Refactor query abstractions into a separate project; implement ServiceProviderQueryDispatcher and FakeQueryHandler for enhanced query dispatch handling.

Odin.sln

@@ -122,6 +122,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Odin.Testing.NUnitUtility",
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Odin.Patterns.Commands.Abstractions", "Patterns\Commands.Abstractions\Odin.Patterns.Commands.Abstractions.csproj", "{7749636C-2D34-4E0B-81C9-9E89C5630858}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Odin.Patterns.Queries.Abstractions", "Patterns\Queries.Abstractions\Odin.Patterns.Queries.Abstractions.csproj", "{6B3B86A4-CB53-4E25-8681-31245BA33961}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -280,6 +282,10 @@ Global
 		{7749636C-2D34-4E0B-81C9-9E89C5630858}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{7749636C-2D34-4E0B-81C9-9E89C5630858}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{7749636C-2D34-4E0B-81C9-9E89C5630858}.Release|Any CPU.Build.0 = Release|Any CPU
+		{6B3B86A4-CB53-4E25-8681-31245BA33961}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{6B3B86A4-CB53-4E25-8681-31245BA33961}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{6B3B86A4-CB53-4E25-8681-31245BA33961}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{6B3B86A4-CB53-4E25-8681-31245BA33961}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(NestedProjects) = preSolution
 		{CE323D9C-635B-EFD3-5B3F-7CE371D8A86A} = {BF440C74-E223-3CBF-8FA7-83F7D164F7C3}
@@ -320,5 +326,6 @@ Global
 		{1491A6A6-ED85-4AD0-A6EE-57F4CAF3192B} = {605A7674-8EA4-458D-9FEB-A86C927AC0F0}
 		{46BCDE44-FA4E-4AE0-A6C0-9AFAE1BE09DF} = {51D0FF83-33F9-490F-A0CB-A32DC1CC48AF}
 		{7749636C-2D34-4E0B-81C9-9E89C5630858} = {605A7674-8EA4-458D-9FEB-A86C927AC0F0}
+		{6B3B86A4-CB53-4E25-8681-31245BA33961} = {605A7674-8EA4-458D-9FEB-A86C927AC0F0}
 	EndGlobalSection
 EndGlobal

Patterns/Queries/IQuery.cs → Patterns/Queries.Abstractions/IQuery.cs

@@ -2,8 +2,7 @@ namespace Odin.Patterns.Queries;
 
 /// <summary>
 /// Defines a query request that returns
-/// one or other for of data or a result.
+/// one or other form of data or a result.
 /// </summary>
 /// <typeparam name="TResult"></typeparam>
 public interface IQuery<out TResult> { }
-

Patterns/Queries.Abstractions/IQueryDispatcher.cs

@@ -0,0 +1,21 @@
+namespace Odin.Patterns.Queries;
+
+/// <summary>
+/// Dispatches queries to their registered query handlers.
+/// </summary>
+public interface IQueryDispatcher
+{
+    /// <summary>
+    /// Dispatches a query that returns a result
+    /// to its registered query handler.
+    /// </summary>
+    /// <typeparam name="TQuery"></typeparam>
+    /// <typeparam name="TQueryResult"></typeparam>
+    /// <param name="query"></param>
+    /// <param name="ct"></param>
+    /// <returns></returns>
+    Task<TQueryResult> DispatchAsync<TQuery, TQueryResult>(
+        TQuery query,
+        CancellationToken ct = default)
+        where TQuery : IQuery<TQueryResult>;
+}

Patterns/Queries/IQueryHandler.cs → Patterns/Queries.Abstractions/IQueryHandler.cs

@@ -1,11 +1,11 @@
 namespace Odin.Patterns.Queries;
 
