diff --git a/Directory.Packages.props b/Directory.Packages.props
index 03deeb6..e57d4bc 100644
--- a/Directory.Packages.props
+++ b/Directory.Packages.props
@@ -22,6 +22,7 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageVersion Include="NetEvolve.Arguments" Version="1.2.12" />
     <PackageVersion Include="NetEvolve.Logging.Abstractions" Version="1.1.0" />
+    <PackageVersion Include="NSubstitute" Version="5.1.0" />
     <PackageVersion Include="Verify.Xunit" Version="25.0.1" />
     <PackageVersion Include="xunit" Version="2.8.1" />
     <PackageVersion Include="xunit.extensibility.core" Version="2.8.1" />
diff --git a/src/NetEvolve.Logging.XUnit/XUnitLogger.cs b/src/NetEvolve.Logging.XUnit/XUnitLogger.cs
index c837f93..80f9340 100644
--- a/src/NetEvolve.Logging.XUnit/XUnitLogger.cs
+++ b/src/NetEvolve.Logging.XUnit/XUnitLogger.cs
@@ -45,7 +45,17 @@ public static XUnitLogger CreateLogger(
         IMessageSink messageSink,
         IExternalScopeProvider? scopeProvider = null,
         IXUnitLoggerOptions? options = null
-    ) => CreateLogger(messageSink, TimeProvider.System, scopeProvider, options);
+    )
+    {
+        Argument.ThrowIfNull(messageSink);
+
+        return new XUnitLogger(
+            message => _ = messageSink.OnMessage(new DiagnosticMessage(message)),
+            TimeProvider.System,
+            scopeProvider,
+            options
+        );
+    }
 
     /// <summary>
     /// Creates a new instance of <see cref="XUnitLogger"/>.
@@ -65,7 +75,12 @@ public static XUnitLogger CreateLogger(
         Argument.ThrowIfNull(messageSink);
         Argument.ThrowIfNull(timeProvider);
 
-        return new XUnitLogger(messageSink, timeProvider, scopeProvider, options);
+        return new XUnitLogger(
+            message => _ = messageSink.OnMessage(new DiagnosticMessage(message)),
+            timeProvider,
+            scopeProvider,
+            options
+        );
     }
 
     /// <summary>
@@ -81,7 +96,12 @@ public static XUnitLogger<T> CreateLogger<T>(
         IExternalScopeProvider? scopeProvider = null,
         IXUnitLoggerOptions? options = null
     )
-        where T : notnull => CreateLogger<T>(messageSink, scopeProvider, options);
+        where T : notnull
+    {
+        Argument.ThrowIfNull(messageSink);
+
+        return new XUnitLogger<T>(messageSink, TimeProvider.System, scopeProvider, options);
+    }
 
     /// <summary>
     /// Creates a new instance of <see cref="XUnitLogger{T}"/>.
@@ -98,7 +118,13 @@ public static XUnitLogger<T> CreateLogger<T>(
         IExternalScopeProvider? scopeProvider = null,
         IXUnitLoggerOptions? options = null
     )
-        where T : notnull => new XUnitLogger<T>(messageSink, timeProvider, scopeProvider, options);
+        where T : notnull
+    {
+        Argument.ThrowIfNull(messageSink);
+        Argument.ThrowIfNull(timeProvider);
+
+        return new XUnitLogger<T>(messageSink, timeProvider, scopeProvider, options);
+    }
 
     /// <summary>
     /// Creates a new instance of <see cref="XUnitLogger"/>.
@@ -111,7 +137,17 @@ public static XUnitLogger CreateLogger(
         ITestOutputHelper testOutputHelper,
         IExternalScopeProvider? scopeProvider = null,
         IXUnitLoggerOptions? options = null
-    ) => CreateLogger(testOutputHelper, TimeProvider.System, scopeProvider, options);
+    )
+    {
+        Argument.ThrowIfNull(testOutputHelper);
+
+        return new XUnitLogger(
+            testOutputHelper.WriteLine,
+            TimeProvider.System,
+            scopeProvider,
+            options
+        );
+    }
 
