Re: marshalling a pointer member in struct from c# - C# Discussion Boards

Re: Problem with Stored Procedure

22-Jun-04 3:03

The last two replies are correct, in that you're not declaring your field as a string, but are instead declaring it as a char.

I wanted to add that the above code can be made much more efficient by not using the indexer to access your param. The SqlParameterCollection.Add overload you're using returns a SqlParameter, so you can either type .Value = value; immediately after your closing parenthesis, or store the return value from Add to a SqlParameter variable and assign the value from there. The latter way works best when executing a batch of statements (add the parameters to the command and loop to add each iteration's values without re-adding the params again; execute the command with each iteration).

This will result in better performance, since a string lookup on a collection can be very slow (mO(n) processing time, where n is the number of items in a collection and m is the number of iterations; compare that to O(m)).

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Anonymous22-Jun-04 3:29

Anonymous

22-Jun-04 3:29

CREATE PROCEDURE spAddNewAddress
@AddressID char, @AddressType char, @HomeAddress varchar,
@City varchar, @State varchar, @Zip varchar, @Country varchar
AS
INSERT INTO Address VALUES(@AddressID, @AddressType, @HomeAddress,
@City, @State, @Zip, @Country)
GO

In the above parameters, a size needs to be added:

@AddressID char(nn), @AddressType char(nn), @HomeAddress varchar(nn),
@City varchar(nn), @State varchar(2), @Zip varchar(9), @Country varchar(nn)

where the "nn" is the size of the field.

marshalling a pointer member in struct from c#

rana7421-Jun-04 14:16

21-Jun-04 14:16

I have a struct explicitly defined in C# as

[
Guid("FE9814A3-D198-4bd0-8475-D055DAA7C659"),
StructLayout(LayoutKind.Sequential)
]
public struct _alloc
{
public _alloc(short a,short b)
{
sx= a;sy=b;
}
public short sx;
public short sy;

}

The following struct contains a pointer member to _alloc in COM IDL

typedef
[
uuid(649B2720-373B-46a4-BC1F-6B53CC38755D),
version(1.0),
helpstring("A Demo struct typedef for Marshal")
]
struct _groupalloc {

_alloc* lx;
short ly;

} _GroupAlloc;

I tried to define it as follows in C#
[
Guid("649B2720-373B-46a4-BC1F-6B53CC38755D"),
StructLayout(LayoutKind.Sequential)
]
public struct _groupalloc
{
public _alloc[] sx;
public short sy;

}

This is to be used in a method definition as follows,to marshal it to COM
int GetComplexStruct(ref _groupalloc _pc);

I get the runtime error - 'cannot marshal field sx of _groupalloc - it cannot be marshalled as member of struct'.
What is the principle behind this?Why cant an array be marshalled as member of a struct - if someone could give pointers to related articles too,apart from the answer to the above;that would be appreciated.

thanks and regards.

Heath Stewart22-Jun-04 2:56

22-Jun-04 2:56

You could declare the member as an IntPtr, then use GCHandle and the Marshal class (specifically it's PtrToStructure method).

If you want to marshal an array, you must also use the MarshalAsAttribute to provide additional information, like the size or size param. When marshaling from unmanaged to managed code, the size of an array is known so this information is needed for the marshaler to construct the managed array.

For more information on marshaling, read Interop Marshaling[^] in the .NET Framework SDK.

If you want more information, read about the classes and methods I mentioned above and continue this thread if you have additional questions.

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rana7422-Jun-04 14:23

22-Jun-04 14:23

Thanks for the response.
I thought that when i am marshalling an array allocated from managed code - i need not give the siz parameter ,as the CLR knows the bounds anyway.
Also in the example i had given ,if I just marshal a _alloc* to COM as in
int GetStructandModify(ref Location _ps);
It just works fine .
However,when it is a member of a struct - it is not marshalled.
Is it something to do with value and reference type?-it would be very helpful if you could clarify this part.

Also ,I tried to marshal it as IntPtr using GCHandle.Alloc and Marshal.AllocCoTaskMem - it works successfully in either case.Also I think GCHandle uses managed memory while AllocCoTaskMem would use the native heap - but I am not sure how to decide which one to be used.

Is there some advice available on parameters to decide whether to use managed or unmanaged memory.
thanks.

Heath Stewart22-Jun-04 15:14

22-Jun-04 15:14

From managed to unmanaged code, the size param is not needed; but, as I said, when going from unmanaged to managed it is because an array is nothing more than an allocated block of memory with no type-safety.

Use GCHandle (pinning the object when it's a reference object) instead of Marshal.AllocCoTaskMem. Use that mainly when COM request the use of CoTaskMemAlloc. A GCHandle pins the object in memory so the GC doesn't move it around, which is what you want here. All you have to do is free the GCHandle, instead of calling Marshal.FreeCoTaskMem. This unpins the object and allows the GC to move it around again.

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rana7422-Jun-04 16:28

22-Jun-04 16:28

Thanks for your advice.
So when a structure pointer is a member of another structure,can i also marshal the pointer member using marshalas too ,instead of using instptr.If so - can you give an example ,as all my attempts have failed.
Also I am yet not clear when I should prefer allocating an object into unmanaged heap(using marshal AllocCoTaskMem) ,rather than pinning it into managed heap and passing the address to unmanged code.

