Add support for .NET Standard 2.0

On .NET Standard 2.0, the library uses Microsoft.Bcl.AsyncInterfaces
as a polyfill for IAsyncEnumerable. Also, nullable reference types
and attributes are disabled on .NET Standard 2.0.

Author: TolikPylypchuk

Matchmaker/Linq/MatchResultExtensions.cs

@@ -1,7 +1,9 @@
 namespace Matchmaker.Linq;
 
 using System;
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
 using System.Diagnostics.CodeAnalysis;
+#endif
 using System.Threading.Tasks;
 
 /// <summary>
@@ -19,7 +21,9 @@ public static class MatchResultExtensions
     /// <returns>
     /// The result's value if it's successful, or the default one otherwise.
     /// </returns>
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
     [return: MaybeNull]
+#endif
     public static T GetValueOrDefault<T>(this MatchResult<T> result) =>
         result!.GetValueOrDefault(default(T));
 
@@ -32,8 +36,15 @@ public static T GetValueOrDefault<T>(this MatchResult<T> result) =>
     /// <returns>
     /// The result's value if it's successful, or the default one otherwise.
     /// </returns>
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
     [return: MaybeNull]
-    public static T GetValueOrDefault<T>(this MatchResult<T> result, [AllowNull] T defaultValue) =>
+#endif
+    public static T GetValueOrDefault<T>(
+        this MatchResult<T> result,
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
+        [AllowNull]
+#endif
+        T defaultValue) =>
         result.IsSuccessful ? result.Value : defaultValue;
 
     /// <summary>
@@ -50,7 +61,9 @@ public static T GetValueOrDefault<T>(this MatchResult<T> result, [AllowNull] T d
     /// <exception cref="ArgumentNullException">
     /// <paramref name="defaultValueProvider" /> is <see langword="null" />.
     /// </exception>
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
     [return: MaybeNull]
+#endif
     public static T GetValueOrDefault<T>(this MatchResult<T> result, Func<T> defaultValueProvider) =>
         defaultValueProvider != null
             ? result.IsSuccessful ? result.Value : defaultValueProvider()
@@ -68,7 +81,9 @@ public static T GetValueOrDefault<T>(this MatchResult<T> result, Func<T> default
     /// <exception cref="ArgumentNullException">
     /// <paramref name="exceptionProvider" /> is <see langword="null" />.
     /// </exception>
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
     [return: MaybeNull]
+#endif
     public static T GetValueOrThrow<T>(this MatchResult<T> result, Func<Exception> exceptionProvider) =>
         exceptionProvider != null
             ? result.IsSuccessful ? result.Value : throw exceptionProvider()
@@ -225,7 +240,12 @@ public static async Task<T> GetValueOrDefault<T>(this Task<MatchResult<T>> futur
     /// <exception cref="ArgumentNullException">
     /// <paramref name="futureResult" /> is <see langword="null" />.
     /// </exception>
-    public static async Task<T> GetValueOrDefault<T>(this Task<MatchResult<T>> futureResult, [AllowNull] T defaultValue)
+    public static async Task<T> GetValueOrDefault<T>(
+        this Task<MatchResult<T>> futureResult,
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
+        [AllowNull]
+#endif
+        T defaultValue)
     {
         var result = await (futureResult ?? throw new ArgumentNullException(nameof(futureResult)));
         return result.IsSuccessful ? result.Value! : defaultValue!;

Matchmaker/MatchException.cs

@@ -2,7 +2,6 @@ namespace Matchmaker;
 
 using System;
 using System.Diagnostics.CodeAnalysis;
-using System.Runtime.Serialization;
 
 /// <summary>
 /// Represents an exception which is thrown when a match expression hasn't found a successful pattern.
@@ -36,13 +35,4 @@ public MatchException(string message)
     public MatchException(string message, Exception innerException)
         : base(message, innerException)
     { }
-
-    /// <summary>
-    /// Initializes a new instance of the <see cref="MatchException" /> class.
-    /// </summary>
-    /// <param name="info">The serialization info.</param>
-    /// <param name="context">The streaming context.</param>
-    protected MatchException(SerializationInfo info, StreamingContext context)
-        : base(info, context)
-    { }
 }

Matchmaker/MatchResult.cs

@@ -1,6 +1,8 @@
 namespace Matchmaker;
 
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
 using System.Diagnostics.CodeAnalysis;
+#endif
 
 /// <summary>
 /// A static class which is used to create match results.
@@ -13,7 +15,11 @@ public static class MatchResult
     /// </summary>
     /// <param name="value">The value of the result.</param>
     /// <returns>A successful match result with the specified value.</returns>
-    public static MatchResult<T> Success<T>([AllowNull] T value) =>
+    public static MatchResult<T> Success<T>(
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
+        [AllowNull]
+#endif
+        T value) =>
         new(true, value);
 
