.NET 7 Preview 5 – Generic Math

In .NET 6 we previewed a feature known as Generic Math. Since then, we have made continuous improvements to the implementation and responded to various feedback from the community in order to ensure that relevant scenarios are possible and the necessary APIs are available.

If you missed out on the original blog post, Generic Math combines the power of generics and a new feature known as static virtuals in interfaces to allow .NET developers to take advantage of static APIs, including operators, from generic code. This means that you get all the power of generics, but now with the ability to constrain the input to number like types, so you no longer need to write or maintain many near identical implementations just to support multiple types. It also means that you get access to all your favorite operators and can use them from generic contexts. That is, you can now have static T Add<T>(T left, T right) where T : INumber<T> => left + right; where-as previously it would have been impossible to define.

Much like generics, this feature will see the most benefits by API authors where they can simplify the amount of code required they need to maintain. The .NET Libraries did just this to simplify the Enumerable.Min and Enumerable.Max APIs exposed as part of LINQ. Other developers will benefit indirectly as the APIs they consume may start supporting more types without the requirement for each and every numeric type to get explicit support. Once an API supports INumber<T> then it should work with any type that implements the required interface. All devs will likewise benefit from having a more consistent API surface and having more functionality available by default. For example, all types that implement IBinaryInteger<T> will support operations like + (Addition), - (Subtraction), << (Left Shift), and LeadingZeroCount.

Generic Math

Lets take a look at an example piece of code that computes a standard deviation. For those unfamiliar, this is a math function used in statistics that builds on two simpler methods: Sum and Average. It is basically used to determine how spread apart a set of values are.

The first method we’ll look at is Sum, which just adds a set of values together. The method takes in an IEnumerable<T> where T must be a type that implements the INumber<T> interface. It returns a TResult with a similar constraint (it must be a type that implements INumber<TResult>). Because two generic parameters are here, it is allowed to return a different type than it takes as an input. This means, for example, you can do Sum<int, long> which would allow summing the values of an int[] and returning a 64-bit result to help avoid overflow. TResult.Zero efficiently gives the value of 0 as a TResult and TResult.CreateChecked converts value from a T into a TResult throwing an OverflowException if it is too large or too small to fit in the destination format. This means, for example, that Sum<int, byte> would throw if one of the input values was negative or greater than 255.

public static TResult Sum<T, TResult>(IEnumerable<T> values)
    where T : INumber<T>
    where TResult : INumber<TResult>
{
    TResult result = TResult.Zero;

    foreach (var value in values)
    {
        result += TResult.CreateChecked(value);
    }

    return result;
}

The next method is Average, which just adds a set of values together (calls Sum) and then divides that by the number of values. It doesn’t introduce any additional concepts beyond what were used in Sum. It does show use of the division operator.

public static TResult Average<T, TResult>(IEnumerable<T> values)
    where T : INumber<T>
    where TResult : INumber<TResult>
{
    TResult sum = Sum<T, TResult>(values);
    return TResult.CreateChecked(sum) / TResult.CreateChecked(values.Count());
}

StandardDeviation is the last method, as indicated above it basically determines how far apart a set of values are. For example, { 0, 50, 100 } has a high deviation of 49.501; { 0, 5, 10 } on the other hand has a much lower deviation of just 4.5092. This method introduces a different constraint of IFloatingPointIeee754 which indicates the return type must be an IEEE 754 floating-point type such as double (System.Double) or float (System.Single). It introduces a new API CreateSaturating which explicitly saturates, or clamps, the value on overflow. That is, for byte.CreateSaturating<int>(value) it would convert -1 to 0 because -1 is less than the minimum value of 0. It would likewise convert 256 to 255 because 256 is greater than the maximum value of 255. Saturation is the default behavior for IEEE 754 floating-point types as they can represent positive and negative infinity as their respective minimum and maximum values. The only other new API is Sqrt which behaves just like Math.Sqrt or MathF.Sqrt and calculates the square root of the floating-point value.

public static TResult StandardDeviation<T, TResult>(IEnumerable<T> values)
    where T : INumber<T>
    where TResult : IFloatingPointIeee754<TResult>
{
    TResult standardDeviation = TResult.Zero;

    if (values.Any())
    {
        TResult average = Average<T, TResult>(values);
        TResult sum = Sum<TResult, TResult>(values.Select((value) => {
            var deviation = TResult.CreateSaturating(value) - average;
            return deviation * deviation;
        }));
        standardDeviation = TResult.Sqrt(sum / TResult.CreateSaturating(values.Count() - 1));
    }

    return standardDeviation;
}

These methods can then be used with any type that implements the required interfaces and in .NET 7 preview 5 we have 20 types that implement these interfaces out of the box. The following table gives a brief description of those types, the corresponding language keyword for C# and F# when that exists, and the primary generic math interfaces they implement. More details on these interfaces and why they exist are provided later on in the Available APIs section.

