如何将 object 转换为 IAsyncEnumerable？

Question

I'm building a logging filter for my SignalR hub. If the invocation is of type IAsyncEnumerable , I want to log every action that it yields.

public class LogFilter : IHubFilter
{
    private static readonly ILogger _log = Log.ForContext<LogFilter>();

    public async ValueTask<object> InvokeMethodAsync(
        HubInvocationContext invocationContext,
        Func<HubInvocationContext, ValueTask<object>> next)
    {
        if (invocationContext.HubMethod.ReturnType.GetGenericTypeDefinition() == typeof(IAsyncEnumerable<>))
        {
            var ret = await next(invocationContext);
            // Cast ret to IAsyncEnumerable.
            // Iterate over items, yield each item and log something.
            return ret;
        }
    }
}

I want to cast object to IAsyncEnumerable , but since IAsyncEnumerable is generic, I'm not sure how to do this.

Answer 1

Since the type parameter of IAsyncEnumerable<T> is covariant you can cast it to IAsyncEnumerable<object> in case the type of T is unknown.

So the easiest way to do this would be using is :

public class LogFilter : IHubFilter
{
    private static readonly ILogger _log = Log.ForContext<LogFilter>();

    public async ValueTask<object> InvokeMethodAsync(HubInvocationContext invocationContext, Func<HubInvocationContext, ValueTask<object>> next)
    {
        if (invocationContext.HubMethod.ReturnType is IAsyncEnumerable<object> enumerable)
        {
            // simply use the enumerable variable, it is already cast to IAsyncEnumerable<object>
            ...
        }
    }
}

Update

As @Dai pointed out, this approach is not working in case of IAsyncEnumerable<T> of value types. It should work fine if you are sure that you are only dealing with reference types.

The reason is explained here .