MetalamaConceptual documentationExtending MetalamaCustom metrics
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Custom metrics

Creating a custom metric

Step 1. Reference the Metalama SDK

Start by referencing Metalama.Framework.Sdk and Metalama.Framework, both privately:

<PackageReference Include="Metalama.Framework.Sdk" Version="$(MetalamaVersion)" PrivateAssets="all" />
<PackageReference Include="Metalama.Framework" Version="$(MetalamaVersion)" PrivateAssets="all" />

Step 2. Create the metric public API

Create a struct that implements the IMetric<T> generic interface. The type parameter should be the type of declaration to which the metric applies (e.g., IMethod or IField). Note that your metric struct can implement several generic instances of the IMetric<T> interface simultaneously.

Typically, your metric struct will have at least one public property. It will also have internal members to update the values, which will be utilized by the metric implementation.


The following example demonstrates a single-value metric.

public struct SyntaxNodesCount : IMetric<IMethodBase>, IMetric<INamedType>, IMetric<INamespace>, IMetric<ICompilation>
    public int Value { get; internal set; }

    internal void Add( in SyntaxNodesCount other ) => this.Value += other.Value;

Step 3. Create the metric implementation

A metric requires several implementation classes:

  1. Create a public class that derives from SyntaxMetricProvider<T>, where T is the metric type created above. In the constructor, pass an instance of the visitor created in the next step.

  2. Inside the metric provider class, create a nested visitor class that derives from SyntaxMetricProvider<T>.BaseVisitor. Override the relevant Visit methods in this class. This class forms the actual implementation of the metric. The visitor should recursively compute the metric for each syntax node in the syntax tree. The visitor is invoked by the metric provider (described below) for each member. The visitor should not implement aggregation at the type or namespace level.

  3. Implement the Aggregate method. This method is used to aggregate the metric from the level of members to the level of types, namespaces, or the whole project.

  4. Annotate this class with the MetalamaPlugInAttribute custom attribute.

Example: counting syntax nodes

The following example implements a metric that counts the number of nodes in a member.

public class SyntaxNodesCountMetricProvider : SyntaxMetricProvider<SyntaxNodesCount>
    public SyntaxNodesCountMetricProvider() : base( new Visitor() ) { }

    protected override void Aggregate( ref SyntaxNodesCount aggregate, in SyntaxNodesCount newValue )
        => aggregate.Add( newValue );

    private class Visitor : BaseVisitor
        public override SyntaxNodesCount DefaultVisit( SyntaxNode node )
            var metric = new SyntaxNodesCount { Value = 1 };

            foreach ( var child in node.ChildNodes() )
                metric.Add( this.Visit( child ) );

            return metric;

Consuming a custom metric

Custom metrics can be consumed in the usual manner.