How get a Span<byte> view of a struct without the unsafe keyword

Question

How can a Span<byte> view (reinterpret cast) be created from a single struct value with no copying, no allocations, and without the unsafe keyword.

I can currently only accomplish this using the unsafe keyword:

public unsafe Span<byte> AsSpan<T>(in T val) where T : unmanaged
{
    void* valPtr = Unsafe.AsPointer(ref Unsafe.AsRef(val));
    return new Span<byte>(valPtr, Marshal.SizeOf<T>());
}

// Alternatively, slightly easier when using 'ref' instead of 'in'
public unsafe Span<byte> AsSpan<T>(ref T val) where T : unmanaged
{
    void* valPtr = Unsafe.AsPointer(ref val);
    return new Span<byte>(valPtr, Marshal.SizeOf<T>());
}

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When dealing with an array rather than a single value - this is easily and safely done using MemoryMarshal.Cast<TTo, TFrom>( ... ), for example:

public Span<byte> AsSpan<T>(Span<T> vals) where T : unmanaged
{
    return MemoryMarshal.Cast<T, byte>(vals);
}

---

Using netstandard2.0, latest language version C# 7.3, and latest RC packages for System.Memory and System.Runtime.CompilerServices.Unsafe:

<PropertyGroup>
   <TargetFramework>netstandard2.0</TargetFramework>
   <LangVersion>7.3</LangVersion>
</PropertyGroup>
<ItemGroup>
   <PackageReference Include="System.Memory" Version="4.5.0" />
   <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.0" />
</ItemGroup>

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Edit: Concerning the replies about memory safety/corruption - the unmanaged generic constraint introduced in C# 7.3 can replace the struct generic constraint and allow this to be done in a memory safe way.

See: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/constraints-on-type-parameters

where T : unmanaged The type argument must not be a reference type and must not contain any reference type members at any level of nesting.

Answer 1

Partial solution:

If targeting netcoreapp rather than netstandard2.0 then there is an API available in netcoreapp2.1 (available for download here as of this comment date).

---

Usage:

using System.Runtime.InteropServices;

public Span<byte> AsSpan<T>(ref T val) where T : unmanaged
{
    Span<T> valSpan = MemoryMarshal.CreateSpan(ref val, 1);
    return MemoryMarshal.Cast<T, byte>(valSpan);
}

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This is not a solution to the question which asks for this capability in netstandard2.0. Nevertheless, this should be helpful to many who stumble across this.