dotnet-cqrs/Svrnty.CQRS.Abstractions/Discovery/QueryMeta.cs

using System;
using System.Linq.Expressions;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;
using Svrnty.CQRS.Abstractions.Attributes;

namespace Svrnty.CQRS.Abstractions.Discovery;

public class QueryMeta : IQueryMeta
{
    private readonly string _name;

    public QueryMeta(Type queryType, Type serviceType, Type queryResultType)
    {
        QueryType = queryType;
        ServiceType = serviceType;
        QueryResultType = queryResultType;

        // Cache reflection and computed value once in constructor
        var nameAttribute = queryType.GetCustomAttribute<QueryNameAttribute>();
        _name = nameAttribute?.Name ?? queryType.Name.Replace("Query", string.Empty);

        // Build compiled delegate for handler invocation
        CompiledInvoker = BuildCompiledInvoker(serviceType, queryType, queryResultType);
    }

    private static Func<object, object, CancellationToken, Task<object?>> BuildCompiledInvoker(
        Type serviceType,
        Type queryType,
        Type resultType)
    {
        // Parameters: (object handler, object query, CancellationToken ct)
        var handlerParam = Expression.Parameter(typeof(object), "handler");
        var queryParam = Expression.Parameter(typeof(object), "query");
        var ctParam = Expression.Parameter(typeof(CancellationToken), "ct");

        // Cast handler to actual handler type
        var handlerCast = Expression.Convert(handlerParam, serviceType);

        // Cast query to actual query type
        var queryCast = Expression.Convert(queryParam, queryType);

        // Get HandleAsync method with correct return type (queries always return Task<TResult>)
        var expectedReturnType = typeof(Task<>).MakeGenericType(resultType);
        var handleMethod = serviceType.GetMethod("HandleAsync",
            BindingFlags.Public | BindingFlags.Instance,
            null,
            new[] { queryType, typeof(CancellationToken) },
            null);

        if (handleMethod == null || handleMethod.ReturnType != expectedReturnType)
            throw new InvalidOperationException($"HandleAsync method with return type {expectedReturnType} not found on {serviceType}");

        // Call handler.HandleAsync(query, ct) - returns Task<TResult>
        var methodCall = Expression.Call(handlerCast, handleMethod, queryCast, ctParam);

        // Use helper method to box the result
        var asyncHelperMethod = typeof(QueryMeta).GetMethod(nameof(InvokeAndBox), System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic)!
            .MakeGenericMethod(resultType);

        var invokeCall = Expression.Call(asyncHelperMethod, methodCall);

        // Compile to delegate
        var lambda = Expression.Lambda<Func<object, object, CancellationToken, Task<object?>>>(
            invokeCall,
            handlerParam,
            queryParam,
            ctParam);

        return lambda.Compile();
    }

    private static async Task<object?> InvokeAndBox<TResult>(Task<TResult> task)
    {
        var result = await task;
        return result;
    }

    public virtual string Name => _name;

    public virtual Type QueryType { get; }
    public virtual Type ServiceType { get; }
    public virtual Type QueryResultType { get; }
    public virtual string Category => "BasicQuery";

    public string LowerCamelCaseName
    {
        get
        {
            // Use virtual Name property so derived classes can override
            var name = Name;
            if (string.IsNullOrEmpty(name))
                return name;

            var firstLetter = char.ToLowerInvariant(name[0]);
            return $"{firstLetter}{name[1..]}";
        }
    }

    /// <summary>
    /// Compiled delegate for invoking the handler without reflection.
    /// Signature: (object handler, object query, CancellationToken ct) => Task<object?>
    /// </summary>
    public Func<object, object, CancellationToken, Task<object?>> CompiledInvoker { get; }
}