Re: Passing null - C# Discussion Boards

2-Sep-02 8:09

I am using the ChangeDisplaySettings function from user32.dll, but when i need to reset to the default regitry settings, msdn says do ChangeDisplaySettings(null, 0); but it doesnt like the null value Frown | :(

what should i do?

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Paul Riley2-Sep-02 8:17

2-Sep-02 8:17

What do you mean by "doesn't like"?

Paul

Nnamdi Onyeyiri2-Sep-02 8:25

2-Sep-02 8:25

it does not compile, as the function definition is as follows.

ChangeDisplaySettings(ref DEVMODE d, int flags);

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leppie2-Sep-02 9:17

2-Sep-02 9:17

DEVMODE is a struct.

1. just make another definition

ChangeDisplaySettings(object dummy, int flags);

or

2. change DEVMODE to a class with a sequential layout. Then you can pass null.

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Nnamdi Onyeyiri2-Sep-02 9:23

2-Sep-02 9:23

i have done the second one, but the app doesnt run now, wen you try and run it, it doesnt, nt even listed in the Process List. Confused | :confused:

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leppie2-Sep-02 12:11

2-Sep-02 12:11

I sent you VS solution, that happened to me too, seemed to be due not changing the enum declaration from ref DEVMODE to just DEVMODE ,its a class now so we dont need to. I however had to add the OutAttribute the the declaration.

Smile | :)

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Rickard Andersson²⁰2-Sep-02 10:45

Rickard Andersson²⁰

2-Sep-02 10:45

How about making the function unsafe?
Then use a pointer as parameter.
But that's maybe not possible when it's an imported function!?

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Display Mode

Nnamdi Onyeyiri2-Sep-02 2:22

2-Sep-02 2:22

have geen trying to get he current displaymode but have been unable, my latest attmpt was to try[^] and convert this articles code, but i had a little difficulty. I found the content of the DEVMODE struct on MSDN and put it into my code. I declare the EnumDisplaySettings like this

[DllImport("User32.dll")]<br />
public static extern bool EnumDisplaySettings(string lpszDeviceName, int iModeNum, DEVMODE lpDevMode);

then use it in code like this:

DEVMODE dm = new DEVMODE();
EnumDisplaySettings(null, -1, dm);

but i get the exception 'object reference not set to an instance of an object' yet clearly, a DEVMODE object has been initiated. Confused | :confused:

any help would be appreciated.

*sudden thought* in the decleration of the DEVMODE struct, on MSDN, a couple of the members where of tpye TCHAR, declared like this.

TCHAR  dmDeviceName[CCHDEVICENAME];<br />
TCHAR  dmFormName[CCHFORMNAME];

i couldnt find what CCHFORMNAME is so i just used the MarshalAs Attribute like this

[MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)]  
public string  dmDeviceName;

for both, could that be the problem?

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leppie2-Sep-02 3:00

2-Sep-02 3:00

I think the exception lies with string lpszDeviceName being null.

You mite want to redefine:

[DllImport("User32.dll")]
public static extern bool EnumDisplaySettings(StringBuidler lpszDeviceName, int iModeNum, DEVMODE lpDevMode);

Then when calling do StringBuilder devname = new StringBuilder( insert const size here).

Also , many times you need to set the size of a struct. In this case, according to MSDN, you will need to.

Nnamdi Onyeyiri wrote:
i couldnt find what CCHFORMNAME is so i just used the MarshalAs Attribute like this

This you will find the value in the header files, so download the Platform SDK if you dont have it already.

Hope this helps

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Nnamdi Onyeyiri2-Sep-02 3:16

2-Sep-02 3:16

leppie wrote:
I think the exception lies with string lpszDeviceName being null.

i dont think so, i tried entering a value for that, and it did not work, it gavethe same exception. i used null, because that is what was used in the article i gave the link for.

leppie wrote:
This you will find the value in the header files, so download the Platform SDK if you dont have it already.

is that installed with the default settings for vs.net install?

leppie wrote:
Also , many times you need to set the size of a struct. In this case, according to MSDN, you will need to.

i didnt know about this, ill have a look now.

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leppie2-Sep-02 3:31

2-Sep-02 3:31

Nnamdi Onyeyiri wrote:
dont think so, i tried entering a value for that, and it did not work, it gavethe same exception. i used null, because that is what was used in the article i gave the link for.

Still try using StringBuilder as u CAN pass null as a value.

Nnamdi Onyeyiri wrote:
is that installed with the default settings for vs.net install?

If u install VC++ , else just look for a dir in program files with many *.h files Smile | :)

I had a look and that is a pretty viscious struct Smile | :)

. Im not sure what a BCHAR is though. Im gonna try do something in C# and see what i come up with. Can you perhaps mail me what u have done so far?