     /// <summary>
     /// Creates a new instance of <see cref="XUnitLogger"/>.
@@ -131,7 +167,7 @@ public static XUnitLogger CreateLogger(
         Argument.ThrowIfNull(testOutputHelper);
         Argument.ThrowIfNull(timeProvider);
 
-        return new XUnitLogger(testOutputHelper, timeProvider, scopeProvider, options);
+        return new XUnitLogger(testOutputHelper.WriteLine, timeProvider, scopeProvider, options);
     }
 
     /// <summary>
@@ -147,8 +183,12 @@ public static XUnitLogger<T> CreateLogger<T>(
         IExternalScopeProvider? scopeProvider = null,
         IXUnitLoggerOptions? options = null
     )
-        where T : notnull =>
-        CreateLogger<T>(testOutputHelper, TimeProvider.System, scopeProvider, options);
+        where T : notnull
+    {
+        Argument.ThrowIfNull(testOutputHelper);
+
+        return new XUnitLogger<T>(testOutputHelper, TimeProvider.System, scopeProvider, options);
+    }
 
     /// <summary>
     /// Creates a new instance of <see cref="XUnitLogger{T}"/>.
@@ -165,36 +205,22 @@ public static XUnitLogger<T> CreateLogger<T>(
         IExternalScopeProvider? scopeProvider = null,
         IXUnitLoggerOptions? options = null
     )
-        where T : notnull =>
-        new XUnitLogger<T>(testOutputHelper, timeProvider, scopeProvider, options);
-
-    private protected XUnitLogger(
-        ITestOutputHelper testOutputHelper,
-        TimeProvider timeProvider,
-        IExternalScopeProvider? scopeProvider,
-        IXUnitLoggerOptions? options
-    )
+        where T : notnull
     {
         Argument.ThrowIfNull(testOutputHelper);
         Argument.ThrowIfNull(timeProvider);
 
-        ScopeProvider = scopeProvider ?? NullExternalScopeProvider.Instance;
-        _timeProvider = timeProvider;
-        _options = options ?? XUnitLoggerOptions.Default;
-
-        _loggedMessages = [];
-
-        _writeToLog = testOutputHelper.WriteLine;
+        return new XUnitLogger<T>(testOutputHelper, timeProvider, scopeProvider, options);
     }
 
-    private protected XUnitLogger(
-        IMessageSink messageSink,
+    internal XUnitLogger(
+        Action<string> writeToAction,
         TimeProvider timeProvider,
         IExternalScopeProvider? scopeProvider,
         IXUnitLoggerOptions? options
     )
     {
-        Argument.ThrowIfNull(messageSink);
+        Argument.ThrowIfNull(writeToAction);
         Argument.ThrowIfNull(timeProvider);
 
         ScopeProvider = scopeProvider ?? NullExternalScopeProvider.Instance;
@@ -203,7 +229,7 @@ private protected XUnitLogger(
 
         _loggedMessages = [];
 
-        _writeToLog = message => _ = messageSink.OnMessage(new DiagnosticMessage(message));
+        _writeToLog = writeToAction;
     }
 
     /// <inheritdoc cref="ILogger.BeginScope{TState}(TState)"/>
diff --git a/src/NetEvolve.Logging.XUnit/XUnitLoggerProvider.cs b/src/NetEvolve.Logging.XUnit/XUnitLoggerProvider.cs
index 7a3556e..c12d41a 100644
--- a/src/NetEvolve.Logging.XUnit/XUnitLoggerProvider.cs
+++ b/src/NetEvolve.Logging.XUnit/XUnitLoggerProvider.cs
@@ -7,6 +7,7 @@
 using NetEvolve.Arguments;
 using NetEvolve.Logging.Abstractions;
 using Xunit.Abstractions;
+using Xunit.Sdk;
 
 [ProviderAlias("XUnit")]
 internal sealed class XUnitLoggerProvider
@@ -14,8 +15,7 @@ internal sealed class XUnitLoggerProvider
         ISupportExternalScope,
         IXUnitLoggerOptions
 {
-    private readonly ITestOutputHelper? _testOutputHelper;
-    private readonly IMessageSink? _messageSink;
+    private readonly Action<string?> _writeToAction;
 
     private readonly IExternalScopeProvider _scopeProvider = NullExternalScopeProvider.Instance;
     private readonly XUnitLoggerOptions _options;
@@ -55,7 +55,7 @@ public XUnitLoggerProvider(
         _timeProvider = timeProvider;
 
