I have a condition in a silverlight application that compares 2 strings, for some reason when I use == it returns false while .Equals() returns true.
Here is the code:
if (((ListBoxItem)lstBaseMenu.SelectedItem).Content.Equals("Energy Attack"))
{
// Execute code
}
if (((ListBoxItem)lstBaseMenu.SelectedItem).Content == "Energy Attack")
{
// Execute code
}
Any reason as to why this is happening?
When == is used on an expression of type object, it'll resolve to System.Object.ReferenceEquals.
Equals is just a virtual method and behaves as such, so the overridden version will be used (which, for string type compares the contents).
When comparing an object reference to a string (even if the object reference refers to a string), the special behavior of the == operator specific to the string class is ignored.
Normally (when not dealing with strings, that is), Equals compares values, while == compares object references.
If two objects you are comparing are referring to the same exact instance of an object, then both will return true, but if one has the same content and came from a different source (is a separate instance with the same data), only Equals will return true. However, as noted in the comments, string is a special case because it overrides the == operator so that when dealing purely with string references (and not object references), only the values are compared even if they are separate instances. The following code illustrates the subtle differences in behaviors:
string s1 = "test";
string s2 = "test";
string s3 = "test1".Substring(0, 4);
object s4 = s3; // Notice: set to object variable!
Console.WriteLine($"{object.ReferenceEquals(s1, s2)} {s1 == s2} {s1.Equals(s2)}");
Console.WriteLine($"{object.ReferenceEquals(s1, s3)} {s1 == s3} {s1.Equals(s3)}");
Console.WriteLine($"{object.ReferenceEquals(s1, s4)} {s1 == s4} {s1.Equals(s4)}");
The output is:
True True True // s1, s2
False True True // s1, s3
False False True // s1, s4
Summary:
Variables
.ReferenceEquals
==
.Equals
s1, s2
True
True
True
s1, s3
False
True
True
s1, s4
False
False
True
== and .Equals are both dependent upon the behavior defined in the actual type and the actual type at the call site. Both are just methods / operators which can be overridden on any type and given any behavior the author so desires. In my experience, I find it's common for people to implement .Equals on an object but neglect to implement operator ==. This means that .Equals will actually measure the equality of the values while == will measure whether or not they are the same reference.
When I'm working with a new type whose definition is in flux or writing generic algorithms, I find the best practice is the following
If I want to compare references in C#, I use Object.ReferenceEquals directly (not needed in the generic case)
If I want to compare values I use EqualityComparer<T>.Default
In some cases when I feel the usage of == is ambiguous I will explicitly use Object.Reference equals in the code to remove the ambiguity.
Eric Lippert recently did a blog post on the subject of why there are 2 methods of equality in the CLR. It's worth the read
http://blogs.msdn.com/ericlippert/archive/2009/04/09/double-your-dispatch-double-your-fun.aspx
== Operator
If operands are Value Types and their values are equal, it returns true else false.
If operands are Reference Types with exception of string and both refer to the same instance (same object), it returns true else false.
If operands are string type and their values are equal, it returns true else false.
.Equals
If operands are Reference Types, it performs Reference Equality that is if both refer to the same instance (same object), it returns true else false.
If Operands are Value Types then unlike == operator it checks for their type first and if their types are same it performs == operator else it returns false.
As far as I understand it the answer is simple:
== compares object references.
.Equals compares object content.
String datatypes always act like content comparison.
I hope I'm correct and that it answered your question.
Firstly, there is a difference. For numbers
> 2 == 2.0
True
> 2.Equals(2.0)
False
And for strings
> string x = null;
> x == null
True
> x.Equals(null)
NullReferenceException
In both cases, == behaves more usefully than .Equals
I would add that if you cast your object to a string then it will work correctly. This is why the compiler will give you a warning saying:
Possible unintended reference comparison; to get a value comparison,
cast the left hand side to type 'string'
Because the static version of the .Equal method was not mentioned so far, I would like to add this here to summarize and to compare the 3 variations.