     /// <summary>

Matchmaker/MatchResult_1.cs

@@ -1,7 +1,9 @@
 namespace Matchmaker;
 
 using System;
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
 using System.Diagnostics.CodeAnalysis;
+#endif
 
 using Matchmaker.Linq;
 
@@ -20,16 +22,23 @@ namespace Matchmaker;
     /// <summary>
     /// The value of the result if it's successful.
     /// </summary>
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
     [AllowNull]
     [MaybeNull]
+#endif
     private readonly T value;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="MatchResult{T}" /> class.
     /// </summary>
     /// <param name="isSuccessful">The value which indicates whether the match result is successful.</param>
     /// <param name="value">The value of the result, if it is successful.</param>
-    internal MatchResult(bool isSuccessful, [AllowNull] T value)
+    internal MatchResult(
+        bool isSuccessful,
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
+        [AllowNull]
+#endif
+        T value)
     {
         this.IsSuccessful = isSuccessful;
         this.value = value;
@@ -47,7 +56,9 @@ internal MatchResult(bool isSuccessful, [AllowNull] T value)
     /// <exception cref="InvalidOperationException">
     /// The result is not successful.
     /// </exception>
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
     [MaybeNull]
+#endif
     public T Value =>
         this.IsSuccessful
             ? this.value
@@ -85,7 +96,11 @@ public bool Equals(MatchResult<T> other) =>
     /// </summary>
     /// <returns>The hash code of this match result.</returns>
     public override int GetHashCode() =>
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
         HashCode.Combine(this.IsSuccessful, this.value);
+#else
+        this.value?.GetHashCode() ?? 0;
+#endif
 
     /// <summary>
     /// Returns the string representation of this match result.

Matchmaker/Match_2.cs

@@ -2,7 +2,10 @@ namespace Matchmaker;
 
 using System;
 using System.Collections.Generic;
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
 using System.Diagnostics.CodeAnalysis;
+#endif
+
 
 using Matchmaker.Linq;
 using Matchmaker.Patterns;
@@ -170,7 +173,9 @@ public Match<TInput, TOutput> Case<TType>(bool fallthrough, Func<TType, TOutput>
     /// <exception cref="MatchException">
     /// The match failed for all cases.
     /// </exception>
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
     [return: MaybeNull]
+#endif
     public TOutput ExecuteOn(TInput input) =>
         this.ExecuteNonStrict(input).GetValueOrThrow(() => new MatchException($"Could not match {input}"));
 

Matchmaker/Matchmaker.csproj

@@ -1,10 +1,23 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.1</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netstandard2.1;net8.0</TargetFrameworks>
     <LangVersion>latest</LangVersion>
     <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net8.0'))">
     <IsTrimmable>true</IsTrimmable>
+    <EnableTrimAnalyzer>true</EnableTrimAnalyzer>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <Nullable>disable</Nullable>
+    <NoWarn>$(NoWarn);nullable</NoWarn>
+  </PropertyGroup>
+
+  <PropertyGroup>
     <PackageTags>csharp pattern-matching functional-programming</PackageTags>
     <PackageId>Matchmaker</PackageId>
     <Product>Matchmaker</Product>
@@ -48,6 +61,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="9.0.5" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>

Matchmaker/Matchmaker.xml

@@ -52,5 +52,5 @@ protected AsyncPattern(string description) =>
     /// The description of this pattern, if it has one. Otherwise, the name of this pattern's type.
     /// </returns>
     public override string ToString() =>
-        String.IsNullOrEmpty(this.Description) ? base.ToString() : this.Description;
+        String.IsNullOrEmpty(this.Description) ? base.ToString()! : this.Description;
 }

Matchmaker/Patterns/Async/AsyncPattern_2.cs

@@ -19,7 +19,9 @@ internal sealed class CachingAsyncPattern<TInput, TMatchResult> : AsyncPattern<T
     /// <summary>
     /// The dictionary which holds this pattern's cache.
     /// </summary>
+#nullable disable
     private readonly ConcurrentDictionary<TInput, Task<MatchResult<TMatchResult>>> cache = new();
+#nullable enable
 
     /// <summary>
     /// The cached result for the <see langword="null" /> input.

Matchmaker/Patterns/Async/CachingAsyncPattern.cs

@@ -18,7 +18,9 @@ internal sealed class CachingPattern<TInput, TMatchResult> : Pattern<TInput, TMa
     /// <summary>
     /// The dictionary which holds this pattern's cache.
     /// </summary>
-    private readonly Dictionary<TInput, MatchResult<TMatchResult>> cache = new();
+#nullable disable
+    private readonly Dictionary<TInput, MatchResult<TMatchResult>> cache = [];
+#nullable enable
 
     /// <summary>
     /// The cached result for the <see langword="null" /> input.