c# - Non-blittable error on a blittable type

Question

I have this struct and this code:

[StructLayout(LayoutKind.Sequential, Pack = 8)]
private class xvid_image_t
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 4)]
    public int[] stride;

    // [MarshalAs(UnmanagedType.ByValArray, SizeConst = 4)]
    // public IntPtr[] plane;
}

public int decore()
{
    xvid_image_t myStruct = new xvid_image_t();
    myStruct.stride = new int[4]; // can be commented out - same result
    GCHandle.Alloc(myStruct, GCHandleType.Pinned);

    // ...
}

When I try to run it I get an ArgumentException saying:

Object contains non-primitive or non-blittable data

After reading this MSDN page saying

The following complex types are also blittable types:

• One-dimensional arrays of blittable types, such as an array of integers. However, a type that contains a variable array of blittable types is not itself blittable.

• Formatted value types that contain only blittable types (and classes if they are marshaled as formatted types). For more information about formatted value types, see Default Marshaling for Value Types.

I don't understand what I am doing wrong. I don't just want to use Marshal, but to understand this too.

So what I actually want is to know:

1. Why?
2. How can I resolve this?
3. Will the solution you provide also work with the commented line in the struct?

I am using .Net 4.5 but a solution for .Net 2.0 is also needed.

Answer 1

Object contains non-primitive or non-blittable data

That's the exception message you get. You are focusing on the "non-blittable" part of the message, but that's not the problem. It is the "non-primitive" part that's the issue. An array is a non-primitive data type.

The CLR is trying to keep you out of trouble here. You could pin the object but then you still have a problem, the array won't be pinned. An object isn't truly pinned when it has fields that need to be pinned as well.

And you have a bigger problem with the UnmanagedType.ByValArray, that requires a structural conversion. In other words, the layout that you need is completely different from the layout of the managed class object. Only the pinvoke marshaller can make this conversion.

You can get what you want without using the pinvoke marshaller by using fixed size buffers, using the fixed keyword. This requires using the unsafe keyword. Make it look like this:

    [StructLayout(LayoutKind.Sequential)]
    unsafe private struct xvid_image_t {
        public fixed int stride[4];
    }

Note that you have to change the declaration to a struct type. It is now a value type, you no longer need to use GCHandle to pin the value when you make it a local variable. Do make sure that whatever unmanaged code takes the structure value, usually by reference, does not store a pointer to the struct. That's going to blow up badly and utterly undiagnosably. The unsafe keyword is appropriate here. If it does store the pointer then you really do have to byte the bullet and use Marshal.AllocHGlobal() and Marshal.StructureToPtr() to ensure the pointer stays valid while the unmanaged code is using it.