MyString.Equals("Somestring")) //Method 1
MyString == "Somestring" //Method 2
String.Equals("Somestring", MyString); //Method 3 (static String.Equals method) - better
where MyString is a variable that comes from somewhere else in the code.
Background info and to summerize:
In Java using == to compare strings should not be used. I mention this in case you need to use both languages and also
to let you know that using == can also be replaced with something better in C#.
In C# there's no practical difference for comparing strings using Method 1 or Method 2 as long as both are of type string. However, if one is null, one is of another type (like an integer), or one represents an object that has a different reference, then, as the initial question shows, you may experience that comparing the content for equality may not return what you expect.
Suggested solution:
Because using == is not exactly the same as using .Equals when comparing things, you can use the static String.Equals method instead. This way, if the two sides are not the same type you will still compare the content and if one is null, you will avoid the exception.
bool areEqual = String.Equals("Somestring", MyString);
It is a little more to write, but in my opinion, safer to use.
Here is some info copied from Microsoft:
public static bool Equals (string a, string b);
Parameters
a String
The first string to compare, or null.
b String
The second string to compare, or null.
Returns Boolean
true if the value of a is the same as the value of b; otherwise, false. If both a and b are null, the method returns true.
Just as an addition to the already good answers: This behaviour is NOT limited to Strings or comparing different numbertypes. Even if both elements are of type object of the same underlying type. "==" won't work.
The following screenshot shows the results of comparing two object {int} - values
I am a bit confused here. If the runtime type of Content is of type string, then both == and Equals should return true. However, since this does not appear to be the case, then runtime type of Content is not string and calling Equals on it is doing a referential equality and this explains why Equals("Energy Attack") fails. However, in the second case, the decision as to which overloaded == static operator should be called is made at compile time and this decision appears to be ==(string,string). this suggests to me that Content provides an implicit conversion to string.
There is another dimension to an earlier answer by #BlueMonkMN. The additional dimension is that the answer to the #Drahcir's title question as it is stated also depends on how we arrived at the string value. To illustrate:
string s1 = "test";
string s2 = "test";
string s3 = "test1".Substring(0, 4);
object s4 = s3;
string s5 = "te" + "st";
object s6 = s5;
Console.WriteLine("{0} {1} {2}", object.ReferenceEquals(s1, s2), s1 == s2, s1.Equals(s2));
Console.WriteLine("\n Case1 - A method changes the value:");
Console.WriteLine("{0} {1} {2}", object.ReferenceEquals(s1, s3), s1 == s3, s1.Equals(s3));
Console.WriteLine("{0} {1} {2}", object.ReferenceEquals(s1, s4), s1 == s4, s1.Equals(s4));
Console.WriteLine("\n Case2 - Having only literals allows to arrive at a literal:");
Console.WriteLine("{0} {1} {2}", object.ReferenceEquals(s1, s5), s1 == s5, s1.Equals(s5));
Console.WriteLine("{0} {1} {2}", object.ReferenceEquals(s1, s6), s1 == s6, s1.Equals(s6));
The output is:
True True True
Case1 - A method changes the value:
False True True
False False True
Case2 - Having only literals allows to arrive at a literal:
True True True
True True True
Adding one more point to the answer.
.EqualsTo() method gives you provision to compare against culture and case sensitive.
Really great answers and examples!
I would just like to add the fundamental difference between the two,
Operators such as == are not polymorphic, while Equals is
With that concept in mind, if you work out any example (by looking at left hand and right hand reference type, and checking/knowing if the type actually has == operator overloaded and Equals being overriden) you are certain to get the right answer.
This is due to value equality (equal method) and referential equality(== operator), as the equal method checks the values while the same == is checked the reference.
== operator overriding code available inside the string class on https://referencesource.microsoft.com/
so now it's easier to understand, the equal method also has 2 implementations one from the string class itself and one from the object class. its impact on performance as well I also run some basic code and try to understand the benchmarks.
