Question

I have the following marshalling code in my project. I have few questions on this.

[DllImport=(Core.dll, SetLastError=true, EntryPoint="CoreCreate", CharSet="CharSet.Ansi", CallingConvention="CallingConvention.Cdecl")]
internal static extern uint CoreCreate(ref IntPtr core);

1. Why ‘internal static extern’ is required? Is this compulsory? Why this is used?
2. What is SetLastError?

[StructLayout(LayoutKind.Sequential,CharSet=CharSet.Ansi)]
internal struct Channel
{
internal byte LogicalChannel;
}

Why LayoutKind.Sequential?

Answer 1

Why ‘internal static extern’ is required?

The internal modifier just set visibility of your method. It’s not required to be internal so you can declare the method private or public as you need and as you would do with any other standard method.

The static modifier is required because it’s not an instance method and that method doesn’t know any class (it hasn’t a this pointer).

Finally extern is required to inform the compiler that the method isn’t implemented here but in another place (and you’ll specify where using attributes). Evey extern method must be declared static too (because it’s a simple function call without any knowledge about objects).

What is SetLastError?

It indicates that method may change the thread’s last-error code value. See the GetLastError() function for details about this. If the called function will change this value then it’s a good thing to set SetLastError to true, from MSDN:

The runtime marshaler calls GetLastError and caches the value returned to prevent it from being overwritten by other API calls. You can retrieve the error code by calling GetLastWin32Error.

In short it saves the value returned by GetLastError() to an internal cache so any other call to system API (even internal to others framework functions) won’t overwrite that value.

Why LayoutKind.Sequential?

Class layout in .NET isn’t required to be sequential in memory (sequential = if A is declared before B then memory layout has A before B). This is not true in C where the declaration order matters (declaration is used by the compiler to understand the layout, in memory, of raw data). If you have to interop with C functions then you have to be sure about the layout of the data you pass them. This is how LayoutKind.Sequential works: it instructs the compiler to respect the declaration sequential order for data in the struct. This is not the only option to interop with unmanaged world, you can even explicitly set the offset (from structure beginning) of each field (see LayoutKind.Explicit).