· c net functional-programming functional-collection-parameters

Functional Collection Parameters: Handling the null collection

One of the interesting cases where I’ve noticed we tend to avoid functional collection parameters in our code base is when there’s the possibility of the collection being null.

The code is on the boundary of our application’s interaction with another service so it is actually a valid scenario that we could receive a null collection.

When using extension methods, although we wouldn’t get a null pointer exception by calling one on a null collection, we would get a ‘source is null’ exception when the expression is evaluated so we need to protect ourself against this.

As a result of defending against this scenario we have quite a lot of code that looks like this:


public IEnumerable<Foo> MapFooMessages(IEnumerable<FooMessage> fooMessages)
{
	var result = new List<Foo>();
	if(fooMessagaes != null)
	{
		foreach(var fooMessage in fooMessages)
		{
			result.Add(new Foo(fooMessage));
		}
	}
	return result;
}

The method that we want to apply here is ‘Select’ and even though we can’t just apply that directly to the collection we can still make use of it.


private IEnumerable<Foo> MapFooMessages(IEnumerable<FooMessage> fooMessages)
{
	if(fooMessages == null) return new List<Foo>();
	return fooMessages.Select(eachFooMessage => new Foo(eachFooMessage));
}

There’s still duplication doing it this way though so I pulled it up into a ‘SafeSelect’ extension method:


public static class ICollectionExtensions
{
       public static IEnumerable<TResult> SafeSelect<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector)
       {
               return source == null ? new List<TResult>() : source.Select(selector) ;
       }
}

We can then make use of this extension method like so:


private IEnumerable<Foo> MapFooMessages(IEnumerable<FooMessage> fooMessages)
{
	return fooMessages.SafeSelect(eachFooMessage => new Foo(fooMessage));
}

The extension method is a bit different to the original way that we did this as I’m not explicitly converting the result into a list at the end which means that it will only be evaluated when the data is actually needed.

In this particular case I don’t think that decision will have a big impact but it’s something interesting to keep in mind.

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