         _loggers = new ConcurrentDictionary<string, XUnitLogger>(StringComparer.Ordinal);
-        _messageSink = messageSink;
+        _writeToAction = message => messageSink.OnMessage(new DiagnosticMessage(message));
     }
 
     public XUnitLoggerProvider(
@@ -74,7 +74,7 @@ public XUnitLoggerProvider(
         _timeProvider = timeProvider;
 
         _loggers = new ConcurrentDictionary<string, XUnitLogger>(StringComparer.Ordinal);
-        _testOutputHelper = testOutputHelper;
+        _writeToAction = testOutputHelper.WriteLine;
     }
 
     /// <inheritdoc cref="ILoggerProvider.CreateLogger(string)"/>
@@ -84,30 +84,7 @@ public ILogger CreateLogger(string categoryName)
 
         return _loggers.GetOrAdd(
             $"{categoryName}_Default",
-            name =>
-            {
-                if (_testOutputHelper is not null)
-                {
-                    return XUnitLogger.CreateLogger(
-                        _testOutputHelper,
-                        _timeProvider,
-                        _scopeProvider,
-                        this
-                    );
-                }
-
-                if (_messageSink is not null)
-                {
-                    return XUnitLogger.CreateLogger(
-                        _messageSink,
-                        _timeProvider,
-                        _scopeProvider,
-                        this
-                    );
-                }
-
-                throw new InvalidOperationException();
-            }
+            name => new XUnitLogger(_writeToAction, _timeProvider, _scopeProvider, this)
         );
     }
 
@@ -116,30 +93,7 @@ public ILogger CreateLogger<T>()
         where T : notnull =>
         _loggers.GetOrAdd(
             $"{typeof(T).FullName}_Default",
-            _ =>
-            {
-                if (_testOutputHelper is not null)
-                {
-                    return XUnitLogger.CreateLogger<T>(
-                        _testOutputHelper,
-                        _timeProvider,
-                        _scopeProvider,
-                        this
-                    );
-                }
-
-                if (_messageSink is not null)
-                {
-                    return XUnitLogger.CreateLogger<T>(
-                        _messageSink,
-                        _timeProvider,
-                        _scopeProvider,
-                        this
-                    );
-                }
-
-                throw new InvalidOperationException();
-            }
+            _ => new XUnitLogger<T>(_writeToAction, _timeProvider, _scopeProvider, this)
         );
 
     /// <inheritdoc cref="ILoggerProvider.CreateLogger(string)"/>
diff --git a/src/NetEvolve.Logging.XUnit/XUnitLogger`T.cs b/src/NetEvolve.Logging.XUnit/XUnitLogger`T.cs
index f129233..81dcd3a 100644
--- a/src/NetEvolve.Logging.XUnit/XUnitLogger`T.cs
+++ b/src/NetEvolve.Logging.XUnit/XUnitLogger`T.cs
@@ -3,6 +3,7 @@
 using System;
 using Microsoft.Extensions.Logging;
 using Xunit.Abstractions;
+using Xunit.Sdk;
 
 /// <inheritdoc cref="XUnitLogger" />
 public sealed class XUnitLogger<T> : XUnitLogger, ILogger<T>
@@ -14,7 +15,14 @@ internal XUnitLogger(
         IExternalScopeProvider? scopeProvider,
         IXUnitLoggerOptions? options
     )
-        : base(messageSink, timeProvider, scopeProvider, options) { }
+        : base(
+            messageSink is not null
+                ? message => messageSink.OnMessage(new DiagnosticMessage(message))
+                : throw new ArgumentNullException(nameof(messageSink)),
+            timeProvider,
+            scopeProvider,
+            options
+        ) { }
 