Sorry for the trouble.

Heath Stewart22-Jun-04 16:46

22-Jun-04 16:46

It's already allocated - why allocate it again? For a reference object, all you need to do is pin it in memory (a value type doesn't need (and can't be) pinned since it will not be moved by the GC, although it could be reclaimed so still use a GCHandle or HandleRef to reference it).

You could try:

// Excuse the names - I don't remember off-hand what you used
[StructLayout(LayoutKind.Sequential)]
internal struct S1
{
  public int x;
  public int y;
}
[StructLayout(LayoutKind.Sequential)]
internal struct S2
{
  [MarshalAs(UnmanagedType.LPArray, SizeParamIndex=1)]public S1[] arr;
  public int size;
}

This will help the marshaler create the array when marshaling from unmanaged code.

Otherwise, you can use the GC handle to pin the S1[] array in memory till after the P/Invoke method returns:

S1[] arr = new S1[];
// Fill S1
GCHandle handle = GCHandle.Alloc(arr, GCHandleType.Pinned);
S2 s2 = new S2();
s2.arr = (IntPtr)handle; // S2.arr is defined as IntPtr
s2.size = arr.Length; // Probably not required here, but oh well
CallMyPInvokeMethod(ref s2);
// Get new data (if any)
handle.Free();

It's crude, but should hopefully give you a good idea.

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rana7422-Jun-04 21:03

22-Jun-04 21:03

If i try to marshal the pointer element using
[MarshalAs(UnmanagedType.LPArray, SizeParamIndex=1)]public S1[] arr;
as you suggested - I am back to the
"This type cannot be marshalled as a structure field" error,which formed my original question.

Heath Stewart22-Jun-04 21:06

Active Directory & Multiple Connections

22-Jun-04 21:06

Which is why I gave you the alternative to try below that.

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inyoursadachine21-Jun-04 14:02

inyoursadachine

21-Jun-04 14:02

I have a C# program the asks the user for a UserID, Password, and Domain and does a WinNT Active Directory query to return a list of groups the specified user belongs to. I start the program and enter user1/password 1 on domain 1 and it works just fine. But when I (in the same invocation) try to enter user2/password2 on domain 1 I get the following COM exception:

"Multiple connections to a server or shared resource by the same user, using more than one user name, are not allowed....."

I've tried closing and disposing the DirectoryEntry object after each use but to no avail. What do I have to do (close) to allow my program to do 2+ separate Active Directory queries for 2+ userid/password combos in the same invocation??

TIA,
Matt

Re: Active Directory & Multiple Connections

Heath Stewart22-Jun-04 2:51

22-Jun-04 2:51

The credential cache has to be cleared, and I'm not sure you have any control over this. The default expiration time is around 15 minutes. This is a function of Windows and not .NET.

The best solution is to not emulate users but to - if possible - log in as a user with administrative rights (or enough privileges to enumerate group membership) and query the groups for a user. This would require only 1 login and would allow you to query as many users as you want. This is the way that most programs (including the management tools in Windows) work.

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Encoding

eggie521-Jun-04 12:39

21-Jun-04 12:39

Now, I don't now anything about encoding... Now lets take this text for example. It's "regular" encoding, correct? Now take this @ for example. Is the @ sign something special?

/\ |_ E X E GG

Christian Graus21-Jun-04 14:29

Christian Graus

21-Jun-04 14:29

yeah, it's big and bold. Perhaps you need to reword your question ?

Christian

I have drunk the cool-aid and found it wan and bitter. - Chris Maunder

eggie521-Jun-04 18:21

21-Jun-04 18:21

I don't understand... Confused | :confused:

/\ |_ E X E GG

Christian Graus21-Jun-04 18:23

Christian Graus

21-Jun-04 18:23

The @ is 'special' in that it's been made larger and bold. Your question was somewhat enigmatic. That's why I suggested you reword it.

Christian

I have drunk the cool-aid and found it wan and bitter. - Chris Maunder

eggie522-Jun-04 8:54

22-Jun-04 8:54

ok, thank you.

/\ |_ E X E GG

Steven Campbell21-Jun-04 15:39

Steven Campbell

21-Jun-04 15:39

Encoding refers to the way that bits are translated to characters. What you refer to as "regular encoding" is ASCII, which is a 7 bit encoding allowing a total of only 128 characters. Unicode encodings (UTF7, UTF8, UTF16) are more flexible, and can represent far more characters. The numbers in the UTF codes refer to the minimum number of bits that are used to represent a single character.

Heath Stewart22-Jun-04 2:48

22-Jun-04 2:48

Encoding is important when working with file streams. For a good article on encoding, read The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)[^].

The '@' character is a literal character (no escapes necessary), and as with everything else, you can read more about it in the C# language documentation. Read the documentation[^] on the string type for more information about @-quoted strings.

For example, "Hello, \"world\"" == @"Hello, ""world""", and

"Hello,\nworld!" == @"Hello,
world!"

(line break intentional)

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eggie514-Sep-04 5:49