order of fields in struct has influence

[drvic10k]

The order in which the fields are defined has influence on the values

Steps too reproduce: take the sample and move the bool a field in the MyStruct down (keep the attribute value)

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi, Pack = 1)]
    public class MyStruct
    {
        public MyStruct(bool a, short b, int c)
        {
            this.a = a;
            this.b = b;
            this.c = c;
        }

        [FieldOffset(1)]
        public short b;

        [FieldOffset(3)]
        public int c;

        [FieldOffset(0)]
        public bool a;
    }

now when getting the MyStruct value from server, you get different values

_symbolValues.Add(this.Symbols["Globals.myStruct1"], new MyStruct(true,42,99));

_symbolValues.Add(this.Symbols["Main.myStruct1"], new MyStruct(true, 260, 300));

[drvic10k]

interestingly, this problem disappears, when the bool is changed to byte

but byte is still interpreted as bool (not sure, but this might be the problem on the powershell script side)

    [StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi, Pack = 1)]
    public class MyStruct
    {
        public MyStruct(byte a, short b, int c)
        {
            this.a = a;
            this.b = b;
            this.c = c;
        }

        [FieldOffset(1)]
        public short b;

        [FieldOffset(3)]
        public int c;

        [FieldOffset(0)]
        public byte a;
    }

_symbolValues.Add(this.Symbols["Main.myStruct1"], new MyStruct(1, 260, 300));

[RalfHeitmann]

There is a second place where the order of the fields have to be changed. In the symbolic definition of the MyStruct datatype. Look for the "AddMember" statements in the demo server code. Beneath of that, your FieldOffset statements are now wrong. You have to align the data properly in memory.

[drvic10k]

another test case:

make the struct definition match the order of fields:

            StructType dtStruct = new StructType("MYSTRUCT", null)
                // Add Members
                .AddAligned(new Member("b", dtInt))
                .AddAligned(new Member("c", dtDInt))
                .AddAligned(new Member("a", dtBool))
                ;

that means, that these 3 things need to be in sync:

1. order in the struct definition
2. attribute value
3. AddAligned calls order

I expected the numbers 2 and 3 to need to match, but number 1 seems strange

is that really the case?

[RalfHeitmann]

StructType dtStruct = new StructType("MYSTRUCT", null)
                // Add Members
                .AddAligned(new Member("b", dtInt)) // Field-Offset 0
                .AddAligned(new Member("c", dtDInt)) // Field-Offset 2
                .AddAligned(new Member("a", dtBool)) // Field-Offset 6
                ;

If you order your type like that you have (because the FieldOffset is calculated by AddAligned internally);

(Field)Offset 0: b (2 Bytes)
(Field)Offset 2: c (4 Bytes)
(Field)Offset 6: a (1 Byte)

Without having tested it yet, in my opinion the Default-Marshaler should work with both cases below:

[FieldOffset(0)]
public short b;

[FieldOffset(2)]
public int c;

[FieldOffset(6)]
public bool a;

or

[FieldOffset(6)]
public bool a;

[FieldOffset(0)]
public short b;

[FieldOffset(2)]
public int c;

Relevant is only the correct FieldOffset for the Memory alignment - not the order.

[drvic10k]

it works as you describe with byte it fails with bool

[RalfHeitmann]

Seems right. For struct types the .NET Interop Default marshaler is used. That means that the boolean is marshalled with 4-bytes by default, see here:

https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.unmanagedtype?view=net-6.0 states

The example defines the type 'dtBool' as size '1' (1-Byte). Therefore there is a misalignment in the FieldOffsets. I'll correct the example in forcing the .NET marshaler to use UnmanagedType.U1 for the boolean.

See also: https://docs.microsoft.com/en-us/dotnet/standard/native-interop/customize-struct-marshalling

[drvic10k]

great, I tried that and it works