I am sharing the results below Please correct or advise if I am wrong somewhere. there are 3 cases and I have run the same code for all the cases and this is the result.
case 1: here I am using string. equal method for comparing 2 strings and both the string have the same values. string.equals(a,b)
1st run:5608195 ticks
2nd run:5529387 ticks
3rd run:5622569 ticks
total ticks: 16760151
case 2: here I am using string. equal() method(overloaded one) for comparing 2 strings and both the string have the same values.
a.equals(b)
1st run: 6738583 ticks
2nd run: 6452927 ticks
3rd run: 7168897 ticks
total ticks=20360407
case 3: here I am using the == operator for comparing 2 strings and both the string have the same values.
a==b
1st run: 6652151 ticks
2nd run: 7514300 ticks
3rd run: 7634606 ticks
total ticks=21801057
class Program
{
private static int count;
static string a = "abcdef";
static string b = "abcdef";
static void Main(string[] args)
{
for (int j = 1; j <= 3; j++)
{
Stopwatch sw = new Stopwatch();
sw.Start();
for (int i = 1; i <= 1000; i++)
{
checkString();
}
sw.Stop();
Console.WriteLine(sw.ElapsedTicks);
}
Console.ReadLine();
}
public static void checkString()
{
for (int i = 1; i <= 100000; i++)
{
if (a==b)
count++;
}
}
}
The == token in C# is used for two different equality-check operators. When the compiler encounters that token, it will check whether either of the types being compared has implemented an equality-operator overload for either the specific combination types being compared(*), or for a combination of types to which both types can be converted. If the compiler finds such an overload it will use it. Otherwise, if the two types are both reference types and they are not unrelated classes (either may be an interface, or they may be related classes), the compiler will regard == as a reference-comparison operator. If neither condition applies, compilation will fail.
Note that some other languages use separate tokens for the two equality-check operators. In VB.NET, for example, the = token is used within expressions solely for the overloadable equality-check operator, and Is is used as a reference-test or null-test operator. An to use = on a type which does not override the equality-check operator will fail, as will attempting to use Is for any purpose other than testing reference equality or nullity.
(*)Types generally only overload equality for comparison with themselves, but it may be useful for types to overload the equality operator for comparison with other particular types; for example, int could have (and IMHO should have but didn't) defined an equality operators for comparison with float, so that 16777217 would not report itself equal to 16777216f. As it is, since no such operator is defined, C# will promote the int to float, rounding it to 16777216f before the equality-check operator sees it; that operator then sees two equal floating-point numbers and reports them as equal, unaware of the rounding that took place.
Note that there are two different types of equality in C#
1- Value Equality (For value types like int, DateTime and struct)
2- Reference Equality (For objects)
There are two basic standard protocols for implement equality checks.
1- The == and != operators.
2- The virtual Equals method.
The == and != are statically resolve, which means C# will make a compile-time decision as to which type will perform the comparison.
For instance the value-type
int x = 50;
int y = 50;
Console.WriteLine (x == y); // True
but for reference type
object x = 50;
object y = 50;
Console.WriteLine (x == y); // False
The Equals() originally resoled at runtime according to operand actual type.
For instance, in the following example, at runtime, it will be decided that the Equals() will apply on int values, the result is true.
object x = 5;
object y = 5;
Console.WriteLine (x.Equals (y)); // True
However, for a reference type, it will use a reference equality check.
MyObject x = new MyObject();
MyObject y = x;
Console.WriteLine (x.Equals (y)); // True
Note that Equals() uses structural comparison for struct, which means it calls Equals on each field of a struct.
Whether Equals() and == are the same depends on the implementation. Because C# allows users to set different behaviors for Equals() and == respectively.
class CompareTest
{
public readonly int val;
public CompareTest(int val)
{
this.val = val;
}
public override bool Equals(object obj)
{
return obj is CompareTest test && this.val == test.val;
}
public override int GetHashCode()
{
return val;
}
public static bool operator == (CompareTest a, object b)
{
return Equals(a, b);
}
public static bool operator != (CompareTest a, object b)
{
return !(a == b);
}
}
In this example, I made Equals() and == have the same behavior. But what if I make them different? For example:
public static bool operator == (CompareTest a, object b)
{
return false;
}
Equals() works normally but == will never works.
