Technical Note: Testing ILogger with NSubstitute
1. The Challenge
Directly verifying ILogger extension methods (e.g., _logger.LogError("...")) with NSubstitute is difficult. These methods resolve to a single, complex generic method on the ILogger interface, making standard Received() calls verbose and brittle.
void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter);
2. The Solution: Inspect the State Argument
The most robust solution is to inspect the state argument passed to the core Log<TState> method. When using structured logging, this state object is an IEnumerable<KeyValuePair<string, object>> that contains the full context of the log call, including the original message template.
3. Implementation: A Reusable Helper Method
To avoid repeating complex verification logic, create a static extension method for ILogger<T>.
LoggerTestExtensions.cs
using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Extensions.Logging;
using NSubstitute;
using FluentAssertions;
public static class LoggerTestExtensions
{
/// <summary>
/// Verifies that a log call with a specific level and message template was made.
/// </summary>
/// <typeparam name="T">The type of the logger's category.</typeparam>
/// <param name="logger">The ILogger substitute.</param>
/// <param name="expectedLogLevel">The expected log level (e.g., LogLevel.Error).</param>
/// <param name="expectedMessageTemplate">The exact message template string to verify.</param>
public static void VerifyLog<T>(this ILogger<T> logger, LogLevel expectedLogLevel, string expectedMessageTemplate)
{
// Find all calls to the core Log method with the specified LogLevel.
var logCalls = logger.ReceivedCalls()
.Where(call => call.GetMethodInfo().Name == "Log" &&
(LogLevel)call.GetArguments()[0]! == expectedLogLevel)
.ToList();
logCalls.Should().NotBeEmpty($"at least one log call with level {expectedLogLevel} was expected.");
// Check if any of the found calls match the message template.
var matchFound = logCalls.Any(call =>
{
var state = call.GetArguments()[2];
if (state is not IEnumerable<KeyValuePair<string, object>> kvp) return false;
return kvp.Any(p => p.Key == "{OriginalFormat}" && p.Value.ToString() == expectedMessageTemplate);
});
matchFound.Should().BeTrue($"a log call with the message template '{expectedMessageTemplate}' was expected but not found.");
}
}
4. How to Use in a Unit Test
Step 1: Arrange
In your test, create a substitute for ILogger<T> and inject it into your System Under Test (SUT).
// In your test class
private readonly ILogger<MyService> _logger;
private readonly MyService _sut;
public MyServiceTests()
{
_logger = Substitute.For<ILogger<MyService>>();
_sut = new MyService(_logger);
}
Step 2: Act Execute the method that is expected to produce a log entry.
[Fact]
public void DoWork_WhenErrorOccurs_ShouldLogError()
{
// Act
_sut.DoWorkThatFails();
// ...
}
Step 3: Assert
Use the VerifyLog extension method for a clean and readable assertion.
// Assert
_logger.VerifyLog(LogLevel.Error, "An error occurred while doing work for ID: {WorkId}");
}
5. Key Advantages of This Approach
- Robustness: It verifies the intent (the message template) rather than the final formatted string, making it resilient to changes in parameter values.
- Readability: The test assertion
_logger.VerifyLog(...)is clean, concise, and clearly states what is being tested. - Reusability: The extension method can be used across the entire test suite.
- Precision: It correctly targets the specific log level and message, avoiding ambiguity.