Add polyfills and centralize null checks

src/Fusillade/ArgumentExceptionHelper.cs

@@ -0,0 +1,52 @@
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
+// Licensed to ReactiveUI and Contributors under one or more agreements.
+// ReactiveUI and Contributors licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
+
+namespace Fusillade.Helpers;
+
+/// <summary>
+/// Provides helper methods for argument validation.
+/// These methods serve as polyfills for ArgumentExceptionHelper.ThrowIfNull and related methods
+/// that are only available in newer .NET versions.
+/// </summary>
+[ExcludeFromCodeCoverage]
+internal static class ArgumentExceptionHelper
+{
+    /// <summary>
+    /// Throws an <see cref="ArgumentNullException"/> if <paramref name="argument"/> is null.
+    /// </summary>
+    /// <param name="argument">The reference type argument to validate as non-null.</param>
+    /// <param name="paramName">The name of the parameter with which <paramref name="argument"/> corresponds.</param>
+    public static void ThrowIfNull([NotNull] object? argument, [CallerArgumentExpression(nameof(argument))] string? paramName = null)
+    {
+        if (argument is null)
+        {
+            throw new ArgumentNullException(paramName);
+        }
+    }
+
+    /// <summary>
+    /// Throws an exception if <paramref name="argument"/> is null or empty.
+    /// </summary>
+    /// <param name="argument">The string argument to validate as non-null and non-empty.</param>
+    /// <param name="paramName">The name of the parameter with which <paramref name="argument"/> corresponds.</param>
+    /// <exception cref="ArgumentNullException"><paramref name="argument"/> is null.</exception>
+    /// <exception cref="ArgumentException"><paramref name="argument"/> is empty.</exception>
+    public static void ThrowIfNullOrEmpty([NotNull] string? argument, [CallerArgumentExpression(nameof(argument))] string? paramName = null)
+    {
+        if (argument is null)
+        {
+            throw new ArgumentNullException(paramName);
+        }
+
+        if (argument.Length == 0)
+        {
+            throw new ArgumentException("The value cannot be an empty string.", paramName);
+        }
+    }
+}

src/Fusillade/Builder/FusilladeSplatBuilderExtensions.cs

@@ -1,10 +1,11 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to ReactiveUI and Contributors under one or more agreements.
 // ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
 using System;
 using Fusillade;
+using Fusillade.Helpers;
 
 namespace Splat.Builder;
 
@@ -20,14 +21,7 @@ public static class FusilladeSplatBuilderExtensions
     /// <returns>The App Instance for Chaining.</returns>
     public static IAppInstance CreateFusilladeNetCache(this IAppInstance builder)
     {
-#if NET8_0_OR_GREATER
-        ArgumentNullException.ThrowIfNull(builder);
-#else
-        if (builder is null)
-        {
-            throw new ArgumentNullException(nameof(builder));
-        }
-#endif
+        ArgumentExceptionHelper.ThrowIfNull(builder);
 
         NetCache.CreateDefaultInstances(builder.Current);
         return builder;

src/Fusillade/ConcatenateMixin.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to ReactiveUI and Contributors under one or more agreements.
 // ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.

src/Fusillade/IRequestCache.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to ReactiveUI and Contributors under one or more agreements.
 // ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.

src/Fusillade/InflightRequest.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to ReactiveUI and Contributors under one or more agreements.
 // ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.

src/Fusillade/LimitingHttpMessageHandler.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to ReactiveUI and Contributors under one or more agreements.
 // ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.

src/Fusillade/NetCache.cs

@@ -1,10 +1,11 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to ReactiveUI and Contributors under one or more agreements.
 // ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
 using System;
 using System.Net.Http;
+using Fusillade.Helpers;
 using Punchclock;
 using Splat;
 
