UnityGame/Library/PackageCache/com.unity.burst/Documentation~/csharp-hpc-overview.md

HPC# overview

Burst uses a high performance subset of C# called High Performance C# (HPC#).

Supported C# features in HPC#

HPC# supports most expressions and statements in C#. It supports the following:

Supported feature	Notes
Extension methods.
Instance methods of structs.
Unsafe code and pointer manipulation.
Loading from static read-only fields.	For more information, see the documentation on Static read-only fields and static constructors.
Regular C# control flows.	if else switch case for while break continue
ref and out parameters
fixed statements
Some IL opcodes.	cpblk initblk sizeof
DLLImport and internal calls.	For more information, see the documentation on DLLImport and internal calls.
try and finally keywords. Burst also supports the associated IDisposable patterns, using and foreach.	If an exception happens in Burst, the behavior is different from .NET. In .NET, if an exception occurs inside a try block, control flow goes to the finally block. However, in Burst, if an exception happens inside or outside a try block, the exception throws as if any finally blocks do not exist. Invoking foreach calls is supported by Burst, but there is a foreach edge case that burst currently does not support (see "Foreach and While" section for more details).
Strings and ProfilerMarker.	For more information, see the documentation on Support for Unity Profiler markers.
throw expressions.	Burst only supports simple throw patterns, for example, throw new ArgumentException("Invalid argument"). When you use simple patterns like this, Burst extracts the static string exception message and includes it in the generated code.
Strings and Debug.Log.	Only partially supported. For more information, see the documentation on String support and Debug.Log.

Burst also provides alternatives for some C# constructions not directly accessible to HPC#:

• Function pointers as an alternative to using delegates within HPC#
• Shared static to access static mutable data from both C# and HPC#

Exception expressions

Burst supports throw expressions for exceptions. Exceptions thrown in the editor can be caught by managed code, and are reported in the console window. Exceptions thrown in player builds will always cause the application to abort. Thus with Burst you should only use exceptions for exceptional behavior. To ensure that code doesn't end up relying on exceptions for things like general control flow, Burst produces the following warning on code that tries to throw within a method not attributed with [Conditional("ENABLE_UNITY_COLLECTIONS_CHECKS")]:

Burst warning BC1370: An exception was thrown from a function without the correct [Conditional("ENABLE_UNITY_COLLECTIONS_CHECKS")] guard. Exceptions only work in the editor and so should be protected by this guard

Foreach and While

Burst supports invoking foreach and while. However, there is an edge case which is currently unsupported - methods that take one or more generic collection parameters T: IEnumerable<U> and invoke foreach or while on at least one of the collections in the method body. To illustrate, the following example methods exemplify this limitation:

public static void IterateThroughConcreteCollection(NativeArray<int> list)
{
    foreach (var element in list)
    {
        // This works
    }
}

public static void IterateThroughGenericCollection<S>(S list) where S : struct, IEnumerable<int>
{
    foreach (var element in list)
    {
        // This doesn't work
    }
}

Note that the uppermost method IterateThroughConcreteCollection()'s parameter is specified to be a concrete collection type, in this case NativeArray<int>. Because it's concrete iterating through it inside the method will compile in Burst. In the method IterateThroughGenericCollection() below it, however, the parameter is specified to be a generic collection type S. Iterating through S inside the method will therefore not compile in Burst. It will instead throw the following error:

Can't call the method (method name) on the generic interface object type (object name). This may be because you are trying to do a foreach over a generic collection of type IEnumerable.

Unsupported C# features in HPC#

HPC# doesn't support the following C# features:

• Catching exceptions catch in a try/catch.
• Storing to static fields except via Shared Static
• Any methods related to managed objects, for example, string methods.