expr>} over an object, such as the built-in @p (properties) object, <name> will be bound to the names of the properties of the object.

To get to the values of the properties, use a second binding:

{#each k, v in @p}{k} = {v}{#delimit},{#end}

This example, if an event has three properties, will produce output like:

Account = "nblumhardt", Cart = ["Tea", "Coffee"], Powerup = 42

The syntax supports {#each <name>[, <name>] in <expr>}, an optional {#delimit} block, and finally an optional {#else} block, which will be evaluated if the array or object is empty.

Recipes

Trim down SourceContext to a type name only:

Substring(SourceContext, LastIndexOf(SourceContext, '.') + 1)

This expression takes advantage of LastIndexOf() returning -1 when no . character appears in SourceContext, to yield a startIndex of 0 in that case.

Write not-referenced context properties (only if there are any):

{#if rest(true) <> {}} <Context: {rest(true)}>{#end}

Access a property with a non-identifier name:

@p['some name']

Any structured value, including the built-in @p, can be indexed by string key. This means that User.Name and User['Name'] are equivalent, for example.

Access a property with inconsistent casing:

ElementAt(@p, 'someName') ci

ElementAt() is a function-call version of the [] indexer notation, which means it can accept the ci case-insensitivity modifier.

Format events as newline-delimited JSON (template, embedded in C# or JSON):

{ {Timestamp: @t, Username: User.Name} }\n

This output template shows the use of a space between the opening { of a hole, and the enclosed object literal with Timestamp and Username fields. The object will be formatted as JSON. The trailing \n is a C# or JSON newline literal (don't escape this any further, as it's not part of the output template syntax).

Working with the raw API

The package provides the class SerilogExpression in the Serilog.Expressions namespace for working with expressions.

if (SerilogExpression.TryCompile("RequestPath like '/health%'", out var compiled, out var error)
{
    // `compiled` is a function that can be executed against `LogEvent`s:
    var result = compiled(someEvent);

    // `result` will contain a `LogEventPropertyValue`, or `null` if the result of evaluating the
    // expression is undefined (for example if the event has no `RequestPath` property).
    if (result is ScalarValue value &&
        value.Value is bool matches &&
        matches)
    {
        Console.WriteLine("The event matched.");
    }
}
else
{
    // `error` describes a syntax error.
    Console.WriteLine($"Couldn't compile the expression; {error}.");
}

Compiled expression delegates return LogEventPropertyValue because this is the most convenient type to work with in many Serilog scenarios (enrichers, sinks, ...). To convert the result to plain-old-.NET-types like string, bool, Dictionary<K,V> and Array, use the functions in the Serilog.Expressions.ExpressionResult class:

    var result = compiled(someEvent);

    // `true` only if `result` is a scalar Boolean `true`; `false` otherwise:
    if (ExpressionResult.IsTrue(result))
    {
        Console.WriteLine("The event matched.");
    }

Implementing user-defined functions

User-defined functions can be plugged in by implementing static methods that:

• Return LogEventPropertyValue?,
• Have arguments of type LogEventPropertyValue? or LogEvent,
• If the ci modifier is supported, accept a StringComparison, and
• If culture-specific formatting or comparisons are used, accepts an IFormatProvider.

For example:

public static class MyFunctions
{
    public static LogEventPropertyValue? IsHello(
        StringComparison comparison,
        LogEventPropertyValue? maybeHello)
    {
        if (maybeHello is ScalarValue sv && sv.Value is string s)
            return new ScalarValue(s.Equals("Hello", comparison));

        // Undefined - argument was not a string.
        return null;
    }
}

In the example, IsHello('Hello') will evaluate to true, IsHello('HELLO') will be false, IsHello('HELLO') ci will be true, and IsHello(42) will be undefined.

User-defined functions are supplied through an instance of NameResolver:

var myFunctions = new StaticMemberNameResolver(typeof(MyFunctions));
var expr = SerilogExpression.Compile("IsHello(User.Name)", nameResolver: myFunctions);
// Filter events based on whether `User.Name` is `'Hello'` :-)

Acknowledgements

Includes the parser combinator implementation from Superpower, copyright Datalust, Superpower Contributors, and Sprache Contributors; licensed under the Apache License, 2.0.