@@ -62,14 +63,7 @@ public static LimitingHttpMessageHandler Speculative
         get => unitTestSpeculative ?? GetCurrent().GetService<LimitingHttpMessageHandler>("Speculative") ?? speculative;
         set
         {
-#if NET8_0_OR_GREATER
-            ArgumentNullException.ThrowIfNull(value, nameof(value));
-#else
-            if (value is null)
-            {
-                throw new ArgumentNullException(nameof(value));
-            }
-#endif
+            ArgumentExceptionHelper.ThrowIfNull(value, nameof(value));
 
             if (ModeDetector.InUnitTestRunner())
             {
@@ -92,14 +86,7 @@ public static HttpMessageHandler UserInitiated
         get => unitTestUserInitiated ?? GetCurrent().GetService<HttpMessageHandler>("UserInitiated") ?? userInitiated;
         set
         {
-#if NET8_0_OR_GREATER
-            ArgumentNullException.ThrowIfNull(value, nameof(value));
-#else
-            if (value is null)
-            {
-                throw new ArgumentNullException(nameof(value));
-            }
-#endif
+            ArgumentExceptionHelper.ThrowIfNull(value, nameof(value));
 
             if (ModeDetector.InUnitTestRunner())
             {
@@ -122,14 +109,7 @@ public static HttpMessageHandler Background
         get => unitTestBackground ?? GetCurrent().GetService<HttpMessageHandler>("Background") ?? background;
         set
         {
-#if NET8_0_OR_GREATER
-            ArgumentNullException.ThrowIfNull(value, nameof(value));
-#else
-            if (value is null)
-            {
-                throw new ArgumentNullException(nameof(value));
-            }
-#endif
+            ArgumentExceptionHelper.ThrowIfNull(value, nameof(value));
 
             if (ModeDetector.InUnitTestRunner())
             {
@@ -152,14 +132,7 @@ public static HttpMessageHandler Offline
         get => unitTestOffline ?? GetCurrent().GetService<HttpMessageHandler>("Offline") ?? offline;
         set
         {
-#if NET8_0_OR_GREATER
-            ArgumentNullException.ThrowIfNull(value, nameof(value));
-#else
-            if (value is null)
-            {
-                throw new ArgumentNullException(nameof(value));
-            }
-#endif
+            ArgumentExceptionHelper.ThrowIfNull(value, nameof(value));
 
             if (ModeDetector.InUnitTestRunner())
             {
@@ -183,14 +156,7 @@ public static OperationQueue OperationQueue
         get => unitTestOperationQueue ?? GetCurrent().GetService<OperationQueue>("OperationQueue") ?? operationQueue;
         set
         {
-#if NET8_0_OR_GREATER
-            ArgumentNullException.ThrowIfNull(value, nameof(value));
-#else
-            if (value is null)
-            {
-                throw new ArgumentNullException(nameof(value));
-            }
-#endif
+            ArgumentExceptionHelper.ThrowIfNull(value, nameof(value));
 
             if (ModeDetector.InUnitTestRunner())
             {

src/Fusillade/OfflineHttpMessageHandler.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to ReactiveUI and Contributors under one or more agreements.
 // ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.

src/Fusillade/Polyfills/CallerArgumentExpressionAttribute.cs

@@ -0,0 +1,40 @@
+// Copyright (c) 2016 - 2026 ReactiveUI and Contributors. All rights reserved.
+// Licensed to ReactiveUI and Contributors under one or more agreements.
+// ReactiveUI and Contributors licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+#if !NET5_0_OR_GREATER
+
+// Polyfill implementation adapted from Simon Cropp's Polyfill library
+// https://github.com/SimonCropp/Polyfill
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+namespace System.Runtime.CompilerServices;
+
+/// <summary>
+/// Indicates that a parameter captures the expression passed for another parameter as a string.
+/// </summary>
+[ExcludeFromCodeCoverage]
+[DebuggerNonUserCode]
+[AttributeUsage(AttributeTargets.Parameter)]
+internal sealed class CallerArgumentExpressionAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="CallerArgumentExpressionAttribute"/> class.
+    /// </summary>
+    /// <param name="parameterName">The name of the parameter whose expression should be captured as a string.</param>
+    public CallerArgumentExpressionAttribute(string parameterName) =>
+        ParameterName = parameterName;
+
+    /// <summary>
+    /// Gets the name of the parameter whose expression should be captured as a string.
+    /// </summary>
+    public string ParameterName { get; }
+}
+
+#else
+using System.Runtime.CompilerServices;
+
+[assembly: TypeForwardedTo(typeof(CallerArgumentExpressionAttribute))]
+#endif