This means that out of the box users get a broad set of support for Generic Math. As the community adopts these interfaces for their own types, the support will continue to grow.

Types Without Language Support

Readers might note that there are a few types here that don’t have an entry in the C# Keyword or F# Keyword column. While these types exist and are supported fully in the BCL, languages like C# and F# do not provide any additional support for them today and so users may surprised when certain language features do not work with them. Some examples are that the language won’t provide support for literals (Int128 value = 0xF_FFFF_FFFF_FFFF_FFFF isn’t valid), constants (const Int128 Value = 0; isn’t valid), constant folding (Int128 value = 5; is evaluated at runtime, not at compile time), or various other functionality that is limited to types that have corresponding language keywords.

The types without language support are:

• System.Half is a 16-bit binary floating-point type that implements the IEEE 754 standard much like System.Double and System.Single. It was originally introduced in .NET 5
• System.Numerics.BigInteger is an arbitrary precision integer type and automatically grows to fit the value represented. It was originally introduced in .NET Framework 4.0
• System.Numerics.Complex can represent the expression a + bi where a and b are System.Double and i is the imaginary unit. It was originally introduced in .NET Framework 4.0
• System.Runtime.InteropServices.NFloat is a variable precision binary floating-point type that implements the IEEE 754 standard and much like System.IntPtr it is 32-bits on a 32-bit platform (equivalent to System.Single) and 64-bits on a 64-bit platform (equivalent to System.Double) It was originally introduced in .NET 6 and is primarily meant for interop purposes.
• System.Int128 is a 128-bit signed integer type. It is new in .NET 7
• System.UInt128 is a 128-bit unsigned integer type. It is new in .NET 7

Breaking Changes Since .NET 6

The feature that went out in .NET 6 was a preview and as such there have been several changes to the API surface based on community feedback. This includes, but is not limited to:

• Renaming System.IParseable to System.IParsable
• Moving all other new numeric interfaces to the System.Numerics namespace
• Introducing INumberBase so that types like System.Numerics.Complex can be represented
• Splitting the IEEE 754 specific APIs into their own IFloatingPointIeee754 interface so types like System.Decimal can be represented
• Moving various APIs lower in the type hierarchy such as the IsNaN or MaxNumber APIs
  • Many of the concepts will return a constant value or be a no-op on various type
  • Despite this, it is still important that they’re available, since the exact type of a generic is unknown and many of these concepts are important for more general algorithms

.NET API reviews are done in the open and are livestreamed for all to view and participate in. Past API review videos can be found on our YouTube channel.

The design doc for the Generic Math feature is available in the dotnet/designs repo on GitHub.

The corresponding PRs updating the document, general discussions around the feature, and links back to the relevant API reviews are also available.

Support in other languages

F# is getting support for static virtuals in interfaces as well and more details should be expected soon in the fsharp/fslang-design repo on GitHub.

A fairly 1-to-1 translation of the C# Sum method using the proposed F# syntax is expected to be:

let Sum<'T, 'TResult when 'T :> INumber<'T> and 'TResult :> INumber<'TResult>>(values : IEnumerable<'T>) =
    let mutable result = 'TResult.Zero
    for value in values do
        result <- result 'TResult.CreateChecked(value)
    result

Available APIs

Numbers and math are both fairly complex topics and the depth in which one can go is almost without limit. In programming there is often only a loose mapping to the math one may have learned in school and special rules or considerations may exist since execution happens in a system with limited resources. Languages therefore expose many operations that make sense only in the context of certain kinds of numbers or which exist primarily as a performance optimization due to how hardware actually works. The types they expose often have well-defined limits, an explicit layout of the data they are represented by, differing behaviors around rounding or conversions, and more.

Because of this there remains a need to both support numbers in the abstract sense while also still supporting programming specific constructs such as floating-point vs integer, overflow, unrepresentable results; and so it was important as part of designing this feature that the interfaces exposed be both fine-grained enough that users could define their own interfaces built on top while also being granular enough that they were easy to consume. To that extent, there are a few core numeric interfaces that most users will interact with such as System.Numerics.INumber and System.Numerics.IBinaryInteger; there are then many more interfaces that support these types and support developers defining their own numeric interfaces for their domain such as IAdditionOperators and ITrigonometricFunctions.

