CODEWITHSUNDEEP
c#cstructpinvokeunions
Pavel Durov
Pavel Durov

Reputation: 1297

Declaring C struct with union on C#

I want to declare _WAITCHAIN_NODE_INFO struct declaration in my code - for WCT usage. I've tried to follow the tutorials from :

https://msdn.microsoft.com/en-us/library/eshywdt7(v=vs.110).aspx

But every time then I use WCT call with my managed struct declaration I get heap corruption.

typedef struct _WAITCHAIN_NODE_INFO {
  WCT_OBJECT_TYPE   ObjectType;
  WCT_OBJECT_STATUS ObjectStatus;
  union {
    struct {
      WCHAR         ObjectName[WCT_OBJNAME_LENGTH];
      LARGE_INTEGER Timeout;
      BOOL          Alertable;
    } LockObject;
    struct {
      DWORD ProcessId;
      DWORD ThreadId;
      DWORD WaitTime;
      DWORD ContextSwitches;
    } ThreadObject;
  };
} WAITCHAIN_NODE_INFO, *PWAITCHAIN_NODE_INFO;

MSDN: https://msdn.microsoft.com/en-us/library/windows/desktop/ms681422(v=vs.85).aspx

The only declaration that doesn't get me a heap corruption is this:

public struct WAITCHAIN_NODE_INFO
{
    public WCT_OBJECT_TYPE ObjectType;
    public WCT_OBJECT_STATUS ObjectStatus;
}

Obviously, I am missing here the union of LockObject and ThreadObject structs.

How can I convert this C struct to C# managed declaration?

Any help will be appreciated.

Upvotes: 1

Views: 1032

Answers (1)

David Heffernan
David Heffernan

Reputation: 613352

Declare the two structs in the union as C# structs in the usual way. Then declare a type for the union, using an explicit layout. 

[StructLayout(LayoutKind.Explicit)] 
public struct _WAITCHAIN_NODE_INFO_UNION
{
    [FieldOffset(0)]
    _WAITCHAIN_NODE_INFO_LOCK_OBJECT LockObject;
    [FieldOffset(0)]
    _WAITCHAIN_NODE_INFO_THREAD_OBJECT ThreadObject;
}

Then add the union to your struct:

[StructLayout(LayoutKind.Sequential)]
public struct WAITCHAIN_NODE_INFO
{
    public WCT_OBJECT_TYPE ObjectType;
    public WCT_OBJECT_STATUS ObjectStatus;
    public _WAITCHAIN_NODE_INFO_UNION Union;
}

When you overlay objects like this, extra requirements are placed on the types involved. You cannot overlay a type containing a string or an array for instance. So the character array will have to be implemented as a value type, for instance a fixed array. This is inconvenient to operate with but MS did not define the types with C# in mind.

Upvotes: 3

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