Accessing fixed size array elements from C# in PowerShell - powershell

Question

Looking for a way to access individual array elements (continuation of this question)
$CSharpCode = #"
using System;
namespace TestStructure
{
public struct TestStructure
{
public byte Field1;
public unsafe fixed byte Field2[4];
}
}
"#
$cp = New-Object System.CodeDom.Compiler.CompilerParameters
$cp.CompilerOptions = '/unsafe'
Add-Type -TypeDefinition $CSharpCode -CompilerParameters $cp
function ConvertTo-Struct
{
# Only accept struct types (sealed value types that are neither primitive nor enum)
param(
[Parameter(Mandatory = $true)]
[ValidateScript({ $_.IsValueType -and $_.IsSealed -and -not($_.IsPrimitive -or $_.IsEnum) })]
[Type]$TargetType,
[Parameter(Mandatory = $true)]
[byte[]]$BinaryData
)
# Start by calculating minimum size of the underlying memory allocation for the new struct
$memSize = [System.Runtime.InteropServices.Marshal]::SizeOf([type]$TargetType)
# Make sure user actually passed enough data to initialize struct
if($memSize -gt $BinaryData.Length){
Write-Error "Not enough binary data to create an instance of [$($TargetType.FullName)]"
return
}
# now we just need some unmanaged memory in which to create our struct instance
$memPtr = [IntPtr]::Zero
try {
$memPtr = [System.Runtime.InteropServices.Marshal]::AllocHGlobal($memSize)
# copy byte array to allocated unmanaged memory from previous step
[System.Runtime.InteropServices.Marshal]::Copy($BinaryData, 0, $memPtr, $memSize)
# then marshal the new memory region as a struct and return
return [System.Runtime.InteropServices.Marshal]::PtrToStructure($memPtr, [type]$TargetType)
}
finally {
# and finally remember to clean up the allocated memory
if($memPtr -ne [IntPtr]::Zero){
[System.Runtime.InteropServices.Marshal]::FreeHGlobal($memPtr)
}
}
}
$testStructure = ConvertTo-Struct -TargetType ([TestStructure.TestStructure]) -BinaryData (1..100 -as [byte[]])
$testStructure.Field1
$testStructure.Field2
this produces the following output:
1
FixedElementField
-----------------
2
only the first array element of $Field2 is visible, can't access others using $testStructure.Field2[x]
Looking for a way to iterate over FixedBuffer of known type / size
$testStructure.Field2.GetType() says <Field2>e__FixedBuffer0
$testStructure.Field2.FixedElementField.GetType() is byte
can't see a way to access other elements of the array.

Answer 1

PowerShell's type adapter doesn't really have anything in place to handle unsafe fixed byte[] fields, effectively raw pointers from the underlying type system's point of view.
The declared size of the underlying memory allocation is only stored in metadata, which you can locate as follows:
# Locate appropriate field metadata
$fieldInfo = $testStructure.GetType().GetField('Field2')
# Discover fixed buffer attribute
$fixedBufferAttribute = $fieldInfo.CustomAttributes.Where({$_.AttributeType -eq [System.Runtime.CompilerServices.FixedBufferAttribute]}) |Select -First 1
Now we can figure out the size and which array element type is expected:
if($fixedBufferAttribute)
{
# Grab array element type + size from FixedBuffer attribute
$elemType,$size = $fixedBufferAttribute.ConstructorArguments
# Create array of appropriate size
$values = $elemType.MakeArrayType()::new($size)
# Copy values from fixed buffer pointer to managed array
try {
$fixedBufferHandle = [System.Runtime.InteropServices.GCHandle]::Alloc($TestStructure.Field2, 'Pinned')
[System.Runtime.InteropServices.Marshal]::Copy($fixedBufferHandle.AddrOfPinnedObject(), $values, 0, $size)
return $values
}
finally {
$fixedBufferHandle.Free()
}
}
You'll find $values now contains the expected byte values 2, 3, 4 and 5