C# 6.0 – Null Propagation Operator

[7/30/2015] This article was written against a pre-release version of C# 6.0 but is still relevant. Be sure to check out the list of my five favorite C# 6.0 features for content written against the release!

One of the most common complaints about C# (especially among the F# crowd) is the amount of code required to properly deal with null values. Consider the following type definitions and invocation chain:

Type Definitions

class PriceBreak(int min, int max, double price)
{
  public int MinQuantity { get; } = min;
  public int MaxQuantity { get; } = max;
  public double Price { get; } = price;
}

class Product(IList<PriceBreak> priceBreaks)
{
  public IList<PriceBreak> PriceBreaks { get; } = priceBreaks;
}

Invocation Chain

var minPrice = product.PriceBreaks[0].Price;

As it’s written the preceding invocation chain is full of problems (we’ll ignore the potential ArgumentOutOfRangeException for this discussion). In fact, it can throw a NullReferenceException when any of the following values are null:

  • Product
  • Product.PriceBreaks
  • Product.PriceBreaks[0]

To properly avoid the exception we need to explicitly check for null. One possible approach would look like this:

double minPrice = 0;

if (product != null
    && product.PriceBreaks != null
    && product.PriceBreaks[0] != null)
{
  minPrice = product.PriceBreaks[0].Price;
}

Now we can be certain that we won’t encounter a NullReferenceException but we’ve greatly increased the complexity of the code. Rather than a single line, we have 5 that actually do something and of that, less than half are providing any value beyond making sure we don’t see an exception. C# 6.0’s null propagation operator (?) gives us the safety of checking for nulls while avoiding the verbose ceremony. The result is much cleaner and closely resembles the invocation chain shown earlier:

var minPrice = product?.PriceBreaks?[0]?.Price;

Introducing the null propagation operator allows us to chain each call while short-circuiting any nulls encountered along the way. In this case, the return type is double? (INullable) because anything preceding Price can be null. As such, we can either defer checking for null until we try to use the value or we can simply use the null coalescing operator with a default to coerce the type conversion as follows:

var minPrice = product?.PriceBreaks?[0]?.Price ?? 0;

One limitation of the null propagation operator is that it can’t work directly with delegate invocation. For instance, the following code is invalid:

var result = myDelegate?("someArgument");

Fortunately, there’s a workaround; rather than invoking the delegate directly as shown above, we can use invoke it via the Invoke method like this:

var result = myDelegate?.Invoke("someArgument");

The null propagation operator should eliminate a great deal of null reference checking code although I still believe F#’s approach of virtually eliminating null is superior. Honestly, my biggest concern about the null propagation operator is that it could encourage Law of Demeter violations but that’s a discussion for another day.

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