 /// <summary>
-/// Defines the handling implementation for a command request that returns a Result.
+/// Defines the handling implementation for a query request that returns a result.
 /// </summary>
 /// <typeparam name="TQuery"></typeparam>
 /// <typeparam name="TQueryResult"></typeparam>
-public interface IQueryHandler<in TQuery, TQueryResult> 
+public interface IQueryHandler<in TQuery, TQueryResult>
     where TQuery : IQuery<TQueryResult>
 {
     /// <summary>
@@ -15,4 +15,4 @@ public interface IQueryHandler<in TQuery, TQueryResult>
     /// <param name="ct"></param>
     /// <returns></returns>
     Task<TQueryResult> HandleAsync(TQuery query, CancellationToken ct = default);
-}
+}

Patterns/Queries.Abstractions/Odin.Patterns.Queries.Abstractions.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+    <PropertyGroup>
+        <TargetFrameworks>net8.0;net9.0;net10.0</TargetFrameworks>
+        <ImplicitUsings>true</ImplicitUsings>
+        <Nullable>enable</Nullable>
+        <PackageIcon>icon.png</PackageIcon>
+        <PackageReadmeFile>README.md</PackageReadmeFile>
+        <RootNamespace>Odin.Patterns.Queries</RootNamespace>
+        <Description>All abstractions for the query dispatch pattern.
+
+        </Description>
+        <WarningsAsErrors>1591;1573;</WarningsAsErrors> <!-- Not to be removed. Documentation is required. -->
+    </PropertyGroup>
+
+    <ItemGroup>
+        <None Include="../../Assets/icon.png" Pack="true" PackagePath=""/>
+        <None Include="..\README.md" Pack="true" PackagePath=""/>
+    </ItemGroup>
+</Project>

Patterns/Queries/FakeQueryHandler.cs

@@ -0,0 +1,32 @@
+namespace Odin.Patterns.Queries;
+
+/// <summary>
+/// Returns the result passed in via the constructor.
+/// </summary>
+/// <typeparam name="TQuery"></typeparam>
+/// <typeparam name="TResult"></typeparam>
+public class FakeQueryHandler<TQuery, TResult>  : IQueryHandler<TQuery, TResult>  
+    where TQuery : IQuery<TResult>
+{
+    private readonly TResult _resultToReturn;
+
+    /// <summary>
+    /// Initialise to return 'result' on HandleAsync.
+    /// </summary>
+    /// <param name="result"></param>
+    public FakeQueryHandler(TResult result)
+    {
+        _resultToReturn = result;
+    }
+
+    /// <summary>
+    /// Does nothing.
+    /// </summary>
+    /// <param name="query"></param>
+    /// <param name="ct"></param>
+    /// <exception cref="NotImplementedException"></exception>
+    public async Task<TResult> HandleAsync(TQuery query, CancellationToken ct = default)
+    {
+        return await Task.FromResult(_resultToReturn);
+    }
+}

Patterns/Queries/Odin.Patterns.Queries.csproj

@@ -5,12 +5,23 @@
         <Nullable>enable</Nullable>
         <PackageIcon>icon.png</PackageIcon>
         <PackageReadmeFile>README.md</PackageReadmeFile>
-        <Description>
+        <Description>Implementation of a query dispatch pattern.
 
         </Description>
         <WarningsAsErrors>1591;1573;</WarningsAsErrors> <!-- Not to be removed. Documentation is required. -->
     </PropertyGroup>
 
+    <ItemGroup>
+        <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions"/>
+        <PackageReference Include="Microsoft.Extensions.Logging.Abstractions"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\..\DesignContracts\Core\Odin.DesignContracts.csproj" />
+        <ProjectReference Include="..\..\Logging\Core\Odin.Logging.csproj" />
+        <ProjectReference Include="..\Queries.Abstractions\Odin.Patterns.Queries.Abstractions.csproj" />
+    </ItemGroup>
+
     <ItemGroup>
         <None Include="../../Assets/icon.png" Pack="true" PackagePath=""/>
         <None Include="..\README.md" Pack="true" PackagePath=""/>

