Class TypeHash

Produces a stable type hash for a Type based on its layout in memory and how that memory should be interpreted.

Inheritance

object

TypeHash

Namespace: Unity.Entities

Assembly: Unity.Entities.dll

Syntax

public class TypeHash

Remarks

You can rename field members and this hash isn't affected. However, if a field's type changes, this hash changes, because the interpretation of the underlying memory will have contextually changed.

The purpose of the stable type hash is to provide a version linking serialized component data to its runtime representation.

As such, the type hash has a few requirements:

• R0: TypeHashes apply only to types that Unity.Entities serializes. The internals of a UnityEngine.Object reference contained in a component mustn't have an effect on the type hash. You can safely change the internals of the UnityEngine.Object reference because they're serialized outside of Unity.Entities.
• R1: Types with the same data layout but are different types should have different hashes.
• R2: If a type's data layout changes, so should the type hash. This includes:
  • A nested field's data layout changes (for example, a new member added)
  • FieldOffsets, explicit size, or pack alignment are changed
  • Different types of the same width swap places (for example if a uint swaps with an int)
  • Note: Unity can't detect if fields of the same type swap (for example, mInt1 swaps with mInt2) This is a semantic difference which you should increase your component [TypeVersion(1)] attribute. You shouldn't try to hash field names to handle this, because you can't control all field names. Also, field names have no effect serialization and doesn't affect hashes.
• R3: You should version the hash in case the semantics of a type change, but the data layout is unchanged
• R4: DOTS Runtime relies on hashes generated from the Editor (used in serialized data) and hashes generated during compilation. These hashes must match. This rule exists because of the following:
  • Tiny swapa out assemblies for 'tiny' versions, which means you should avoid any hashing using the AssemblyName or handle it specially for the known swapped assemblies.
    • This means you should avoid Type.AssemblyQualifiedName in hashes, but as well, closed-form generic types include the assembly qualified name for GenericArguments in Type.FullName which causes issues. For example, typeof(ComponentWithGeneric.GenericField).FullName == Unity.Entities.ComponentWithGeneric.GenericField1[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]`
  • System.Reflection and Mono.Cecil provide different Type names (they use a different format). Generating hashes from System.Reflection to match hashes using Mono.Cecil must account for this difference.

Methods

Name	Description
CalculateMemoryOrdering(Type, out bool, Dictionary<Type, ulong>)	Calculates a MemoryOrdering for the input type.
CalculateStableTypeHash(Type, Dictionary<Type, ulong>)	Calculates a stable type hash for the input type.
CalculateStableTypeHash(Type, IEnumerable<CustomAttributeData>, Dictionary<Type, ulong>)	Calculates a stable type hash for the input type.
CombineFNV1A64(ulong, ulong)	Combines a FNV1A64 hash with a value.
FNV1A64(int)	Generates a FNV1A64 hash.
FNV1A64(string)	Generates a FNV1A64 hash.
FNV1A64<T>(T)	Generates a FNV1A64 hash.