Develop Reference

Shortcut &= acting like & not &&

public static void Main()
{
var result = false;
Xpto xpto = null;
result &= xpto.Some;
}
public class Xpto
{
public bool Some { get; set; }
}
I'm using C# 6.0 VS2015; the code above triggers a null exception.
The &= operator is behaving like the & operator. I would expect it to behave similar to && and skip evaluation of the right side if value already false (see explanation of "short-circuit" behavior in The conditional-AND operator (&&) article).
Is this behavior intended or is it a bug?

Take a look at the documentation: &= Operator:
x &= y
is equivalent to x = x & y
(...) The & operator performs a bitwise logical AND operation on integral operands and logical AND on bool operands.
I'm assuming you're trying to avoid the null check.
The code below will not throw an exception because you stated the result variable is false; in this case, the language optimization will short-circuit the expression and will not evaluate the second part:
result = result && xpto.Some;
If you're a on liner person, you can make it more same by writing:
result = result && (xpto == null ? true : xpto.Some);
or, in plain C#:
if (xpto != null)
result = result && xpto.Some;

&& operator overloading and assignments in C# - Clarification?

Following this very interesting issue which was originated from this question -
I want to take 1 steps back please (removed the dynamic environment) :
Looking at this code : ( a variant of this one)
void Main()
{
int a;
int b = 100;
Console.WriteLine(X.M(1, out a));
}
public class X
{
public int H=0;
public static X M(int x, out int y)
{
Console.WriteLine("x = "+x);
y = x;
return new X(x);
}
public X(){}
public X(int h)
{
H=h;
}
public static bool operator false(X x) {Console.WriteLine("in false operator for "+ x.H); return true; }
public static bool operator true(X x) {Console.WriteLine("in true operator for "+ x.H); return true; }
public static X operator &(X a, X b) {Console.WriteLine("in & operator for "+ a.H+","+b.H); return new X(); }
public static implicit operator bool (X x) {Console.WriteLine("in bool operator for "+ x.H);return true; }
}
The result is :
x = 1
in bool operator for 1
True
This is understood :
The x = 1 is from the method itself ( using Console.Writeline)
in bool operator for 1 is from the implicit operator from X to Bool
(so - Console.WriteLine treats the whole expression as Console.Writeline(bool))
The last "True" is from the "return true" in the operator bool (X x)
OK - So let's change
Console.WriteLine(X.M(1, out a));
to
Console.WriteLine(X.M(1, out a) && X.M(2, out b));
Now - the result is :
x = 1
in false operator for 1
in bool operator for 1
True
2 questions :
Why does this in false operator for 1 executes ? I don't see any reason for false to be present here.
I could understand why the right part in X.M(1, out a) && X.M(2, out b) won't executes ONLY if the left part is false - but again I don't see how the left part can be false. It does return true (according to my first code)
NB
I've read many times the answers from the post :
Jon said :
The second && is a normal && between two bool expressions - because
Nop returns bool, and there's no &(X, bool) operator... but there is a
conversion from X to bool.
So it's more like:
bool first = X.M(1, out a) && X.M(2, out b);
if (first && Nop(a, b))
Now first is true even though only the first operand of && has been
evaluated... so b really hasn't been assigned.
Still I don't understand : "first is true(????) even though only the first operand of && has been evaluated"

Firstly, don't forget that this is deliberately bizarre code, used to find a corner case. If you ever find a type that behaves like this in a genuine program, find the author and have a quiet word with them.
Still I don't understand : "first is true(????) even though only the first operand of && has been evaluated"
Yes, because of the way the && operand is handled in the case where the operands aren't bool. It's specified in section 7.12.2 of the C# spec:
The operation x && y is evaluated as T.false(x) ? x : T.&(x, y) where T.false(x) is an invocation of the operator false declared in T, and T.&(x, y) is an invocation of the selected operator in &. In other words, x is first evaluated and operator false is invoked on the result to determine if x is definitely false. Then, if x is definitely false, the result of the operation is the value previously computed for x. Otherwise, y is evaluated, and the selected operator & is invoked on the value previously computed for x and the value computed for y to produce the result of the operation.
So, in order:
X.M(1, out a) is invoked, to get a result - call it op1 for the moment
Next X.false(op1) is invoked, and returns true
Then by the above, the result of the expression X.M(1, out a) && X.M(2, out b) is op1
Next, the value of the conversion from op1 to bool is invoked, and that returns true. This is due to overload resolution for Console.WriteLine.
To answer your specific confusion:
but again I don't see how the left part can be false.It does return true (according to my first code)
It returns a value which is somewhat contradictory - it's false in that the false operator returns true, but it's true in that the conversion to bool returns true. Once you understand that it's the value returned by the false operator which determines whether or not to evaluate the second operand of &&, it should all be clear.

