NServiceBus 7 - Validate if the Azure Service Bus unicast destination exists

ValidateDestinationExistsBehavior.cs

// Usage:
//
// var endpointConfiguration = new EndpointConfiguration("MyEndpoint")
// ...
// var connectionString = Environment.GetEnvironmentVariable("AzureServiceBus_ConnectionString");
// var managementClient = new ManagementClient(connectionString);
// endpointConfiguration.Pipeline.Register(new ValidateDestinationExistsBehavior (managementClient), nameof(ValidateDestinationExistsBehavior));
//
using Microsoft.Azure.ServiceBus.Management;
using NServiceBus.Pipeline;
using NServiceBus.Routing;

class ValidateDestinationExistsBehavior : IBehavior<IOutgoingPhysicalMessageContext, IOutgoingPhysicalMessageContext>
{
    const int CacheLimit = 2500;
    readonly HashSet<string> KnownDestinations = new HashSet<string>();
    readonly ManagementClient ManagementClient;

    public ValidateDestinationExistsBehavior(ManagementClient managementClient)
    {
        ManagementClient = managementClient;
    }

    public async Task Invoke(IOutgoingPhysicalMessageContext context, Func<IOutgoingPhysicalMessageContext, Task> next)
    {
        var routingStrategy = context.RoutingStrategies.Single();
        var targetEndpoint = routingStrategy.Apply(new Dictionary<string, string>());

        var unicastAddress = targetEndpoint as UnicastAddressTag;

        if (unicastAddress is not null)
        {
            var destination = unicastAddress.Destination;

            if (!KnownDestinations.Contains(destination))
            {
                var exists = await ManagementClient.QueueExistsAsync(destination)
                    .ConfigureAwait(false);

                if (!exists)
                {
                    throw new InvalidOperationException("Destination does not exist.")
                    {
                        Data = { { "Destination", destination } }
                    };
                }

                // If we allow unbounded collection this could cause a memory leak. Ensuring we have check for a maximum number of items.
                if (KnownDestinations.Count > CacheLimit)
                {
                    throw new InvalidOperationException($"Known destination cache exceeds {CacheLimit} destinations.");
                }

                KnownDestinations.Add(destination);
            }
        }

        await next(context)
            .ConfigureAwait(false);
    }
}