Unsafe.As in C# — Reinterpret Memory Without Copying (.NET 8 Examples)

A .NET 8 console application demonstrating System.Runtime.CompilerServices.Unsafe.As<TFrom, TTo>() — a zero-copy way to reinterpret memory in C#, treating the bytes of one type as another without any data conversion or allocation.

What Is Unsafe.As?

Unsafe.As<TFrom, TTo>(ref TFrom source) returns a managed reference (ref TTo) that points to the same memory as the input. It is a pure compile-time reinterpretation — no copy, no boxing, no conversion. The project enables <AllowUnsafeBlocks>true</AllowUnsafeBlocks> for the unsafe pointer arithmetic in Example 1.

Examples

Program.cs contains four numbered examples; the first two (reading and then modifying a long through an int reference) are shown together below.

Example 1 — long reinterpreted as int

Reinterprets a long value as a reference to its first 4 bytes as int. Modifying through the int reference updates the original long value’s low 32 bits.

Original long value: 42
As int (first 4 bytes): 42
As int (second 4 bytes): 0
After modifying through int reference, long value: 100

Example 2 — PointFloat reinterpreted as PointInt

Reinterprets a struct PointFloat { float X; float Y; } as PointInt { int X; int Y; }. The displayed integer values are the raw IEEE 754 bit patterns of the float values.

Original: (1, 2)
As PointInt: (1065353216, 1073741824)

Example 3 — byte[] reinterpreted as long

Gets a ref long to the first element of a byte[8] array and reads or writes all 8 bytes at once through a single 64-bit write.

Original bytes: [01, 02, 03, 04, 05, 06, 07, 08]
As long: 578437695752307201
After modification: [01, 02, 03, 04, 05, 06, 07, 08]

The bytes look unchanged after the modification because the code writes 0x0807060504030201, which is the same value: on a little-endian CPU (x86/x64 and Arm64 on Windows), the lowest byte 01 is stored first. Writing a different value would visibly change the array.

Requirements

Build and Run

cd UnsafeAs
dotnet run

Project Structure

UnsafeAs/
├── Program.cs         # Four Unsafe.As examples (shown as three sections above)
├── PointFloat.cs      # struct PointFloat { float X; float Y; }
├── PointInt.cs        # struct PointInt { int X; int Y; }
├── UnsafeAs.csproj    # AllowUnsafeBlocks = true
└── UnsafeAs.sln

Notes

Unsafe.As is intended for performance-critical scenarios such as binary serialisation, network protocol parsing, and SIMD-style operations. Misuse can corrupt memory or produce incorrect results; always ensure source and target types have compatible sizes and alignment.

The outputs shown above were confirmed by running the project.

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