Fix: Cryptic "No constructor" Error When ILoggerFactory Is Not Registered (Issue #1153) by jbogard · Pull Request #1157 · LuckyPennySoftware/MediatR

jbogard · 2026-02-22T19:10:40Z

Fix: Cryptic "No constructor" Error When ILoggerFactory Is Not Registered (Issue #1153)

Context

After upgrading to MediatR 14.x, users get a confusing exception when resolving IMediator:

No constructor for type 'MediatR.Licensing.LicenseAccessor' can be instantiated using services from the service container and default values.

Why it happens:

ServiceRegistrar.AddRequiredServices registers LicenseAccessor and LicenseValidator via bare generic TryAddSingleton<T>() (lines 473–474 in ServiceRegistrar.cs), with no factory lambda.
The DI container auto-constructs using the longest satisfiable constructor, but ILoggerFactory — required by both of LicenseAccessor's constructors — is not registered by the application.
serviceProvider.GetService<LicenseAccessor>() in CheckLicense throws (not returns null) because the type is registered but its constructor cannot be satisfied.
The error names an internal type users have never heard of, with no hint they need to call services.AddLogging().

Files to Modify

src/MediatR/Registration/ServiceRegistrar.cs — change how LicenseAccessor and LicenseValidator are registered (lines 473–474)
src/MediatR/MicrosoftExtensionsDI/MediatRServiceCollectionExtensions.cs — remove now-dead fallback code in CheckLicense (lines 65–70)

Implementation

1. Replace bare registrations with factory lambdas in `ServiceRegistrar.cs`

Before (lines 473–474):

services.TryAddSingleton<LicenseAccessor>();
services.TryAddSingleton<LicenseValidator>();

After:

services.TryAddSingleton<LicenseAccessor>(static sp =>
{
    var loggerFactory = sp.GetService<ILoggerFactory>()
        ?? throw new InvalidOperationException(
            "MediatR requires ILoggerFactory to be registered. " +
            "Call services.AddLogging() before services.AddMediatR().");
    var config = sp.GetService<MediatRServiceConfiguration>();
    return config != null
        ? new LicenseAccessor(config, loggerFactory)
        : new LicenseAccessor(loggerFactory);
});
services.TryAddSingleton<LicenseValidator>(static sp =>
{
    var loggerFactory = sp.GetService<ILoggerFactory>()
        ?? throw new InvalidOperationException(
            "MediatR requires ILoggerFactory to be registered. " +
            "Call services.AddLogging() before services.AddMediatR().");
    return new LicenseValidator(loggerFactory);
});

This produces a clear, actionable error at IMediator resolution time instead of the internal-type-naming cryptic message.

2. Simplify `CheckLicense` in `MediatRServiceCollectionExtensions.cs`

The ?? new LicenseAccessor(...) and ?? new LicenseValidator(...) fallbacks are now dead code: both types are always registered (via TryAddSingleton), so GetService<T>() will never return null — it either succeeds or throws. Simplify to GetRequiredService calls.

Before (lines 65–70):

var licenseAccessor = serviceProvider.GetService<LicenseAccessor>() ?? new LicenseAccessor(
    serviceProvider.GetRequiredService<MediatRServiceConfiguration>(),
    serviceProvider.GetRequiredService<ILoggerFactory>()
);
var licenseValidator = serviceProvider.GetService<LicenseValidator>()
                       ?? new LicenseValidator(serviceProvider.GetRequiredService<ILoggerFactory>());

After:

var licenseAccessor = serviceProvider.GetRequiredService<LicenseAccessor>();
var licenseValidator = serviceProvider.GetRequiredService<LicenseValidator>();

Verification

dotnet test test/MediatR.Tests/MediatR.Tests.csproj

All 166 existing tests pass (they already register NullLoggerFactory.Instance). The fix only changes behavior when ILoggerFactory is absent — producing a clear actionable message instead of the cryptic "No constructor" error.

Copilot

Pull request overview

This PR attempts to fix issue #1153 by providing clearer error messages when ILoggerFactory is not registered before AddMediatR() is called. The PR aims to replace the cryptic "No constructor for type 'MediatR.Licensing.LicenseAccessor' can be instantiated" error with an actionable message directing users to call services.AddLogging().