Furthermore, although I made them have the same behavior, but there is still one difference: which == function will be called depends on the left value:
Compare Test a = new CompareTest(1);
object b = new CompareTest(1);
CompareTest c = new CompareTest(1);
Debug.Log("AB " + (a == b)); // true
Debug.Log("BA " + (b == a)); // false! because it calls object's == function
Debug.Log("AC " + (a == c)); // true
Debug.Log("CA " + (c == a)); // true
When we create any object there are two parts to the object one is the content and the other is reference to that content.
== compares both content and reference;
equals() compares only content
http://www.codeproject.com/Articles/584128/What-is-the-difference-between-equalsequals-and-Eq
==
The == operator can be used to compare two variables of any kind, and it simply compares the bits.
int a = 3;
byte b = 3;
if (a == b) { // true }
Note : there are more zeroes on the left side of the int but we don't care about that here.
int a (00000011) == byte b (00000011)
Remember == operator cares only about the pattern of the bits in the variable.
Use == If two references (primitives) refers to the same object on the heap.
Rules are same whether the variable is a reference or primitive.
Foo a = new Foo();
Foo b = new Foo();
Foo c = a;
if (a == b) { // false }
if (a == c) { // true }
if (b == c) { // false }
a == c is true
a == b is false
the bit pattern are the same for a and c, so they are equal using ==.
Equal():
Use the equals() method to see if two different objects are equal.
Such as two different String objects that both represent the characters in "Jane"
The only difference between Equal and == is on object type comparison. in other cases, such as reference types and value types, they are almost the same(either both are bit-wise equality or both are reference equality).
object:
Equals: bit-wise equality
==: reference equality
string: (equals and == are the same for string, but if one of string changed to object, then comparison result will be different)
Equals: bit-wise equality
== : bit-wise equality
See here for more explanation.
Why we have different output for below cases:
object obj = "Int32";
string str1 = "Int32";
string str2 = typeof(int).Name;
Console.WriteLine(obj == str1); // true
Console.WriteLine(str1 == str2); // true
Console.WriteLine(obj == str2); // false !?
The answer to this is quite simple:
When you compare an object with a string, a reference comparison is used, which will only be true if both objects have the same reference.
When you compare strings a string comparison is used, which will be true if the string contents are the same regardless of whether they are the same reference.
In your third comparison you are using an object comparison where you are comparing two strings with identical contents but with different references, so it will return false.
The added complication is that the first two strings have the same reference because they are compile-time constants and have been interned by the compiler so that they refer to the same string in memory.
I've annotated your original code with this explanation:
object obj = "Int32"; // As a compile-time constant string, this will be interned.
string str1 = "Int32"; // This is also interned, so has the same reference as obj
string str2 = typeof(int).Name; // Same contents as str1, but a different reference
// (created at runtime, so it wasn't interned)
Console.WriteLine(obj == str1); // Reference comparison: true because the references are the same
Console.WriteLine(str1 == str2); // String comparison: true because the string contents are the same.
Console.WriteLine(obj == str2); // Reference comparison: false because the references different.
Also:
If you are using Resharper it actually warns you for the first and last comparison, saying: "Possible unintended reference comparison".
You can get the same result by declaring str2 as follows:
string str2 = string.Concat("Int", "32");
The obj == str2 line used reference comparison. Whereas the obj == str1 line did not.
Why?
The string type in .NET is an implicit reference type. But it's not at the same time. It's one of those types that is technically a reference type, but it's been programmed to act like a value type, and is immutable, meaning it is not directly modified.
When you create a new string, a reference is allocated for it and that reference is what your string variable holds. You get to do all sorts of things with that string, but you can never change it. If you reassign the value it will merely create a new reference.
In this instance, the obj == str1 line used reference comparison, but the references actually matched. Because they were both hard-coded, the compiler, and .NET, can use the same reference for each. (As we said before, strings are immutable.) You should read the interning link Matthew posted for more information on that.
So, if the strings match, why did .NET create a different reference?
Consider the significant amount of objects you could create in memory. If, at any time, you created a new string and .NET went through all of the other strings to find one that matched, your programme would be phenomenally slow. You would barely get any real work done.
