Whats the difference between using String.Equals(str1,str2) and str1 == str2

Question

Possible Duplicate:
C# difference between == and .Equals()

In my daily code routine I use them a lot, but really don't know how exactly they are different from each other.

if(String.Equals(str1, str2))

and

if(str1 == str2)

Please don't close. This is **not a duplicate** of difference between `==` and `object.Equals` -- although that is a good link. Both `string.Equals(a,b)` and `string.==` are *not* polymorphic, which is the big difference between `==` and `object.Equals`. — , Jan 06 '11 at 22:48
@Woot4Moo I disagree. This is a *specific* question about two *specific* ways of doing a *string comparison*. The "duplicate question" talks about `==`/`object.Equals` in *general*. Perhaps `string.Equals` could also be mentioned, but this would be lost in a merge. — , Jan 06 '11 at 22:52
@pst that's fine to disagree, but if you read the other question that is proposed by ChrisF you can see that it clearly illustrates the difference. Therefore merging or deletion of this duplicate would work. — Woot4Moo, Jan 06 '11 at 22:53
@Woot4Moo I read the other question, and the responses. None are a satisfactory answer which covers [`static bool string.Equals(string,string)`](http://msdn.microsoft.com/en-us/library/1hkt4325.aspx) -- this method is not the same as [`string.Equals(string)`](http://msdn.microsoft.com/en-us/library/858x0yyx.aspx). Since the other question is *general*, and this is *specific*, a merge/close is inadvisable in my mind. — , Jan 06 '11 at 22:54
If you read the responses `pst` you would see that `BlueMonk` and `Merhdad` both have the pieces to answer the question. — Woot4Moo, Jan 06 '11 at 22:56
@Woot4Moo Please read my last comment. BlueMonk and Merhdad are talking about the *non-static* [`string.Equals`](http://msdn.microsoft.com/en-us/library/858x0yyx.aspx) method. This question is about the *static* [`string.Equals`](http://msdn.microsoft.com/en-us/library/1hkt4325.aspx) method. — , Jan 06 '11 at 22:58
@pst if this is about the static version the OP didn't do a good job explaining it. — Woot4Moo, Jan 06 '11 at 22:59
@Woot4Moo Please note the question contains `String.Equals(str1, str2)` (perhaps better as `string.Equals(str1, str2)`) and not `str1.Equals(str2)`. — , Jan 06 '11 at 23:00

Aliostad · Accepted Answer · 2011-01-06T22:59:09.323

6

(UPDATE)

They are in fact exactly the same.

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

so == calls the Equals.

public static bool Equals(string a, string b)
{
    return ((a == b) || (((a != null) && (b != null)) && EqualsHelper(a, b)));
}

EqualsHelper is an unsafe method:

UPDATE What it does, it loops through the characters using integer pointers and compares them as integers (4byte at a time). It does it 10 at a time and then one at a time.

private static unsafe bool EqualsHelper(string strA, string strB)
{
    int length = strA.Length;
    if (length != strB.Length)
    {
        return false;
    }
    fixed (char* chRef = &strA.m_firstChar)
    {
        fixed (char* chRef2 = &strB.m_firstChar)
        {
            char* chPtr = chRef;
            char* chPtr2 = chRef2;
            while (length >= 10)
            {
                if ((((*(((int*) chPtr)) != *(((int*) chPtr2))) || (*(((int*) (chPtr + 2))) != *(((int*) (chPtr2 + 2))))) || ((*(((int*) (chPtr + 4))) != *(((int*) (chPtr2 + 4)))) || (*(((int*) (chPtr + 6))) != *(((int*) (chPtr2 + 6)))))) || (*(((int*) (chPtr + 8))) != *(((int*) (chPtr2 + 8)))))
                {
                    break;
                }
                chPtr += 10;
                chPtr2 += 10;
                length -= 10;
            }
            while (length > 0)
            {
                if (*(((int*) chPtr)) != *(((int*) chPtr2)))
                {
                    break;
                }
                chPtr += 2;
                chPtr2 += 2;
                length -= 2;
            }
            return (length <= 0);
        }
    }
}

edited Jan 06 '11 at 22:59

answered Jan 06 '11 at 22:44

Aliostad

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So what exactly does the bit extra do? At first glance it looks like a bitwise comparison. But wouldn't `==` return true in that case anyways? Is it something locale related? – CodesInChaos Jan 06 '11 at 22:48
so is there any cases where str1 == str2 wont equals String.Equals(str1,srt2)? – Rami Alshareef Jan 06 '11 at 22:51
@Remon if you look at the question that this is a `proposed` duplicate of you will find your answer. – Woot4Moo Jan 06 '11 at 22:55
1

The update doesn't really explain when == and EqualsHelper give different results. – CodesInChaos Jan 06 '11 at 22:55
1

They are exactly the same. Please see my latest update. – Aliostad Jan 06 '11 at 22:56
`==` in the `Equals` in fact is the reference comparison. – Aliostad Jan 06 '11 at 22:58
1

So it's a reflector bug that decodes it as `a==b` instead of `(object)a==(object)b`? In the form reflector decompiles it it looks like infinite recursion to me. – CodesInChaos Jan 06 '11 at 22:59
Sure, it cannot be recursion, it is just as you said `(object)a==(object)b` – Aliostad Jan 06 '11 at 23:02
Then can somebody who as an account there post it in the reflector forum and ask why it's decompiled like this/report it as a bug? – CodesInChaos Jan 06 '11 at 23:04

score 2 · Answer 2 · answered Jan 06 '11 at 23:03

2

They are absolutely the same. Here is what ildasm shows for ==

  IL_0002:  call       bool System.String::Equals(string,
                                              string)

Also read the documentation: http://msdn.microsoft.com/en-us/library/system.string.op_equality.aspx It says

This operator is implemented using the Equals method

answered Jan 06 '11 at 23:03

Stilgar

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intermediate language disassembler - a disassembler for .net byte-code – CodesInChaos Jan 06 '11 at 23:07
2

ildasm is the tool that shows the IL code for a .NET assembly. It is part of the SDK and its location depends on the actual version of the framework. For example C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin – Stilgar Jan 06 '11 at 23:08

Whats the difference between using String.Equals(str1,str2) and str1 == str2

2 Answers2

(UPDATE)