When does overloaded false operator ever gets executed and what is it good for?

I've been searching for actual working code where an overloaded false operator actually gets executed.
This question (What's the false operator in C# good for?) is somewhat the same, but the accepted answer links to an url which is returning a 404 error. I've also looked at How does operator overloading of true and false work? and some other questions.
What I've found in almost all answers is that false only gets executed when you use a short circuited and like x && y. This is evaluated as T.false(x) ? x : T.&(x, y).
Ok, so I have the following code. The struct contains an int and considers itself true if the int is greater than zero.:
public struct MyStruct {
private int _i;
public MyStruct(int i) {
_i = i;
}
public static bool operator true(MyStruct ms) {
return ms._i > 0;
}
public static bool operator false(MyStruct ms) {
return ms._i <= 0;
}
public override string ToString() {
return this._i.ToString();
}
}
Now I would hope the following program will execute and use the overloaded false operator.
class Program {
private static void Main() {
MyStruct b1 = new MyStruct(1); // to be considered true
MyStruct b2 = new MyStruct(-1); // to be considered false
Console.WriteLine(b1 && b2);
Console.WriteLine(b2 && b1);
}
}
However, it does not even compile. It says it cannot apply operator '&&' to operands of type 'MyStruct' and 'MyStruct'.
I know I can implement an overload of the & operator. So let's do that. The & must return a MyStruct, so I can not make it return a bool.
public static MyStruct operator &(MyStruct lhs, MyStruct rhs) {
return new MyStruct(lhs._i & rhs._i);
}
Now the code does compile. Its output is 1 and -1. So the result of b1 && b2 is not the same as that of b2 && b1.
If I debug the code, I see that b1 && b2 first executes the false operator on b1, which returns false. Then it performs the & operator on b1 and b2, which performs a bitwise and on 1 and -1, resulting in 1. So it indeed is first checking if b1 is false.
The second expression, b2 && b1 first executes the false operator on b2, which returns true. Combined with the fact I'm using short circuiting, it doesn't do anything with b1 and just prints out the value of b2.
So yes, the false operator is executed when you use short circuiting. However, it does not execute the true or false operator on the second argument, but instead executes the overloaded & operator on the operands.
When can this ever be useful? Or how can I make my type so that it can check if the two variables both are true?

The contents of the URL you mentioned that was 404 can be found here:
http://web.archive.org/web/20080613013350/http://www.ayende.com/Blog/archive/2006/08/04/7381.aspx
The article that the author is referring to is here:
http://web.archive.org/web/20081120013852/http://steve.emxsoftware.com/NET/Overloading+the++and++operators
To avoid the same issue again, here is the highlights from the article:
A couple months ago I posted about our query API along with an explanation of how it works. Our query API allows us to express our queries using strongly typed C# syntax:
List<Customer> customers = repository.FindAll(Customer.Columns.Age == 20 & Customer.Columns.Name == “foo”);
One of the things I pointed out in my previous posts was that I couldn’t overload the && and || operators directly since the framework doesn’t allow such craziness…at least not directly.
In particular, it is not possible to overload member access, method invocation, or the =, &&, ||, ?:, checked, unchecked, new, typeof, as, and is operators.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/csspec/html/vclrfcsharpspec_7_2_2.asp
Over the last month I’ve done a little investigation on the topic to see if and how I can get && and || to behave the way I want. This evening I came across the Conditional logical operators page on MSDN which gave me the answer I was looking for:
The operation x && y is evaluated as T.false(x) ? x : T.&(x, y), where T.false(x) is an invocation of the operator false declared in T, and T.&(x, y) is an invocation of the selected operator &. In other words, x is first evaluated and operator false is invoked on the result to determine if x is definitely false. Then, if x is definitely false, the result of the operation is the value previously computed for x. Otherwise, y is evaluated, and the selected operator & is invoked on the value previously computed for x and the value computed for y to produce the result of the operation.
The operation x || y is evaluated as T.true(x) ? x : T.|(x, y), where T.true(x) is an invocation of the operator true declared in T, and T.|(x, y) is an invocation of the selected operator |. In other words, x is first evaluated and operator true is invoked on the result to determine if x is definitely true. Then, if x is definitely true, the result of the operation is the value previously computed for x. Otherwise, y is evaluated, and the selected operator | is invoked on the value previously computed for x and the value computed for y to produce the result of the operation.
Since we already have the & and | operators in place it was simply a matter of overloading the true and false operators to both return false. This results in the & and | operators always being called which in turn results in the two criteria objects being turned into an AndCriteria/OrCriteria!
So now we can express our criteria expressions using the && and || syntax we’re accustomed to.
repository.FindAll(Customer.Columns.Age == 20 && Customer.Columns.Name == “foo”);
repository.FindAll(Customer.Columns.FirstName == “Foo” || Customer.Columns.LastName == “Bar”);
The relevant operator overloads are shown below.
public static bool operator true(Criteria<T> criteria) {
return false;
}
public static bool operator false(Criteria<T> criteria) {
return false;
}
public static Criteria<T> operator &(Criteria<T> lhs, Criteria<T> rhs) {
return new AndCriteria<T>(lhs, rhs);
}
public static Criteria<T> operator |(Criteria<T> lhs, Criteria<T> rhs) {
return new OrCriteria<T>(lhs, rhs);
}

