How to use async/await with hub.On in SignalR client

Question

I have a .Net Windows Service (client) that's communicating with a SignalR Hub (server). Most of the client methods will take time to complete. When receiving a call from the server, how do I (or do I need to) wrap the target method/hub.On to avoid the warning:

"Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the await operator to the result of the call"

On the client, this is a sample of the start up / setup code:

IHubProxy _hub
string hubUrl = @"http://localhost/";

var connection = new HubConnection(hubUrl, hubParams);
_hub = connection.CreateHubProxy("MyHub");
await connection.Start();

_hub.On<Message>("SendMessageToClient", i => OnMessageFromServer(i.Id, i.Message));
_hub.On<Command>("SendCommandToClient", i => OnCommandFromServer(i.Id, i.Command));

Also on the client, this is a sample of the methods:

public static async Task<bool> OnMessageFromServer(string Id, string message)
{
    try
    {
        var result = await processMessage(message);  //long running task
    }
    catch (Exception ex)
    {
        throw new Exception("There was an error processing the message: ", ex);
    }
    return result;
}

public static async Task<bool> OnCommandFromServer(string Id, string command)
{
    try
    {
        var result = await processCommand(command);  //long running task
    }
    catch (Exception ex)
    {
        throw new Exception("There was an error processing the message: ", ex);
    }
    return result;
}

Ultimately, I think the _hub.On is registering the callback, not the actual execution (invoke) from the server. I think I need to get in the middle of the actual execution, await the result of On[X]FromServer and return the result.

************* updated example with corrected code*********************

IHubProxy _hub
string hubUrl = @"http://localhost/";

var connection = new HubConnection(hubUrl, hubParams);
_hub = connection.CreateHubProxy("MyHub");
await connection.Start();

//original
//_hub.On<Message>("SendMessageToClient", i => OnMessageFromServer(i.Id, i.Message));
//_hub.On<Command>("SendCommandToClient", i => OnCommandFromServer(i.Id, i.Command));

//new async 
_hub.On<Message>("SendMessageToClient", 
    async (i) => await OnMessageFromServer(i.Id, i.Message));

_hub.On<Message>("SendCommandToClient", 
    async (i) => await OnCommandFromServer(i.Id, i.Message));

//expanding to multiple parameters
_hub.On<Message, List<Message>, bool, int>("SendComplexParamsToClient", 
    async (p1, p2, p3, p4) => 
       await OnComplexParamsFromServer(p1.Id, p1.Message, p2, p3, p4));    

And then the target method signature would be something like

public static async Task<bool> OnComplexParamsFromServer(string id, string message,
                 List<Message> incommingMessages, bool eatMessages, int repeat)
{
    try
    {
        var result = await processCommand(message);  //long running task
        if (result) 
        {
             // eat up your incoming parameters
        }
    }
    catch (Exception ex)
    {
        throw new Exception("There was an error processing the message: ", ex);
    }
    return result;
}

Thanks to @AgentFire for the quick response!!!

Answer 1

This is a void-awaitable pattern, use it like this:

_hub.On<Message>("SendMessageToClient", async i => await OnMessageFromServer(i.Id, i.Message))