Cheers Smile | :)

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Nnamdi Onyeyiri2-Sep-02 3:39

2-Sep-02 3:39

leppie wrote:
I had a look and that is a pretty viscious struct . Im not sure what a BCHAR is though. Im gonna try do something in C# and see what i come up with. Can you perhaps mail me what u have done so far?

sure, but ill need your email address Smile | :)

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leppie2-Sep-02 4:02

2-Sep-02 4:02

DEVMODE dm = new DEVMODE();
dm.dmSize = (short)Marshal.SizeOf(typeof(DEVMODE));
int mode = 0;
while (EnumDisplaySettings(null, mode++, ref dm) != 0)
{
	Console.WriteLine("{0}x{1}, {2}bits {3}hz",
		dm.dmPelsWidth, dm.dmPelsHeight, dm.dmBitsPerPel, dm.dmDisplayFrequency);
}

[DllImport("user32.dll", CharSet=CharSet.Ansi)]
static extern int EnumDisplaySettings(StringBuilder devname, int mode, ref DEVMODE devmode);

public struct DEVMODE 
{
	[MarshalAs(UnmanagedType.ByValTStr, SizeConst=32)]
	public string dmDeviceName; 
	public short dmSpecVersion; 
	public short dmDriverVersion; 
	public short dmSize; 
	public short dmDriverExtra; 
	public int  dmFields; 
	public short dmOrientation;
	public short dmPaperSize;
	public short dmPaperLength;
	public short dmPaperWidth;
	public short  dmScale; 
	public short  dmCopies; 
	public short  dmDefaultSource; 
	public short  dmPrintQuality; 
	public short  dmColor; 
	public short  dmDuplex; 
	public short  dmYResolution; 
	public short  dmTTOption; 
	public short  dmCollate; 
	[MarshalAs(UnmanagedType.ByValTStr, SizeConst=32)]
	public string  dmFormName; 
	public short  dmLogPixels; 
	public int  dmBitsPerPel; 
	public int  dmPelsWidth; 
	public int  dmPelsHeight; 
	public int  dmDisplayFlags; //funny union??
	//public int  dmNup;
	public int  dmDisplayFrequency; 
	public int  dmICMMethod;
	public int  dmICMIntent;
	public int  dmMediaType;
	public int  dmDitherType;
	public int  dmReserved1;
	public int  dmReserved2;
	public int  dmPanningWidth;
	public int  dmPanningHeight;
}

This works Smile | :)

Note the union ???? looks like it combines 2 int 's ???

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Nnamdi Onyeyiri2-Sep-02 4:51

2-Sep-02 4:51

thanks a lot! ive been puzzled over this for the last few hours. Big Grin | :-D

leppie wrote:
This works Note the union ???? looks like it combines 2 int 's ???

im not sure what you mean.

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leppie2-Sep-02 5:02

2-Sep-02 5:02

Im not really sure what a union in C++ does, it appears to join 2 variables into one, hence the commented dmNup. Can anyone C++ person please explain this in C# terms?

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Paul Riley2-Sep-02 5:57

2-Sep-02 5:57

A union uses the same piece of memory for two different variables.

For example

struct INTCHARS
{
    char hi;
    char midhi;
    char midlo;
    char lo;
};


union FOURBYTES
{
    INTCHARS ch;
    int i;
};


FOURBYTES four;
four.ch.hi = 1;
four.ch.midhi = 1;
four.ch.midlo = 1;
four.ch.lo = 1;
four.i = 0;

Now each part of four.ch is 0.

Going back a while since I used one of these. Smile | :)

Paul

leppie2-Sep-02 6:12

2-Sep-02 6:12

Thx,

From what i can see is that unions are class like structures in a struct or class allowing easy retriaval/setting of inner variables. I have been using SequintialLayout classes todo this. Im not sure if we are talking the same thing, but I guess thats why the are not included in C#.

Cheers Smile | :)

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Paul Riley2-Sep-02 8:00

2-Sep-02 8:00

The big difference between a union and a struct is that changing one inner variable in a union will change all of them because they share the same space in memory.

As C++ came into play, unions became less useful, classes and pointers could be used to the same effect.

When PCs with more than 50 bytes of spare memory came out, unions again lost some usefulness. Wink | ;)

Likewise properties haven't done unions any favours, you can now reference the same variable in different ways through get/set properties.

So I wouldn't be surprised if C# didn't have an equivalent, though there are times when it wouldn't go amiss.

Paul

Paul Ingles2-Sep-02 7:52

Paul Ingles

2-Sep-02 7:52

From what I understand (I also was looking into unions recently -- trying to DllImport DnsQuery_A) a union is a UDT which allows for one datatype or another, so if you specify it with a String or Int32 it would let the value be one of the two. At least that was what I took it to mean Smile | :)

I'm not aware of a native C# equivalent but would be glad to know of one!

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James T. Johnson2-Sep-02 9:38

James T. Johnson

2-Sep-02 9:38

As others have said a union just uses the same memory space to hold each of the types (thus only allowing one of them to be used at a time).

Typically when you are dealing with a union you have to break it out into its own datatype so that you can properly use it in code. Otherwise you could be having to use the same variable for two vastly different things such as a pointer to a string and a byte which will be hard to do when dealing with a single member.

To create a union in C#:

[StructLayout(LayoutKind.Explicit)]
public struct myUnion
{
  [FieldOffset(0)] string FieldA;
  [FieldOffset(0)] bool FieldB;
  [FieldOffset(4)] int FieldC;
  [FieldOffset(4)] long FieldD;
  [FieldOffset(12)] int Foo;
  [FieldOffset(12)] char Bar;
  [FieldOffset(12)] float Baz;
}

The numbers used will position where in memory each field will be placed, using the technique you can also create multiple unions as I did above (FieldA and FieldB; FieldC and FieldD; and Foo, Bar, and Baz; are all unions, where only one of the members can be used).

Hope that makes sense.

James
"And we are all men; apart from the females." - Colin Davies

Paul Riley2-Sep-02 9:52