[EC81] Rule proposal : Specify struct layouts

[vdebellabre]

Category : Performance

Severity : Warning

Why is this an issue ?

Structs that don't contain reference types (directly nor indirectly) have the Sequential layout by default. This is to prevent breaking any project that uses interop code and requires that the fields stay ordered. In other cases however, field reordering is usually fine and there are little reasons to use the Sequential layout over the Automatic one, as it will optimize the layout for you, potentially saving memory and improving performance in the process.

You can read a much more in-depth analysis of struct packing here : http://www.catb.org/esr/structure-packing/

When can it be ignored ?

It can be ignored if you know you require the Sequential layout for all of your structs, most likely for interop purposes.

Examples

public struct MyStruct // Compliant, no field
{
}

public struct MyStruct // Compliant, only one field, no ordering possible
{
    public int A;
}

public struct MyStruct // Non compliant, specify the layout. Will take 24 bytes because of Sequential
{
    public int A;
    public double B;
    public int C;
}

[StructLayout(LayoutKind.Sequential)]
public struct MyStruct // Compliant, specified layout. Will take 24 bytes because of Sequential
{
    public int A;
    public double B;
    public int C;
}

[StructLayout(LayoutKind.Auto)]
public struct MyStruct // Compliant, specified layout. Will take 16 bytes thanks to Auto
{
    public int A;
    public double B;
    public int C;
}

public struct MyStruct // Compliant, string is a reference type, LayoutKind.Auto will be forced. Will take 16 bytes
{
    public int A;
    public string B;
    public int C;
}