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Re: How to compare string data by value and by reference

8-Jan-18 21:24

Strings are reference types, but string comparisons are made as value types:

string a = "Hello World";
string b = "Hello";
string c = " World";
string d = b + c;
Console.WriteLine("{0}:{1}:{2}:{3}", a == d, a.Equals(d), (object)a == (object) d, object.ReferenceEquals(a, d));

Will give you:

True:True:False:False

Because the first two tests use the string specific comparison, and the other two specifically do reference comparisons.

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Bernhard Hiller9-Jan-18 21:41

Bernhard Hiller

9-Jan-18 21:41

With strings, there is another fact to consider. When the compiler can detect that they have the same value, the compiler will make sure that they have the same reference also. It can do that optimization because strings are immutable.

In the example given by OriginalGriff, the compiler did not detect that and thus the compiler created two different references.

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Re: How to compare string data by value and by reference

OriginalGriff10-Jan-18 0:38

Re: How to compare string data by value and by reference

10-Jan-18 0:38

That's where it gets complicated (and why I didn't mention it - he is a beginner, after all).
The compiler can detect and intern strings, but the runtime doesn't unless you specifically ask it to:

string a = "Hello World";
string b = "Hello";
string c = " World";
string d = b + c;
Console.WriteLine("{0}:{1}:{2}:{3}", a == d, a.Equals(d), (object)a == (object)d, object.ReferenceEquals(a, d));
d = string.Intern(d);
Console.WriteLine("{0}:{1}:{2}:{3}", a == d, a.Equals(d), (object)a == (object)d, object.ReferenceEquals(a, d));

Will give you:

True:True:False:False
True:True:True:True

As the "new" string is now in the Intern pool.

However ... in the release version of my original code you will probably get different results, as the optimiser will likely compact that code and intern the strings anyway!

There is a good description of the intern pool here: Understanding C#: String.Intern makes strings interesting - O'Reilly Broadcast[^]

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Mou_kol10-Jan-18 21:18

Re: How to compare string data by value and by reference

10-Jan-18 21:18

i have a very small question sir.

string a = "Hello World";
string b = "Hello World";

when we compare like if(a==b) then what will happen?

== operator compare by value or reference ?

when two string variable having same value and if == operator compare by vale then why value is checked by reference ?

please help me to understand this small area. thanks a lot in advance sir.

OriginalGriff10-Jan-18 22:02

Re: How to compare string data by value and by reference

10-Jan-18 22:02

At this stage, I'd say "don't worry about it" - because it starts to get pretty complicated. When you write code like that, two additional factors come into play: optimisation and interning.

Optimisation is not normally applied to debug builds - they are optimised for "debuggability" not performance and that means that large sections of the compiler do not get used that can make significant changes to your code. For example, the compiler can look at this:

public bool IsSame()
   {
   string a = "Hello World";
   string b = "Hello World";
   return a == b;
   }

And "know" that it will always produce the same result: true. So the compiler can "throw away" the whole method and replace it with inline code for "true" wherever your code references it.
That makes it very difficult to say "it's this type of comparison" because the comparison may never occur at all, depending on what happens in the surrounding code! It doesn't do that for debug builds, because it makes it a lot harder for you the developer to work with in the debugger - you can't set a breakpoint on code which has been optimised out! Laugh | :laugh:

Interning is another complexity, and one I was hoping to avoid - and why I wrote my original code exactly the way I did. Interning basically means that because strings are immutable, the compiler "knows" that the two string literals are the same, so it only ever keeps one copy of it. So the comparison changes: it becomes a comparison between two identical references, so the question of "value or reference comparison?" becomes moot: they are the same. To see what I mean, try modifying my original code:

string a = "Hello World";
string b = "Hello";
string c = " World";
string d = b + c;
string e = "Hello World";
Console.WriteLine("{0}:{1}:{2}:{3}", a == d, a.Equals(d), (object)a == (object)d, object.ReferenceEquals(a, d));
Console.WriteLine("{0}:{1}:{2}:{3}", a == e, a.Equals(e), (object)a == (object)e, object.ReferenceEquals(a, e));

Run that, and you will get different results for the two WriteLines:

True:True:False:False
True:True:True:True

The strings have been interned, so the references are the same.
If you want to know more - and this stuff can get quite complicated - I'd point you are the same link I did Bernhard: Understanding C#: String.Intern makes strings interesting - O'Reilly Broadcast[^] - but to be honest, this isn't stuff you need to know 99% of the time as it makes absolutely no different to how you code your app!

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Mou_kol11-Jan-18 3:42

Re: How to compare string data by value and by reference

11-Jan-18 3:42

Sir i was asked in interview that if we execute this code a==b then will it be compare by value or reference ? i said value and they told me nothing. i guess i was wrong.

so tell me when we compare a==b does this comparison occur by value or ref if two string variable having same data. thanks

Richard Deeming11-Jan-18 4:05

Richard Deeming

11-Jan-18 4:05

If both variables are declared as string, then it compares by value.

String.Equality Operator[^]:
Determines whether two specified strings have the same value.

Reference Source[^]:

public static bool operator == (String a, String b) {
    return String.Equals(a, b);
}

public static bool Equals(String a, String b) {
    if ((Object)a==(Object)b) {
        return true;
    }
 
    if ((Object)a==null || (Object)b==null) {
        return false;
    }
 
    if (a.Length != b.Length)
        return false;
 
    return EqualsHelper(a, b);
}

(Where EqualsHelper[^] essentially compares the strings character by character, but using unsafe code to improve the performance.)

