Useful snippets

Mock and callback a generic method with unavailable type using Moq

This is as much a note to self as a blog post. I needed to test a method that was dependent on a API with a callback action where T was scoped internal in the library that I was testing. Rather than making the type public I went ahead and tried to mock it anyway ūüėÄ

The code that we want to test is called from third party API like

await hubProxyFactory
            .Create(hubUrl, configureConnection, async p =>
                {
                    proxy = p;
                    await SendQueuedSubscriptions();
                    p.On<Message>("onEvent", OnEvent);
                },
                Reconnected, FaultedConnection, ConnectionComplete);

Message is a private type and OnEvent is of type Action<Message>. Its the OnEvent method we want to test. So we need to mock On<T> without knowing T and we need to save a reference to the OnEvent even though T is unknown. It looks like this

                    mock.Setup(x => x.On("onEvent", It.IsAny<Action<It.IsAnyType>>())).Callback(
                        (string e, MulticastDelegate callback) =>
                        {
                            onEvent = callback;
                        });
(more…)

Convention based Concurrency Management in Entity Framework Core

Who does not love convention over configuration? Whenever it makes sense I try to use it in my role as a system architect. It helps my programmers write more robust code out of the box.

Writing concurrency safe code is a corner stone in writing robust code today,  without it data quality can not be guaranteed. And when things go wrong you want to know who and when entities were updated so you can investigate what have gone wrong.

So what I did at my latest assignment was to force this onto the entities by convention using two markup interfaces ICreated and IUpdated.¬† (more…)

Using regex in Visual Studio

This is as much a note to self as a blog post :p

One developer in my team had done null == somevariable everywhere in our solution. I think unified code styles are important. So I wanted to remove them. Standard replace does not work here since we need to move the null after the variable name. So we need to use a regex.

Regex

We search for null and then any character as few as possible until we hit a space or a ending parenthesis. We then replace with group 1 ($1) which is the variable name == null and then group 2 ($2) which is either a space or a ending parenthesis.

This ensures above replace works with both if(null == somevar) and code like if(null == somevar && someotherstuff)

Update:
Here is a version on steroids that can do both equals and not equals
Regex v2

A proper thread safe memory cache

The Core 2.2 IMemoryCache is in theory thread safe. But if you call GetOrCreateAsync from multiple threads the factory Func will be called multiple times. Which could be a bad thing. A very simple fix to this is using a semaphore.

Declare it and only let one concurrent request be granted.

private readonly SemaphoreSlim _cacheLock = new SemaphoreSlim(1);

Let one request the cache and when done release the semaphore.

await _cacheLock.WaitAsync();
var data = await _cache.GetOrCreateAsync(key, entry => ...);
_cacheLock.Release();

Stub User.Identity.IsAuthenticated in ASP Core

I’m writing this article strictly because google do not have any obvious solutions in the hope it will be indexed and presented for fellow devs.

We use identity server and to make things easier in dev I want to stub it. ClaimsIdentity takes a second argument AuthenticationType. Its important you set this property. You can set it to what ever you like. Once set IsAuthenticated will return true.

            if (env.IsDevelopment())
            {
                app.Use(async (ctx, next) =>
                {
                   ctx.User = new ClaimsPrincipal(new ClaimsIdentity(new[] { new Claim(ClaimTypes.Name, "local") }, "Authenticated"));
                   await next.Invoke();
                });
            }

SQL CE and namespaces

We use SQL CE in our unit test project to test our EF6 queries. This works pretty well and Code first setup makes sure each test gets a fresh CE database to work with.

We ran into a problem where two EF types had the same table name but inside different schemas. CE does not support schemas and this resulted in naming conflicts when setting up database. My solution was to create a little convention.

public class BakeSchemaIntoTableNameConvention : IStoreModelConvention<EntityType>
{
    public void Apply(EntityType item, DbModel model)
    {
        var entitySet = model.StoreModel.Container.EntitySets.Single(es => es.ElementType == item);
        entitySet.Table = $"{entitySet.Schema}_{entitySet.Table}";
    }
}

It bakes the schema into the table name like dbo_MyTable. You add the convention like.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Conventions.Add<BakeSchemaIntoTableNameConvention>();

    modelBuilder.Configurations.AddFromAssembly(typeof (MyContext).Assembly);
}

You do not want to add this for production code so what I did was create a public event on the context. And call it from the OnModelCreating method.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    ModelCreating?.Invoke(this, modelBuilder);

    modelBuilder.Configurations.AddFromAssembly(typeof (MyContext).Assembly);
}

Call it from your Unittest project like.

ctx.ModelCreating += (s, e) => e.Conventions.Add<BakeSchemaIntoTableNameConvention>();

Microsoft.Rest.ServiceClient opt out retry

If you use Swagger generated REST proxies then you have probably come across the pretty new Microsoft.Rest namespace and namely the abstract class ServiceClient. I noticed a strange behavior when using clients that subclass this base class. The default behavior for this class is to retry when a 500 status code is returned.
I can not understand the reason for this being Opt out, its obvious a feature like this should be Opt in. So keep in mind when using this class you must always call SetRetryPolicy to disable the retry strategy.

var service = new MyService(uri, credentials);
service.SetRetryPolicy(new RetryPolicy(new HttpStatusCodeErrorDetectionStrategy(), 0));

Task.WhenAny with take predicate

Not a very common scenario but still it happens, you want to execute a request to several Services, and only one has the correct data. The built in Task.WhenAny only supports to await the first Task complete. That wont do if you want to wait for the first Task to complete that satisfy a specific condition.

Nito.AsyncEx is a helper library for Task programming, it comes with an extension method called OrderByCompletion. It takes a collection of Task<T> and return a new collection that will be ordered on completion status. Using this method its a simple task to create a WhenAny based on a predicate.

public async static Task<TResult> First<TResult>(this IEnumerable<Task<TResult>> source, Predicate<TResult> predicate)
{
    var ordered = source
        .OrderByCompletion();

    foreach (var task in ordered)
    {
        var result = await task;
        if (predicate(result)) return result;
    }

    throw new Exception("Sequence contains no matching element");
}

Used like

var result = await services
   .Select(s => s.GetFooAsync())
   .First(f => f.IsSuccess);
}

Entity framework 6 and fluent mapping

If you google for EF and fluent mapping this is the first hit you get which is not strange since its the official MSDN page about fluent mapping in EF6.

They only discuss overriding the OnModelCreating method and configure the mapping inline in that method. And this is the most common way of dealing with fluent mapping out there in the community. But there is a much better and seperated way of doing it which MSDN fail to show.

System.Data.Entity.ModelConfiguration.EntityTypeConfiguration

This little class is your salvation when working with Fluent mapping in large enterprise systems. Implement it like. (more…)

Seperate KO View formating and ViewModel state

A normal scenario with Knockout is that you want date and number observables to be formatted to the current culture of the client, this is easily implemented in a Knockout extender, something like.

ko.extenders.numeric = function(observable, options) {
    var textValue = null;
    return ko.computed({
        write: function(value) {
            textValue = value;
            var parsed = Number.parseFloat(value);
            if(!isNaN(parsed)) {
                observable(parsed);
            } else {
                observable(null);
            }
        },
        read: function() {
            var value = observable();
            if(value == null) {
                return textValue;
            }

            return value.toLocaleString(); //This  can be replaced with for example the Globalize plugin 
        }
    });
};

Use it like

this.amount = ko.observable().extend({ numeric: true });

The problem with this approach is that the ViewModel will no longer work against numbers it will work against formatted strings, so this.amount() will return a string instead of a number. This is very bad practice in a View/ViewModel seperation stand point. (more…)