Patterns/Queries/ServiceProviderQueryDispatcher.cs

@@ -0,0 +1,105 @@
+using Microsoft.Extensions.DependencyInjection;
+using Odin.DesignContracts;
+using Odin.Logging;
+
+namespace Odin.Patterns.Queries;
+
+/// <summary>
+/// Dispatches queries by resolving their matching query handler from an <see cref="IServiceProvider"/>.
+/// </summary>
+public sealed class ServiceProviderQueryDispatcher : IQueryDispatcher
+{
+    private readonly IServiceProvider _serviceProvider;
+    private readonly ILoggerWrapper<ServiceProviderQueryDispatcher> _logger;
+
+    /// <summary>
+    /// Creates a new dispatcher.
+    /// </summary>
+    /// <param name="serviceProvider">The service provider used to resolve query handlers.</param>
+    /// <param name="logger">The logger used to record dispatch activity.</param>
+    public ServiceProviderQueryDispatcher(
+        IServiceProvider serviceProvider,
+        ILoggerWrapper<ServiceProviderQueryDispatcher> logger)
+    {
+        Precondition.RequiresNotNull(serviceProvider);
+        Precondition.RequiresNotNull(logger);
+
+        _serviceProvider = serviceProvider;
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public async Task<TQueryResult> DispatchAsync<TQuery, TQueryResult>(
+        TQuery query,
+        CancellationToken ct = default)
+        where TQuery : IQuery<TQueryResult>
+    {
+        Precondition.RequiresNotNull(query);
+
+        Type queryType = typeof(TQuery);
+        Type handlerInterfaceType = typeof(IQueryHandler<TQuery, TQueryResult>);
+
+        _logger.LogDebug(
+            "Dispatching query {QueryType} using handler interface {HandlerInterface}.",
+            FormatTypeName(queryType),
+            FormatTypeName(handlerInterfaceType));
+
+        IQueryHandler<TQuery, TQueryResult> handler =
+            ResolveHandler<IQueryHandler<TQuery, TQueryResult>>(queryType, handlerInterfaceType);
+
+        try
+        {
+            TQueryResult result = await handler.HandleAsync(query, ct).ConfigureAwait(false);
+
+            _logger.LogDebug(
+                "Query {QueryType} completed successfully using handler {HandlerType}.",
+                FormatTypeName(queryType),
+                FormatTypeName(handler.GetType()));
+
+            return result;
+        }
+        catch (OperationCanceledException ex) when (ct.IsCancellationRequested)
+        {
+            _logger.LogDebug(
+                ex,
+                "Query {QueryType} was cancelled while executing handler {HandlerType}.",
+                FormatTypeName(queryType),
+                FormatTypeName(handler.GetType()));
+            throw;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(
+                ex,
+                "Query {QueryType} failed while executing handler {HandlerType}.",
+                FormatTypeName(queryType),
+                FormatTypeName(handler.GetType()));
+            throw;
+        }
+    }
+
+    private THandler ResolveHandler<THandler>(Type queryType, Type handlerInterfaceType)
+        where THandler : class
+    {
+        THandler[] handlers = _serviceProvider.GetServices<THandler>().ToArray();
+
+        if (handlers.Length == 1)
+        {
+            return handlers[0];
+        }
+
+        string message = handlers.Length == 0
+            ? $"No query handler was registered for query type '{FormatTypeName(queryType)}'. " +
+              $"Expected handler interface: '{FormatTypeName(handlerInterfaceType)}'."
+            : $"Multiple query handlers were registered for query type '{FormatTypeName(queryType)}'. " +
+              $"Expected handler interface: '{FormatTypeName(handlerInterfaceType)}'. Found {handlers.Length} registrations.";
+
+        _logger.LogError(message);
+        throw new InvalidOperationException(message);
+    }
+
+    private static string FormatTypeName(Type type)
+    {
+        return type.FullName ?? type.Name;
+    }
+}