FakeLoggerFixture

Assembly: FEFF.TestFixtures.AspNetCore.dll
Namespace: FEFF.TestFixtures.AspNetCore
Source: FakeLoggerFixture.cs

Overview

The FakeLoggerFixture is an extension to AppManagerFixture.
It adds the Microsoft.Extensions.Logging.Testing.FakeLoggerProvider to the logging subsystem of the application under test, allowing to capture and verify application logs during test execution.

Key Features

Log Capture: Collects all log entries generated by the application during test execution
Log Verification: Access captured logs through FakeLogCollector for assertions and debugging
Automatic Registration: Automatically registers the fake logger provider with the application's DI container
Integration with AppManagerFixture: Works seamlessly with other ASP.NET Core fixtures

Basic Usage

First, define your test application entry point with logging:

// ASP.NET Core test application entry point (Program.cs)
public class Program
{
    public static void Main(string[] args)
    {
        var builder = WebApplication.CreateBuilder(args);
        var app = builder.Build();

        // Example endpoint that uses ILogger
        app.MapGet("/logging", (ILogger<Program> logger) =>
        {
            logger.LogInformation("log-1");
            logger.LogWarning("log-2");
            return Results.Ok();
        });

        app.Run();
    }
}

Then use the fixture in your tests:

using FEFF.TestFixtures.AspNetCore;
using Microsoft.Extensions.Logging.Testing;

public class LoggingTests
{
    protected IAppClientFixture Client { get; } = 
        TestContext.Current.GetFeffFixture<AppClientFixture<Program>>();

    // FakeLoggerFixture is automatically integrated with AppManagerFixture
    protected FakeLoggerFixture<Program> FakeLoggerFx { get; } = 
        TestContext.Current.GetFeffFixture<FakeLoggerFixture<Program>>();

    [Fact]
    public async Task Endpoint__should_log_messages()
    {
        // Act
        var resp = await Client.LazyValue.PostAsync("/logging", null, TestContext.Current.CancellationToken);
        resp.StatusCode.Should().Be(HttpStatusCode.OK);

        // Assert - verify captured logs
        var snapshot = FakeLoggerFx.Collector.GetSnapshot();

        JToken.FromObject(snapshot)
            .Should().ContainSubtree(
            """
            [
            {
                "Level": 2, // LogLevel.Information
                "Message": "log-1",
            },
            {
                "Level": 3, // LogLevel.Warning
                "Message": "log-2",
            }
            ]
            """);
    }
}

Type Arguments

Type Argument	Constraint	Description
`TEntryPoint`	`class`	The application entry point type. Typically the `Startup` or `Program` class.

Key Members

Member	Type	Description
`Collector`	`FakeLogCollector`	Gets the log collector for retrieving captured log entries. Use `GetSnapshot()` to access all captured logs.
`LoggerProvider`	`ILoggerProvider`	Gets the `FakeLoggerProvider` instance. Primarily used internally for registration.

Log Verification

The Microsoft.Extensions.Logging.Testing.FakeLogCollector provides methods to access captured logs:

Method	Description
`GetSnapshot()`	Returns an array of all captured log entries

Each log entry in the snapshot contains:

Property	Type	Description
`Category`	`string`	The logger category (typically the class name)
`Level`	`LogLevel`	The log level (Information, Warning, Error, etc.)
`Message`	`string`	The log message
`Exception`	`Exception`	Any exception associated with the log entry
`Scopes`	`ReadOnlyList<Dictionary<string, object>>`	Log scopes attached to the entry
`Timestamp`	`DateTimeOffset`	When the log was recorded
`LevelEnabled`	`bool`	Indicates if the log level was enabled

Example: Filtering Logs

var snapshot = FakeLoggerFx.Collector.GetSnapshot();

// Filter by log level
var warnings = snapshot.Where(x => x.Level == LogLevel.Warning).ToList();

// Filter by category
var myServiceLogs = snapshot.Where(x => x.Category.Contains("MyService")).ToList();

// Filter by message content
var errorLogs = snapshot.Where(x => x.Message.Contains("error")).ToList();

// Verify specific log exists
snapshot.Should().ContainSingle(x => 
    x.Level == LogLevel.Error && 
    x.Message == "Connection failed");

Link	Description
API: FakeLoggerFixture	API reference
FakeLoggerFixtureTests.cs	Unit tests for `FakeLoggerFixture`
API Integration Example	Integration test examples using `FakeLoggerFixture`
Microsoft.Extensions.Logging.Testing Documentation	Official documentation for fake logging types
ASP.NET Core Logging	ASP.NET Core logging overview

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