This kind of structure is of variable size (because of the last member). You must call the API once to get the size, and a second time with the properly allocated buffer.

Here is a definition that works:

int size = 0;
if (!SetupGetInfInformation(@"D:\TestDriverFile\intcoed.inf", INFINFO_INF_NAME_IS_ABSOLUTE,
    IntPtr.Zero, 0, ref size)) // pass NULL the first time
    throw new Win32Exception(Marshal.GetLastWin32Error());

// now, size contains the required buffer size
var ptr = Marshal.AllocHGlobal(size);
if (!SetupGetInfInformation(@"D:\TestDriverFile\intcoed.inf", INFINFO_INF_NAME_IS_ABSOLUTE,
    ptr, size, ref size))
    throw new Win32Exception(Marshal.GetLastWin32Error());

// now, ptr contains a pointer to a SP_INF_INFORMATION structure
var InfStyle = Marshal.ReadInt32(ptr);
var InfCount = Marshal.ReadInt32(ptr, 4);
... etc...

[DllImport("setupapi.dll", SetLastError = true)]
internal static extern bool SetupGetInfInformation(
    string InfSpec,
    int SearchControl,
    IntPtr ReturnBuffer,
    int ReturnBufferSize,
    ref int RequiredSize
    );