Unit Testing ILogger with FakeLogger in C# — .NET 8 MSTest Example

A .NET 8 solution demonstrating how to unit test logging in C# with FakeLogger<T> from the Microsoft.Extensions.Diagnostics.Testing package. It shows how to assert on log messages and log levels written through ILogger<T> — without a mocking library, custom test doubles, or capturing console output.

Overview

The project shows how to verify logging behaviour in tests without mocking or capturing log output manually. FakeLogger<T> records all log calls and exposes them through LatestRecord, making it straightforward to assert on log messages and log levels.

Projects

Project Description
Fake.Data Contains the Calculate class with a Divide method that logs results and errors
Fake.Test MSTest project with two tests validating the logging behaviour of Calculate

The Class Under Test

Calculate.Divide(int dividend, int divisor) performs integer division and uses ILogger<Program> to:

  • Log the inputs and result on success
  • Log an Error-level entry with the exception on divide-by-zero
public int? Divide(int dividend, int divisor)

Returns null if division fails (e.g. division by zero).

Tests

Test What It Verifies
CanDevide After Divide(10, 2), the latest log message contains "Dividing answer 5"
CanLogError After Divide(10, 0), the latest log record has LogLevel.Error

Requirements

NuGet Packages

Fake.Data

Package Version
Microsoft.Extensions.Logging 8.0.0
Microsoft.Extensions.Logging.Abstractions 8.0.1
Microsoft.Extensions.Logging.Console 8.0.0

Fake.Test

Package Version
Microsoft.Extensions.Diagnostics.Testing 8.5.0
Microsoft.NET.Test.Sdk 17.9.0
MSTest.TestAdapter 3.3.1
MSTest.TestFramework 3.3.1
coverlet.collector 6.0.2

Build and Test

cd Fake
dotnet build
dotnet test

Project Structure

Fake/
├── Fake.Data/
│   ├── Calculate.cs        # Class under test — Divide method with ILogger
│   └── Fake.Data.csproj
├── Fake.Test/
│   ├── CalculateTest.cs    # Two MSTest tests using FakeLogger<T>
│   └── Fake.Test.csproj
└── Fake.sln

Fake.Data also contains a small console Program.cs that runs the calculator with a console logger, so you can see the same log messages outside the tests.

FAQ

Why use FakeLogger<T> instead of mocking ILogger? ILogger.Log is a generic method that is awkward to verify with mocking libraries. FakeLogger<T> is a real ILogger<T> implementation that records every entry, so tests can read LatestRecord and assert on the message, level, and exception directly.

Does FakeLogger work with xUnit or NUnit? Yes. It comes from the Microsoft.Extensions.Diagnostics.Testing package and does not depend on a test framework; this demo happens to use MSTest.

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