So, .NET optimizes this away. If you change your code a bit, you'll see what I mean.
object obj = "Int32";
string str1 = typeof(int).Name;
string str2 = typeof(int).Name;
Console.WriteLine(obj == str1); // false
Console.WriteLine(str1 == str2); // true
Console.WriteLine(obj == str2); // false !?
The str1 == str2 line still returns true, because it's actually comparing the strings, whereas the obj == str1 line is now false, because it's comparing the references of those as well.
In C# strings are immutable and managed. In theory that would mean the concatenation of any strings A and B would cause the allocation of a new buffer however this is all pretty obfuscated. When you concatenate with the identity (the empty string) the reference maintains intact. Is this a compile time optimization or is the overloaded assignment operator making the decision to not realloc at runtime? Furthermore, how does the runtime/compiler handle s2's value/allocation when I modify the value of s1? My program would indicate that the memory at the original address of s1 remains intact (and s2 continues pointing there) while a relloc occurs for the new value and then s1 is pointed there, is this an accurate description of what happens under the covers?
Example program;
static void Main(string[] args)
{
string s1 = "Some random text I chose";
string s2 = s1;
string s3 = s2;
Console.WriteLine(Object.ReferenceEquals(s1, s2)); // true
s1 = s1 + "";
Console.WriteLine(Object.ReferenceEquals(s1, s2)); // true
Console.WriteLine(s2);
s1 = s1 + " something else";
Console.WriteLine(Object.ReferenceEquals(s1, s2)); // false cause s1 got realloc'd
Console.WriteLine(Object.ReferenceEquals(s2, s3));
Console.WriteLine(s2);
Console.ReadKey();
}
When you concatenate with the identity (the empty string) the reference maintains intact. Is this a compile time optimization or is the overloaded assignment operator making the decision to not realloc at runtime?
It is both a compile time optimization and also an optimization performed in the implementation of the overloaded concatenation operator. If you concat two compile time literals, or concat a string known to be null or empty at compile time, the concatenation is done at compile time, and then potentially interned, and will therefore be reference equal to any other compile time literal string that has the same value.
Additionally, String.Concat is implemented such that if you concat a string with either null or an empty string, it just returns the other string (unless the other string was null, in which case it returns an empty string). The test you already have demonstrates this, as you're concatting a non-compile time literal string with an empty string and it's staying reference equal.
Of course if you don't believe your own test, you can look at the source to see that if one of the arguments is null then it simply returns the other.
if (IsNullOrEmpty(str0)) {
if (IsNullOrEmpty(str1)) {
return String.Empty;
}
return str1;
}
if (IsNullOrEmpty(str1)) {
return str0;
}
When you concatenate with the identity (the empty string) the reference maintains intact. Is this a compile time optimization or is the overloaded assignment operator making the decision to not realloc at runtime?
This is a run-time optimization. Here is how it is implemented in Mono:
public static String Concat(String str0, String str1) {
Contract.Ensures(Contract.Result() != null);
Contract.Ensures(Contract.Result().Length ==
(str0 == null ? 0 : str0.Length) +
(str1 == null ? 0 : str1.Length));
Contract.EndContractBlock();
// ========= OPTIMIZATION BEGINS ===============
if (IsNullOrEmpty(str0)) {
if (IsNullOrEmpty(str1)) {
return String.Empty;
}
return str1;
}
if (IsNullOrEmpty(str1)) {
return str0;
}
// ========== OPTIMIZATION ENDS =============
int str0Length = str0.Length;
String result = FastAllocateString(str0Length + str1.Length);
FillStringChecked(result, 0, str0);
FillStringChecked(result, str0Length, str1);
return result;
}
The compiler may produce additional optimizations of its own - for example, concatenating two string literals produces a new literal value at compile time, without calling string.Concat. This is not different from C#'s handling of other expressions that include compile-time constants of other data types, though.
Furthermore, how does the runtime/compiler handle s2's value/allocation when I modify the value of s1?
s1 and s2 are independent references to the same string object, which is immutable. Reassigning another object to one of them does not change the other reference.
