Question

Functions such as CreateProcess have signatures taking pointers to structs. In C I would just pass NULL as a pointer for the optional parameters, instead of creating a dummy struct object on the stack and passing a pointer to the dummy.

In C#, I have declared it as (p/invoke)

[DllImport("kernel32.dll", CharSet = CharSet.Auto)]
        public static extern bool CreateProcess(
            string lpApplicationName,
            string lpCommandLine,
            ref SECURITY_ATTRIBUTES lpProcessAttributes,
            ref SECURITY_ATTRIBUTES lpThreadAttributes,
            bool bInheritHandles,
            CreateProcessFlags dwProcessCreationFlags,
            IntPtr lpEnvironment,
            string lpCurrentDirectory,
            ref STARTUPINFO lpStartupInfo,
            ref PROCESS_INFORMATION lpProcessInformation);

But if I try to pass null for the lpProcessAttributes argument or lpThreadAttributes argument, I get a compiler error:

Error 2 Argument 3: cannot convert from ‘<null>’ to ‘ref
Debugging.Wrappers.SECURITY_ATTRIBUTES’

How can I modify the above function signature so that I can just pass null for the SECURITY_ATTRIBUTES arguments, without this compiler error? (And also be able to pass a real struct if I want to?)

Answer 1

null is only valid for Reference types in .Net. your SECURITY_ATTRIBUTES is a struct, which is a ValueType. Rather than passing null, you need to pass an empty SECURITY_ATTRIBUTES structure. (just say new SECURITY_ATTRIBUTES()) in your call.

A cleaner method is to add a static Empty property to your struct, and just pass SECURITY_ATTRIBUTES.Empty

[StructLayout(LayoutKind.Sequential)]
public struct SECURITY_ATTRIBUTES {
    public int nLength;
    public IntPtr lpSecurityDescriptor;
    public int bInheritHandle;

    public static SECURITY_ATTRIBUTES Empty {
        get {
            return new SECURITY_ATTRIBUTES {
                nLength = sizeof(int)*2 + IntPtr.Size,
                lpSecurityDescriptor = IntPtr.Zero,
                bInheritHandle = 0,
            };
        }
    }
}

Or better yet, rather than using P/Invoke to create a Process, check out the System.Diagnostics.Process class, which should probably do what you need it to.