Initialise an unsafe fixed double array in struct

Question

This question is a follow up from Marshalling C# structure to C++ Using StructureToPtr. I have the following struct:

[StructLayout(LayoutKind.Explicit, Size = 120, CharSet = CharSet.Unicode)]
public unsafe struct DynamicState
{
    [FieldOffset(0)]
    public fixed double Position[3];

    [FieldOffset(24)]
    public fixed double Velocity[3];

    [FieldOffset(48)]
    public fixed double Acceleration[3];

    [FieldOffset(72)]
    public fixed double Attitude[3];

    [FieldOffset(96)]
    public fixed double AngularVelocity[3];
}

If I try and initialise an array like this:

var dynamicState = new DynamicState();
double[] array = new double[] { 1, 2, 3 };

fixed (double* pArray = array)
{
    dynamicState.Acceleration = pArray;
}

I get the following error:The left-hand side of an assignment must be a variable, property or indexer.

What is the correct way to initialise an unsafe array that is part of a struct?

Jon Skeet · Accepted Answer · 2012-10-05 22:34:44Z

2

Well the simple approach seems to work:

for (int i = 0; i < 3; i++)
{
    dynamicState.AngularVelocity[i] = array[i];
}

It may not be as far as you're looking for though. Is this a performance-critical piece of code?

This may be better:

Marshal.Copy(array, 0, new IntPtr(dynamicState.AngularVelocity), array.Length);

I can't say I've much experience with unmanaged code, but it's worth at least looking at those options...

answered Oct 5, 2012 at 22:34

Jon Skeet

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3 Comments

Seth Over a year ago

It is performance sensitive. However will the compiler optimize that with dynamicstate being unsafe?

user586399 Over a year ago

He may PInvoke C memset function. It is so fast (although it is not very good with floating-point numbers)

Jon Skeet Over a year ago

@Seth: I don't like to guess too much with optimizations :) See whether the second one works for you - it may well be faster.

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