It is a decision by the String.Concat function not to concat the string. It checks whether s1 is null and assigns "" to s1 if yes.
s1 = s1 + "";
gets optimized by the comiler.
s1 = s1 ?? "";
If you want to learn more check out this link
String concatenation is specified to return a string whose sequence of characters is the concatenation of the sequences encapsulated by the string representations of the things being concatenated. In cases where no existing string contains the proper sequence of characters, the concatenation code will need to create a new one; further, even in cases where an existing string might contain the proper sequence of characters, it will usually be faster for the computer to create a new string than try to find the existing one. I believe, however, that concatenation is allowed to return an existing string in any case where it can quickly find one that contains the proper characters, and in the case of concatenating a zero-length string to a non-zero-length string, finding a string which contains the proper characters is easy.
Because of behavioral details like the above, in most cases the only legitimate application of ReferenceEquals with strings is in situations where a true result is interpreted to say "the strings definitely contain the same characters" and a "false" result to say "the strings might not contain the same characters". It should not be interpreted as saying anything about where the strings came, how they were created, or anything like that.
When you concatenate with the identity (the empty string) the
reference maintains intact. Is this a compile time optimization or is
the overloaded assignment operator making the decision to not realloc
at runtime?
Neither. It's the Concat method that does that decision. The code is actually compiled into:
s1 = String.Concat(s1, "");
The Concat method contains this code, that makes it return the first parameter if the second is empty:
if (IsNullOrEmpty(str1)) {
return str0;
}
Ref: Microsoft reference source: String.Concat(string, string)
My program would indicate that the memory at the original address of
s1 remains intact (and s2 continues pointing there) while a relloc
occurs for the new value and then s1 is pointed there
That is correct.
I know there are a lot of ways to compare VALUE and REFERENCES in C#, but I'm still a bit confused about what type performs what when you try to compare either VALUE or REFERENCE.
String examples:
string str = "hello";
string str2 = "hello";
if (str == str2)
{
Console.WriteLine("Something");
} // Is this a comparison of value?
if (str.Equals(str2))
{
Console.WriteLine("Something");
} // Is this a comparison of value?
string.ReferenceEquals(str, str2); // Comparison of reference (True)
Console.WriteLine((object)str1 == (object)str2); // Comparison of reference (True)
Equals and == will compare by reference by default if they're not overriden / overloaded in a subclass. ReferenceEquals will always compare by reference.
Strings are a confusing data type to use for experimenting with this, because they overload == to implement value equality; also, since they're immutable, C# will generally reuse the same instance for the same literal string. In your code, str and str2 will be the same object.
#Inerdia is right with what he says but I'd like to point out the reason why the line string.ReferenceEquals(str, str2) returns true in your code example. Because you are defining both of the strings at compile time, the compiler can optimise the code so they can both point to the same instance of the string. Since strings are immutable the compiler knows it can do this even though String is a reference type. But If you change your code to dynamically generate one of the strings (as shown below) the compiler can't perform this optimisation. So in your code example if you change your code to:
string str = "hello";
string str2 = new StringBuilder().Append("he").Append("llo").ToString();
Then the string.ReferenceEquals(str, str2) line will now return false as this time the compiler cant know to re-use the same instance (reference of the string).
Equality and Comparision of ReferenceTypes and strings:
Reference types work like this:
System.Object a = new System.Object();
System.Object b = new System.Object();
a == b; //returns true
a.Equals(b); //returns false
b = a;
a == b; //returns true
a.Equals(b); //returns true
Since strings are Reference types they should do the same, shouldn't they? But they don't!
C# Documentation defines string equality like this:
Although string is a reference type, the equality operators (== and
!=) are defined to compare the values of string objects, not
references (7.9.7 String equality operators). This makes testing for
string equality more intuitive.
https://msdn.microsoft.com/en-us/library/362314fe%28v=vs.71%29.aspx
https://msdn.microsoft.com/en-us/library/aa664728%28v=vs.71%29.aspx
This has implications for you test code.
if (str == str2)
{
Console.WriteLine("Something");
} // This is comparision of value even though string is a referenceType
if (str.Equals(str2))
{
Console.WriteLine("Something");
} // This is comparison by value too, because Equals is overrided in String class.