     internal XUnitLogger(
         ITestOutputHelper testOutputHelper,
@@ -22,5 +30,20 @@ internal XUnitLogger(
         IExternalScopeProvider? scopeProvider,
         IXUnitLoggerOptions? options
     )
-        : base(testOutputHelper, timeProvider, scopeProvider, options) { }
+        : base(
+            testOutputHelper is not null
+                ? testOutputHelper.WriteLine
+                : throw new ArgumentNullException(nameof(testOutputHelper)),
+            timeProvider,
+            scopeProvider,
+            options
+        ) { }
+
+    internal XUnitLogger(
+        Action<string> writeToAction,
+        TimeProvider timeProvider,
+        IExternalScopeProvider? scopeProvider,
+        IXUnitLoggerOptions? options
+    )
+        : base(writeToAction, timeProvider, scopeProvider, options) { }
 }
diff --git a/tests/NetEvolve.Logging.XUnit.Tests.Unit/NetEvolve.Logging.XUnit.Tests.Unit.csproj b/tests/NetEvolve.Logging.XUnit.Tests.Unit/NetEvolve.Logging.XUnit.Tests.Unit.csproj
index aa354f3..4d394ed 100644
--- a/tests/NetEvolve.Logging.XUnit.Tests.Unit/NetEvolve.Logging.XUnit.Tests.Unit.csproj
+++ b/tests/NetEvolve.Logging.XUnit.Tests.Unit/NetEvolve.Logging.XUnit.Tests.Unit.csproj
@@ -14,6 +14,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="NSubstitute" />
     <PackageReference Include="xunit" />
     <PackageReference Include="xunit.runner.visualstudio">
       <PrivateAssets>all</PrivateAssets>
diff --git a/tests/NetEvolve.Logging.XUnit.Tests.Unit/XUnitLoggerOfTTests.cs b/tests/NetEvolve.Logging.XUnit.Tests.Unit/XUnitLoggerOfTTests.cs
new file mode 100644
index 0000000..f1cf8aa
--- /dev/null
+++ b/tests/NetEvolve.Logging.XUnit.Tests.Unit/XUnitLoggerOfTTests.cs
@@ -0,0 +1,41 @@
+namespace NetEvolve.Logging.XUnit.Tests.Unit;
+
+using System;
+using Xunit;
+using Xunit.Abstractions;
+
+public partial class XUnitLoggerOfTTests
+{
+    [Fact]
+    public void Constructor_WithMessageSinkNull_ThrowArgumentNullException()
+    {
+        IMessageSink messageSink = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "messageSink",
+            () => _ = new XUnitLogger<XUnitLoggerOfTTests>(messageSink, null!, null!, null!)
+        );
+    }
+
+    [Fact]
+    public void Constructor_WithTestOutputHelperNull_ThrowArgumentNullException()
+    {
+        ITestOutputHelper testOutputHelper = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "testOutputHelper",
+            () => _ = new XUnitLogger<XUnitLoggerOfTTests>(testOutputHelper, null!, null!, null!)
+        );
+    }
+
+    [Fact]
+    public void Constructor_WithWriteToActionNull_ThrowArgumentNullException()
+    {
+        Action<string> writeToAction = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "writeToAction",
+            () => _ = new XUnitLogger<XUnitLoggerOfTTests>(writeToAction, null!, null!, null!)
+        );
+    }
+}
diff --git a/tests/NetEvolve.Logging.XUnit.Tests.Unit/XUnitLoggerTests.cs b/tests/NetEvolve.Logging.XUnit.Tests.Unit/XUnitLoggerTests.cs
index 55b4671..c25d68d 100644
--- a/tests/NetEvolve.Logging.XUnit.Tests.Unit/XUnitLoggerTests.cs
+++ b/tests/NetEvolve.Logging.XUnit.Tests.Unit/XUnitLoggerTests.cs
@@ -1,7 +1,10 @@
 namespace NetEvolve.Logging.XUnit.Tests.Unit;
 
+using System;
 using Microsoft.Extensions.Logging;
+using NSubstitute;
 using Xunit;
+using Xunit.Abstractions;
 