Changes:

Add factory lambdas for LicenseAccessor and LicenseValidator registration to detect missing ILoggerFactory early
Simplify CheckLicense method by removing fallback instantiation logic
Add Microsoft.Extensions.Logging import to ServiceRegistrar.cs

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/MediatR/Registration/ServiceRegistrar.cs	Adds factory lambdas for LicenseAccessor and LicenseValidator with explicit ILoggerFactory validation and clear error messages
src/MediatR/MicrosoftExtensionsDI/MediatRServiceCollectionExtensions.cs	Only contains a whitespace fix; intended simplification of CheckLicense method was not implemented

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Copilot · 2026-02-22T19:14:55Z

src/MediatR/Registration/ServiceRegistrar.cs

+        services.TryAddSingleton(static sp =>
+        {
+            var loggerFactory = sp.GetService<ILoggerFactory>()
+                ?? throw new InvalidOperationException(
+                    "MediatR requires ILoggerFactory to be registered. " +
+                    "Call services.AddLogging() before services.AddMediatR().");
+            var config = sp.GetService<MediatRServiceConfiguration>();
+            return config != null
+                ? new LicenseAccessor(config, loggerFactory)
+                : new LicenseAccessor(loggerFactory);
+        });


The TryAddSingleton call is missing the type parameter. This will not register LicenseAccessor in the DI container correctly. Change line 474 to: services.TryAddSingleton<LicenseAccessor>(static sp =>

Copilot · 2026-02-22T19:14:55Z

src/MediatR/Registration/ServiceRegistrar.cs

+        services.TryAddSingleton(static sp =>
+        {
+            var loggerFactory = sp.GetService<ILoggerFactory>()
+                ?? throw new InvalidOperationException(
+                    "MediatR requires ILoggerFactory to be registered. " +
+                    "Call services.AddLogging() before services.AddMediatR().");
+            return new LicenseValidator(loggerFactory);
+        });


The TryAddSingleton call is missing the type parameter. This will not register LicenseValidator in the DI container correctly. Change line 485 to: services.TryAddSingleton<LicenseValidator>(static sp =>

Copilot · 2026-02-22T19:14:56Z

src/MediatR/MicrosoftExtensionsDI/MediatRServiceCollectionExtensions.cs

@@ -66,7 +66,7 @@ internal static void CheckLicense(this IServiceProvider serviceProvider)
                serviceProvider.GetRequiredService<MediatRServiceConfiguration>(),
                serviceProvider.GetRequiredService<ILoggerFactory>()
            );
-            var licenseValidator = serviceProvider.GetService<LicenseValidator>() 
+            var licenseValidator = serviceProvider.GetService<LicenseValidator>()
                                   ?? new LicenseValidator(serviceProvider.GetRequiredService<ILoggerFactory>());


The PR description states that this method should be simplified to use GetRequiredService for both licenseAccessor and licenseValidator, removing the fallback logic with ?? new LicenseAccessor(...) and ?? new LicenseValidator(...). However, this change was not implemented - only a trailing whitespace was removed from line 69. The fallback logic on lines 65-70 should be replaced with: var licenseAccessor = serviceProvider.GetRequiredService<LicenseAccessor>(); and var licenseValidator = serviceProvider.GetRequiredService<LicenseValidator>(); as described in the PR description.

@copilot open a new pull request to apply changes based on this feedback

Copilot · 2026-02-22T19:15:45Z

@jbogard I've opened a new pull request, #1158, to work on those changes. Once the pull request is ready, I'll request review from you.

…ogic Co-authored-by: jbogard <104498+jbogard@users.noreply.github.com>

Simplify CheckLicense to use GetRequiredService instead of fallback constructors

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

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Copilot · 2026-02-22T19:32:04Z

src/MediatR/Registration/ServiceRegistrar.cs

+            var config = sp.GetService<MediatRServiceConfiguration>();
+            return config != null
+                ? new LicenseAccessor(config, loggerFactory)
+                : new LicenseAccessor(loggerFactory);


MediatRServiceConfiguration is always registered just above via services.TryAddSingleton(serviceConfiguration), so sp.GetService<MediatRServiceConfiguration>() should never be null here. This makes the conditional and the new LicenseAccessor(loggerFactory) branch effectively dead code; consider switching to GetRequiredService<MediatRServiceConfiguration>() and always using the (config, loggerFactory) constructor to simplify.

