When iterating over a collection of DataGridViewCells in a DataGridViewRow, I was testing for equality of a specific cell like so:
var transactionLogId = dataGridView1.Rows[dataGridView1.SelectedCells[0].RowIndex].Cells[0].Value;
if (dgvRow.Cells[0].Value == transactionLogId) {
// Snip
}
And to my dismay, it never entered the nested block. Though both equal the same (5177518), and both dgvRow.Cells[0].Value.GetType().Name and transactionLogId.GetType().Name are Int64, it doesn't work unless I append a .ToString() to both sides of the equality check.
Curious as to what I'm missing here. Thanks!
Operators are static calls, which means that overload resolution will resolve based strictly on the operand's compile time types.
In your case, overload resolution is resolving to the following operator overload:
==(object, object)
because those are the compile time types of the operands. The == for objects simply performs a reference equality check, that is, if both objects are the same object, which they clearly are not.
The same happens in the following code:
object o1 = 1;
object o2 = 1;
var equal = o1 == o2; //false
Using Equals in this case is the correct thing to do, because it is a virtual call and it will therefore resolve to the runtime type of the callee and give you the correct result.
Then why does this work when you call ToString() on both operands? Simply because the overload resolution resolves to the ==(string, string) overload implented in the string class which performs value equality.
UPDATE
Reading comments by Machine Learning there seems to be a misunderstanding concerning the reason why == is returning false in the OP's code. I want to clarify that boxing has nothing to do with it; consider the following example:
var str1 = "a";
var str2 = 'a'.ToString(); //to avoid string interning by the compiler.
var equals = str1 == str2; //true
object o1 = str1;
object o2 = str2;
equals = o1 == o2; //false
The observed behavior is exactly the same and there is no boxing/unboxing happening here.
They are both boxed values so the == operator will never be true.
GetType returns the type of the corresponding unboxed instances anyway.
You need to use Equals.
I'm wondering how you could compare two boxed integers (either can be signed or unsigned) to each other for equality.
For instance, take a look at this scenario:
// case #1
object int1 = (int)50505;
object int2 = (int)50505;
bool success12 = int1.Equals(int2); // this is true. (pass)
// case #2
int int3 = (int)50505;
ushort int4 = (ushort)50505;
bool success34 = int3.Equals(int4); // this is also true. (pass)
// case #3
object int5 = (int)50505;
object int6 = (ushort)50505;
bool success56 = int5.Equals(int6); // this is false. (fail)
I'm stumped on how to reliably compare boxed integer types this way. I won't know what they are until runtime, and I can't just cast them both to long, because one could be a ulong. I also can't just convert them both to ulong because one could be negative.
The best idea I could come up with is to just trial-and-error-cast until I can find a common type or can rule out that they're not equal, which isn't an ideal solution.
In case 2, you actually end up calling int.Equals(int), because ushort is implicitly convertible to int. This overload resolution is performed at compile-time. It's not available in case 3 because the compiler only knows the type of int5 and int6 as object, so it calls object.Equals(object)... and it's natural that object.Equals will return false if the types of the two objects are different.
You could use dynamic typing to perform the same sort of overload resolution at execution time - but you'd still have a problem if you tried something like:
dynamic x = 10;
dynamic y = (long) 10;
Console.WriteLine(x.Equals(y)); // False
Here there's no overload that will handle long, so it will call the normal object.Equals.
One option is to convert the values to decimal:
object x = (int) 10;
object y = (long) 10;
decimal xd = Convert.ToDecimal(x);
decimal yd = Convert.ToDecimal(y);
Console.WriteLine(xd == yd);
This will handle comparing ulong with long as well.
I've chosen decimal as it can exactly represent every value of every primitive integer type.
Integer is a value type. When you compare two integers types, compiller checks their values.
Object is a reference type. When you compare two objects, compiller checks their references.
The interesting part is here:
object int5 = (int)50505;
Compiller perfoms boxing operation, wraps value type into reference type, and Equals will compare references, not values.
This question already has answers here:
C# difference between == and Equals()
(20 answers)
Why would you use String.Equals over ==? [duplicate]
(8 answers)
Closed 7 years ago.
Here's part of the data from my DataSet:
Take a look at the 16th row's data and the 17 row's data (partno column), the value is "CTS04", but when I do the checking later on, below scenario happens:
The partno column data type is string, and 17 row's data was created manually using below code:
DataRow newRow = ds.Tables[tableToAdd].Rows.Add();
newRow["partno"] = ds.Tables[tableToAdd].Rows[i - 1]["partno"];
So, logically, I'm just trying to copy the partno from the above row, but why when I use "==" operator to check the value, it returns false?
DataRow[index] returns the content typed as an object. So when you do <object1> == <object2>, the equality comparison of the object type is used, and that will only be true if there is a reference equality between those two objects.
Since your values are actually strings though, you want to use a string based comparison. You can only do that using == if you cast them to strings. Or use .Equals() which is overloaded by the string type to make the comparison:
object a = "foo";
object b = new StringBuilder("foo").ToString(); // this is to make sure that we get a new string object instead of the constant "foo" that is created at compile-time
Console.WriteLine(a == b); // false
Console.WriteLine((string)a == (string)b); // true
Console.WriteLine(a.Equals(b)); // true
For more information see Why would you use String.Equals over ==? and C# difference between == and Equals().
For objects, the equality operator (==) performs a "ReferenceEquals". In simple types like numers and strings, the operator performs value compare.
ds.Tables[tableToAdd].Rows[i - 1]["partno"] does not return a simple value.
try ds.Tables[tableToAdd].Rows[i - 1]["partno"].GetType();
== comparing the Data Rows not the Value of that rows. Both these rows are different so result is false. In second case it compared value and because value is same the result is true.
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.
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.