If either variable is declared as object, then it will compare by reference.

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- Homer

Re: How to compare string data by value and by reference

Mou_kol15-Jan-18 22:00

Re: How to compare string data by value and by reference

15-Jan-18 22:00

it seems to me that if check two string variable

String.Equals(a, b); OR a == b then it will be checking by value but if i check this way
if ((Object)a==(Object)b) then this type of checking is done by reference.

do i understand properly now ? am i right now ?

Richard Deeming16-Jan-18 1:21

Richard Deeming

16-Jan-18 1:21

Yes, that's correct.

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- Homer

Re: How to compare string data by value and by reference

Mou_kol16-Jan-18 1:44

Re: How to compare string data by value and by reference

16-Jan-18 1:44

thanks you sir for your great help.

OriginalGriff11-Jan-18 4:19

Re: How to compare string data by value and by reference

11-Jan-18 4:19

For a string, a == b is always a value comparison, but ... it starts with a reference comparison!

1) operator == calls string.Equals.
2) The first thing that Equals does is this:

if ((Object)a==(Object)b) return true;

Which is a reference comparison, as you'd expect. A good percentage of the time, this will make the process faster.
3) It then checks for null in either side, and returns false if either is null.
4) It then compares the lengths - any difference is a return false.
5) It then calls another method which repeats those two tests (if it's the debug build) and throws an exception via Contract.Requires
6) It then uses unsafe code with pointers to do a value comparison (but quickly, using 12 bytes at a time for AMD processors, and 10 for Intel - don't ask)

So technically, "a == b" is a value comparison, but it has a few additional tests to speed things up, including a reference comparison.

But you really aren't supposed to know that, the traditional answer is "value comparison for strings". It may be that they wanted you to expand on "value type" and maybe explain why it's not a reference comparison?

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Mycroft Holmes11-Jan-18 10:32

Mycroft Holmes

11-Jan-18 10:32

Wow, my reply in the interview would have been "I don't know, nor do I care". It must be a specialised requirement if this depth of knowledge is required for the position (I do LOB apps only).

Never underestimate the power of human stupidity
RAH

Re: How to compare string data by value and by reference

OriginalGriff11-Jan-18 11:19

C# Windows Form Application with 86x unmanaged dll

11-Jan-18 11:19

You should probably care about the value type comparison that is specific to strings - it does affect performance after all. And knowing that release optimisation could change how things work could be important.

But interning strings - particularly at run time - you don't normally need to know about. (The compile time interning is pretty much irrelevant as it's a "side effect" of immutability.)

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Zeyad Jalil8-Jan-18 20:33

Zeyad Jalil

8-Jan-18 20:33

Dear All,

I have an unmanaged DLL developed by Delphi language and it's support 86x and 64x and I want to use the DLL in my c# windows form application which use any cpu as a platform target but this error occurred
"System.BadImageFormatException: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)
at ScaleDLL.frmMain.AclasSDK_Initialize(Pointer Adjuct)
at ScaleDLL.frmMain.frmMain_Load(Object sender, EventArgs e) in F:\Dot Net Projects\Dlls\ScaleDLL\ScaleDLL\frmMain.cs:line 147"

and we can't change the platform target to 86x because we use other libraries and it is not work when change the platform and we need to use this dll on the AnyCpU platform.

Please help me ASAP.
Thank You

Re: C# Windows Form Application with 86x unmanaged dll

OriginalGriff8-Jan-18 21:16

Re: C# Windows Form Application with 86x unmanaged dll

8-Jan-18 21:16

It's not simple, not at all.
Because you are using 64 bit libraries already, you can't access 32 bit libraries on the same process - even if you compile for "any cpu" that doesn't mean that a process can "switch" dynamically between the two.
If you want to cleanly use your legacy DLL, you will need to either convert everything else to x86, convert the legacy DLL to 64 bit, or ... start reading: Accessing 32-bit DLLs from 64-bit code | Reflections on IT[^] Do not expect this to be a clean, neat, and easy solution.

And no, I've never needed to do it - so I can't help you here...

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Gerry Schmitz10-Jan-18 8:23

Gerry Schmitz

10-Jan-18 8:23

Interprocess Communication Between 32-bit and 64-bit Applications (Windows)

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Game

Member 136022178-Jan-18 10:55

Member 13602217

8-Jan-18 10:55

Hi!how to create wordsearch in c#?

Pete O'Hanlon8-Jan-18 11:09

Pete O'Hanlon

8-Jan-18 11:09

With classes, some variables, methods and a whole host of semi-colons. Hopefully you'll throw some algorithms in there as well.

This space for rent

Dave Kreskowiak8-Jan-18 11:26

Dave Kreskowiak

8-Jan-18 11:26

Asking questions is a skill
CodeProject Forum Guidelines
Google: C# How to debug code
Seriously, go read these articles.

Dave Kreskowiak

Swinkaran8-Jan-18 11:38

Swinkaran

8-Jan-18 11:38

Looks like you are trying to finish a task with less effort.
You need to familiarize yourself with 2 important things. C# String and Algorithms. Then you will know the answer.
Here are few usefull links,

C# Strings methods - MSDN
[^]
String (C# Reference) | Microsoft Docs[^]

Searching Algorithms In C#[^]

PIEBALDconsult8-Jan-18 13:15

PIEBALDconsult

8-Jan-18 13:15

Any way you like.

Chris Quinn8-Jan-18 21:26

Chris Quinn

8-Jan-18 21:26

Your answer is contained in the text below:

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