Which interfaces get used will be dependent on the needs of the declaring API and what functionality it relies on. There are a range of powerful APIs exposed to help users efficiently understand the value they’ve been and decide the appropriate way to work with it including handling edge cases (such as negatives, NaNs, infinities, or imaginary values), having correct conversions (including throwing, saturating, or truncating on overflow), and being extensible enough to version the interfaces moving forward by utilizing Default Interface Methods.

Numeric Interfaces

The types most users will interact with are the numeric interfaces. These define the core interfaces describing number-like types and the functionality available to them.

While there are a few different types here, most users will likely work directly with INumber<TSelf>. This roughly corresponds to what some users may recognize as a “real” number and means the value has a sign and well-defined order, making it IComparable. INumberBase<TSelf> convers more advanced concepts including “complex” and “imaginary” numbers.

Most of the other interfaces, such as IBinaryNumber, IFloatingPoint, and IBinaryInteger, exist because not all operations make sense for all numbers. That is, there are places where APIs only makes sense for values that are known to be binary-based and other places where APIs only make sense for floating-point types. The IAdditiveIdentity, IMinMaxValue, and IMultiplicativeIdentity interfaces exist to cover core properties of number like types. For IMinMaxValue in particular, it exists to allow access to the upper (MaxValue) and lower (MinValue) bounds of a type. Certain types like System.Numerics.BigInteger may not have such bounds and therefore do not implement this interface.

IFloatingPoint<TSelf> exists to cover both IEEE 754 types such as System.Double, System.Half, and System.Single as well as other types such as System.Decimal. The number of APIs provided by it is much lesser and it is expected most users who explicitly need a floating-point-like type will use IFloatingPointIeee754. There is not currently any interface to describe “fixed-point” types but such a definition could exist in the future if there is enough demand.

These interfaces expose APIs previously only available in System.Math, System.MathF, and System.Numerics.BitOperations. This means that functions like T.Sqrt(value) are now available to anything implementing IFloatingPointIeee754<T> (or more specifically the IRootFunctions<T> interface covered below).

Some of the core APIs exposed by each interface includes, but is not limited to the below.

Functions

The function interfaces define common mathematical APIs that may be more broadly applicable than to a specific numeric interface. They are currently all implemented by IFloatingPointIeee754 and may also get implemented by other relevant types in the future.

Parsing and Formatting

Parsing and formatting are core concepts in programming. They are typically used to support converting user input to a given type or to display a given type to the user.

Operators

Central to Generic Math is the ability to expose operators as part of an interface. .NET 7 provides the following interfaces which expose the core operators supported by most languages. This also includes new functionality in the form of user-defined checked operators and unsigned right shift.

User-Defined Checked Operators

User-defined checked operators allow a different implementation to be provided which will throw System.OverflowException rather than silently truncating their result. These alternative implementations are available to C# code by using the checked keyword or setting <CheckForOverflowUnderflow>true</CheckForOverflowUnderflow> in your project settings. The versions that truncate are available by using the unchecked keyword or ensuring CheckForOverflowUnderflow is false (this is the default experience for new projects).

Some types, such as floating-point types, may not have differing behavior as they saturate to PositiveInfinity and NegativeInfinity rather than truncating. BigInteger is another type that does not have differing behavior between the unchecked and checked versions of the operators as the type simply grows to fit the value. 3rd party types may also have their own unique behavior.

Developers can declare their own user-defined checked operators by placing the checked keyword after the operator keyword. For example, public static Int128 operator checked +(Int128 left, Int128 right) declares a checked addition operator and public static explicit operator checked int(Int128 value) declares a checked explicit conversion operator.

Unsigned Right Shift

Unsigned right shift (>>>) allows shifting to occur that doesn’t carry the sign. That is, for -8 >> 2 the result is -2 while -8 >>> 2 is +1073741822.

This is somewhat easier to visualize when looking at the hexadecimal or binary representation. For x >> y the sign of the value is preserved and so for positive values 0 is shifted in while for negative values 1 is shifted in instead. However, for x >>> y the sign of the value is ignored and 0 is always shifted in. This is similar to first casting the value to an unsigned type of the same sign and then doing the shift, that is it is similar to (int)((uint)x >> y) for int.

Closing

The amount of functionality now available in a generic context is quite large, allowing your code to be simpler, more maintainable, and more expressive. Generic Math will empower every developer to achieve more, and we are excited to see how you decide to utilize it!