Suggested change

var config = sp.GetService<MediatRServiceConfiguration>();

return config != null

? new LicenseAccessor(config, loggerFactory)

: new LicenseAccessor(loggerFactory);

var config = sp.GetRequiredService<MediatRServiceConfiguration>();

return new LicenseAccessor(config, loggerFactory);

Copilot · 2026-02-22T19:32:04Z

src/MediatR/Registration/ServiceRegistrar.cs

+            var loggerFactory = sp.GetService<ILoggerFactory>()
+                ?? throw new InvalidOperationException(
+                    "MediatR requires ILoggerFactory to be registered. " +
+                    "Call services.AddLogging() before services.AddMediatR().");


New behavior is being introduced here (custom InvalidOperationException when ILoggerFactory is missing). There isn’t currently a test asserting this specific failure mode/message (the suite generally calls AddFakeLogging). Adding a focused test that omits logging and asserts the thrown message would help prevent regressions back to the cryptic "No constructor" error.

Copilot · 2026-02-22T19:32:04Z

src/MediatR/Registration/ServiceRegistrar.cs

+            var loggerFactory = sp.GetService<ILoggerFactory>()
+                ?? throw new InvalidOperationException(
+                    "MediatR requires ILoggerFactory to be registered. " +
+                    "Call services.AddLogging() before services.AddMediatR().");


The exception message says to call services.AddLogging() before services.AddMediatR(), but registration order doesn’t actually matter as long as ILoggerFactory is registered before the provider is built/resolution happens (e.g., tests call AddRequiredServices before AddFakeLogging). Consider rewording to avoid implying a required ordering (e.g., “Ensure ILoggerFactory is registered (services.AddLogging())”).

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

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test/MediatR.Tests/ServiceFactoryTests.cs

Applying fix

e1f6caa

jbogard linked an issue Feb 22, 2026 that may be closed by this pull request

No constructor for type 'MediatR.Licensing.LicenseAccessor' can be instantiated using services from the service container and default values. #1153

Closed

jbogard requested a review from Copilot February 22, 2026 19:11

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Copilot AI mentioned this pull request Feb 22, 2026

Simplify CheckLicense to use GetRequiredService instead of fallback constructors #1158

Merged

Copilot AI and others added 2 commits February 22, 2026 19:21

Simplify CheckLicense to use GetRequiredService instead of fallback l…

e0ae1ac

…ogic Co-authored-by: jbogard <104498+jbogard@users.noreply.github.com>

Merge pull request #1158 from LuckyPennySoftware/copilot/sub-pr-1157

25ffe81

Simplify CheckLicense to use GetRequiredService instead of fallback constructors

jbogard changed the title ~~Applying fix~~ Fix: Cryptic "No constructor" Error When ILoggerFactory Is Not Registered (Issue #1153) Feb 22, 2026

jbogard requested a review from Copilot February 22, 2026 19:29

Copilot started reviewing on behalf of jbogard February 22, 2026 19:29 View session

Explicitly registering with types

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Copilot AI reviewed Feb 22, 2026

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jbogard added this to the 14.1.0 milestone Feb 22, 2026

jbogard requested a review from Copilot February 22, 2026 19:33

Copilot started reviewing on behalf of jbogard February 22, 2026 19:34 View session

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test/MediatR.Tests/ServiceFactoryTests.cs Show resolved Hide resolved

jbogard merged commit aaefe9f into main Feb 22, 2026
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jbogard deleted the 1153-no-constructor-for-type-mediatrlicensinglicenseaccessor-can-be-instantiated-using-services-from-the-service-container-and-default-values branch February 22, 2026 19:51

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Fix: Cryptic "No constructor" Error When ILoggerFactory Is Not Registered (Issue #1153)#1157

Fix: Cryptic "No constructor" Error When ILoggerFactory Is Not Registered (Issue #1153)#1157
jbogard merged 5 commits intomainfrom
1153-no-constructor-for-type-mediatrlicensinglicenseaccessor-can-be-instantiated-using-services-from-the-service-container-and-default-values

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jbogard commented Feb 22, 2026

Fix: Cryptic "No constructor" Error When ILoggerFactory Is Not Registered (Issue #1153)

Context

Files to Modify

Implementation

1. Replace bare registrations with factory lambdas in ServiceRegistrar.cs

2. Simplify CheckLicense in MediatRServiceCollectionExtensions.cs

Verification

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1. Replace bare registrations with factory lambdas in `ServiceRegistrar.cs`

2. Simplify `CheckLicense` in `MediatRServiceCollectionExtensions.cs`