how char is implememted in C# structs?

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5.00/5 (2 votes)

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I created a simple C# console application that contains this code.

C#

struct CharTest 
{
    char a;
    char b;
}

I expect sizeof(CharTest) to be two times sizeof(char). but surprisingly I see in watch window that It is 2. I don't know why. Can any one tell me what happens?

Posted 17-Dec-10 23:57pm

mr.abzadeh

Updated 18-Dec-10 0:02am

JOAT-MON

v2

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JOAT-MON 18-Dec-10 6:07am

Wrapped code in <pre> tags.

JOAT-MON 18-Dec-10 6:51am

Doesn't make sense...can you post your test code where you are calling the sizeof() so we can take a look?

mr.abzadeh 18-Dec-10 8:15am

I did not call sizeof() in my code. I tried to pass a sequential class containing char fields to unsafe code and It didnot work. there is a conflict between ANSI char fields and [StructLayout(LayoutKind.Sequential)] attribute. I need to use type unicode char so that its size shouldnot change and
passing it to unsafe code in DLL make sense. Please refer to my comment 2 for Your EDIT 2

3 solutions

Solution 1

Why would you expect 1 when you have two chars defined in the struct?

EDIT ==============

~~I think it might have something to do with culture settings. Unicode characters are two bytes, and ascii is just 1.~~

EDIT 2 =============

Well, calling a Marshal.SizeOf on a CharTest object reports a size of 2. Doing a sizeof(char) returns 2. Therefore, I agree with the OP that Marshal.SizeOf would be expected to return 4, but it doesn't.

Here's my code:

C#

public struct MyStruct
{
    public char a;
    public char b;
    public int SizeOf()
    {
        return (sizeof(char) * 2);
    }
}

and then do this:

C#

MyStruct ms = new MyStruct();
ms.a = 'a';
ms.b = 'b';
int sz = System.Runtime.InteropServices.Marshal.SizeOf(ms); // returns 2
int sz2 = ms.SizeOf(); // returns 4

For the record, I'm also using VS2010...

Posted 18-Dec-10 0:15am

#realJSOP

Updated 18-Dec-10 4:52am

v5

Comments

mr.abzadeh 18-Dec-10 6:24am

because sizeof(char) is 2

JOAT-MON 18-Dec-10 7:22am

Did a little research on it...see EDIT 2 in my answer.

Solution 3

It gives 4 to me for a struct and 2 for a char (unicode). This seems correct to me. I am using .Net 4.0.

Posted 18-Dec-10 0:58am

dan!sh

Comments

mr.abzadeh 18-Dec-10 7:37am

Please refer to Answer 2.

Thanks for your intereset.

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JOAT-MON · Accepted Answer · 2010-12-18T00:25:00

Solution 2

My watch window says:
- sizeof(char) = 2
- sizeof(CharTest) = 4

EDIT-----------------------

Just replicated it by using:

C#

System.Runtime.InteropServices.Marshal.SizeOf ( new CharTest() )

EDIT 2---------------------

InteropServices.Marshal.SizeOf is affected by the CharSet that applies to the struct. By default, C# applies ANSI multple-byte CharSet to types. ANSI multiple-byte will use one or two bytes to store character, and on a new instance of the struct the char variables are initialized to their default value which is "\0" or null, thereby only using one byte.

Because of the default CharSet for C#, this struct does not have a predefined size and the sizeof() can only be used in unsafe block where it apparently calculates the max potential of the struct, whereas Marshal.SizeOf is applied to an instance of the struct and is measuring actual size in use.

Posted 18-Dec-10 0:25am

JOAT-MON

Updated 18-Dec-10 1:28am

v5

Comments

mr.abzadeh 18-Dec-10 6:27am

I use Visual studio 2010. what version of VS you use?

JOAT-MON 18-Dec-10 6:40am

VS 2010

mr.abzadeh 18-Dec-10 7:36am

You are right. This is the case as in native C++. but c# documentation says that the type 'char' in C# is unicode. I encountered this problem when passing a class or struct to afunction in Native DLL.
.

How can we define unicode char in C# if char is ANSI multibyte?

mr.abzadeh 18-Dec-10 7:57am

I used this code to pass a class to an unsafe DLL

using System.Runtime.InteropServices;
namespace ns1 {
[StructLayout(LayoutKind.Sequential)]
class Class1 {
.
.
char ch1;
char ch2;
.
.
};

so, ANSI implementation of char conflicts with [StructLayout(LayoutKind.Sequential)] attribute and type 'char' should not be used when passing struct or class to unsafe code because char type in structs or classes is multibyte ansi with predefined size of 1 byte, and changes to 2 bytes when assigning a 2 byte char. It would be a good thing if compiler annonced me for this conflict.

How can I have unicode chars in structs or classes?

JOAT-MON 18-Dec-10 8:19am

In your [StructLayout()] attribute there are named parameters. One of them is CharSet. Set it to CharSet.Unicode.

mr.abzadeh 18-Dec-10 8:24am

It's all right. Thanks for your Precise answers. good bye

Henry Minute 18-Dec-10 9:06am

Good answer.

Since the OP was kind enough to mark your suggestion as the answer, I can only assume that they forgot to also '5' it. Have one on me. :)

JOAT-MON 18-Dec-10 12:27pm

Thanks, Henry!