Keep in mind you as a programmer (Or your tricky coworker) can override .Equals(), changing it's behaviour, what you see above is what should happen. It's not necessarily in line with your codebase-reality, when in doubt check out the definition by marking .Equals() and hitting F12.
Addendum for x.Equals
The behavior of object.Equals() should these rules:
List item
x.Equals(x) returns true.
x.Equals(y) returns the same value as y.Equals(x).
if (x.Equals(y) && y.Equals(z)) returns true, then x.Equals(z) returns true.
Successive invocations of x.Equals(y) return the same value as long as the objects referenced by x and y are not modified.
x.Equals(null) returns false.
https://msdn.microsoft.com/ru-ru/library/ms173147%28v=vs.80%29.aspx
Whenever you are in doubt you can call x.ReferenceEquals, it's defined as following:
Unlike the Object.Equals(Object) method and the equality operator, the
Object.ReferenceEquals(Object) method cannot be overridden. Because of
this, if you want to test two object references for equality and you
are unsure about the implementation of the Equals method, you can call
the method.
https://msdn.microsoft.com/de-de/library/system.object.referenceequals%28v=vs.110%29.aspx
Thus:
System.Object a = new System.Object();
System.Object b = a;
System.Object.ReferenceEquals(a, b); //returns true
In your example the compiler merges your strings in optimization thus:
string str = "hello";
string str2 = "hello";
string.ReferenceEquals(str, str2); // Comparison of reference (True)
This behaviour is only duo to compiler optimization in your example, if we randomize the code it will return false:
string str = "hello";
string str2 = "hello";
if(throwCoin)
{
str2 = "bye";
}
string.ReferenceEquals(str, str2); // Comparison of reference (False)
string.ReferenceEquals(str, str2);
It obviously compares references.
str.Equals(str2)
Tries to compare references at first. Then it tries to compare by value.
str == str2
Does the same as Equals.
A good way to compare strings is to use string.Compare. If you want to ignore case, there is a parameter in place for that too.
Excerpt from .net sources:
public bool Equals(string value)
{
if (this == null)
throw new NullReferenceException();
else if (value == null)
return false;
else if (object.ReferenceEquals((object) this, (object) value))
return true;
else
return string.EqualsHelper(this, value);
}
So in general it is comparision of references first and if they don't match, it compares values.
I am relatively new to C#, and I noticed something interesting today that I guess I have never noticed or perhaps I am missing something. Here is an NUnit test to give an example:
object boolean1 = false;
object booloan2 = false;
Assert.That(boolean1 == booloan2);
This unit test fails, but this one passes:
object string1 = "string";
object string2 = "string";
Assert.That(string1 == string2);
I'm not that surprised in and of itself that the first one fails seeing as boolean1, and boolean2 are different references. But it is troubling to me that the first one fails, and the second one passes. I read (on MSDN somewhere) that some magic was done to the String class to facilitate this. I think my question really is why wasn't this behavior replicated in bool? As a note... if the boolean1 and 2 are declared as bool then there is no problem.
What is the reason for these differences or why it was implemented that way? Is there a situation where you would want to reference a bool object for anything except its value?
It's because the strings are in fact referring the same instance. Strings are interned, so that unique strings are reused. This means that in your code, the two string variables will refer to the same, interned string instance.
You can read some more about it here: Strings in .NET and C# (by Jon Skeet)
Update
Just for completeness; as Anthony points out string literals are interned, which can be showed with the following code:
object firstString = "string1";
object secondString = "string1";
Console.WriteLine(firstString == secondString); // prints True
int n = 1;
object firstString = "string" + n.ToString();
object secondString = "string" + n.ToString();
Console.WriteLine(firstString == secondString); // prints False
Operator Overloading.
The Boolean class does not have an overloaded == operator. The String class does.
As Fredrik said, you are doing a reference compare with the boolean comparison. The reason the string scenario works is because the == operator has been overloaded for strings to do a value compare. See the System.String page on MSDN.