 public partial class XUnitLoggerTests
 {
@@ -28,4 +31,144 @@ public void LogLevelToString_Theory_Expected(string expected, LogLevel logLevel)
             { "CRIT", LogLevel.Critical },
             { "NONE", LogLevel.None }
         };
+
+    [Fact]
+    public void CreateLogger_WithMessageSinkNull_ThrowsArgumentNullException()
+    {
+        IMessageSink messageSink = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "messageSink",
+            () => XUnitLogger.CreateLogger(messageSink)
+        );
+    }
+
+    [Fact]
+    public void CreateLogger_WithMessageSinkNullAndTimeProviderNull_ThrowsArgumentNullException()
+    {
+        IMessageSink messageSink = null!;
+        TimeProvider timeProvider = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "messageSink",
+            () => XUnitLogger.CreateLogger(messageSink, timeProvider)
+        );
+    }
+
+    [Fact]
+    public void CreateLogger_WithMessageSinkSubstituteAndTimeProviderNull_ThrowsArgumentNullException()
+    {
+        var messageSink = Substitute.For<IMessageSink>();
+        TimeProvider timeProvider = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "timeProvider",
+            () => XUnitLogger.CreateLogger(messageSink, timeProvider)
+        );
+    }
+
+    [Fact]
+    public void CreateLoggerGeneric_WithMessageSinkNull_ThrowsArgumentNullException()
+    {
+        IMessageSink messageSink = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "messageSink",
+            () => XUnitLogger.CreateLogger<XUnitLoggerTests>(messageSink)
+        );
+    }
+
+    [Fact]
+    public void CreateLoggerGeneric_WithMessageSinkNullAndTimeProviderNull_ThrowsArgumentNullException()
+    {
+        IMessageSink messageSink = null!;
+        TimeProvider timeProvider = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "messageSink",
+            () => XUnitLogger.CreateLogger<XUnitLoggerTests>(messageSink, timeProvider)
+        );
+    }
+
+    [Fact]
+    public void CreateLoggerGeneric_WithMessageSinkSubstituteAndTimeProviderNull_ThrowsArgumentNullException()
+    {
+        var messageSink = Substitute.For<IMessageSink>();
+        TimeProvider timeProvider = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "timeProvider",
+            () => XUnitLogger.CreateLogger<XUnitLoggerTests>(messageSink, timeProvider)
+        );
+    }
+
+    [Fact]
+    public void CreateLogger_WithTestOutputHelperNull_ThrowsArgumentNullException()
+    {
+        ITestOutputHelper testOutputHelper = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "testOutputHelper",
+            () => XUnitLogger.CreateLogger(testOutputHelper)
+        );
+    }
+
+    [Fact]
+    public void CreateLogger_WithTestOutputHelperNullAndTimeProviderNull_ThrowsArgumentNullException()
+    {
+        ITestOutputHelper testOutputHelper = null!;
+        TimeProvider timeProvider = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "testOutputHelper",
+            () => XUnitLogger.CreateLogger(testOutputHelper, timeProvider)
+        );
+    }
+
+    [Fact]
+    public void CreateLogger_WithTestOutputHelperSubstituteAndTimeProviderNull_ThrowsArgumentNullException()
+    {
+        var testOutputHelper = Substitute.For<ITestOutputHelper>();
+        TimeProvider timeProvider = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "timeProvider",
+            () => XUnitLogger.CreateLogger(testOutputHelper, timeProvider)
+        );
+    }
+
+    [Fact]
+    public void CreateLoggerGeneric_WithTestOutputHelperNull_ThrowsArgumentNullException()
+    {
+        ITestOutputHelper testOutputHelper = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "testOutputHelper",
+            () => XUnitLogger.CreateLogger<XUnitLoggerTests>(testOutputHelper)
+        );
+    }
+
+    [Fact]
+    public void CreateLoggerGeneric_WithTestOutputHelperNullAndTimeProviderNull_ThrowsArgumentNullException()
+    {
+        ITestOutputHelper testOutputHelper = null!;
+        TimeProvider timeProvider = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "testOutputHelper",
+            () => XUnitLogger.CreateLogger<XUnitLoggerTests>(testOutputHelper, timeProvider)
+        );
+    }
+
+    [Fact]
+    public void CreateLoggerGeneric_WithTestOutputHelperSubstituteAndTimeProviderNull_ThrowsArgumentNullException()
+    {
+        var testOutputHelper = Substitute.For<ITestOutputHelper>();
+        TimeProvider timeProvider = null!;
+
+        _ = Assert.Throws<ArgumentNullException>(
+            "timeProvider",
+            () => XUnitLogger.CreateLogger<XUnitLoggerTests>(testOutputHelper, timeProvider)
+        );
+    }
 }