EDIT-
Reading the linked article I was able to get the following output which uses both the true and false operators:
op false on 1
op & on 1 -1
op true on 1
op true on -1
FALSE
op false on -1
op true on -1
FALSE
op true on 1
op true on 1
TRUE
op true on -1
op & on -1 1
op true on -1
op true on 1
TRUE
With the code:
class Program
{
static void Main(string[] args)
{
MyStruct b1 = new MyStruct(1); // to be considered true
MyStruct b2 = new MyStruct(-1); // to be considered false
Console.WriteLine((b1 && b2) ? "TRUE" : "FALSE");
Console.WriteLine((b2 && b1) ? "TRUE" : "FALSE");
Console.WriteLine((b1 || b2) ? "TRUE" : "FALSE");
Console.WriteLine((b2 || b1) ? "TRUE" : "FALSE");
Console.ReadLine();
}
}
public struct MyStruct
{
private int _i;
public MyStruct(int i)
{
_i = i;
}
public static bool operator true(MyStruct ms)
{
Console.WriteLine("op true on {0}", ms);
return ms._i > 0;
}
public static bool operator false(MyStruct ms)
{
Console.WriteLine("op false on {0}", ms);
return ms._i <= 0;
}
public static MyStruct operator &(MyStruct lhs, MyStruct rhs)
{
Console.WriteLine("op & on {0} {1}", lhs, rhs);
if (lhs)
{
return rhs;
}
else
{
return new MyStruct(-1); //-1 is false
}
}
public static MyStruct operator |(MyStruct lhs, MyStruct rhs)
{
Console.WriteLine("op & on {0} {1}", lhs, rhs);
if (lhs)
{
return lhs;
}
else
{
return rhs;
}
}
public override string ToString()
{
return this._i.ToString();
}
}
I'm not sure what you mean when you say the first code can't compile, although it doesnt use the operator true/false, I ran the following code in 2010 express and got the output:
op bool on 1
op bool on -1
False
op bool on -1
False
op bool on -1
op bool on 1
True
op bool on 1
True
Code:
class Program
{
static void Main(string[] args)
{
MyStruct b1 = new MyStruct(1); // to be considered true
MyStruct b2 = new MyStruct(-1); // to be considered false
Console.WriteLine(b1 && b2);
Console.WriteLine(b2 && b1);
Console.WriteLine(b2 || b1);
Console.WriteLine(b1 || b2);
Console.ReadLine();
}
}
public struct MyStruct
{
private int _i;
public MyStruct(int i)
{
_i = i;
}
public static bool operator true(MyStruct ms)
{
Console.WriteLine("op true on {0}", ms);
return ms._i > 0;
}
public static bool operator false(MyStruct ms)
{
Console.WriteLine("op false on {0}", ms);
return ms._i <= 0;
}
public static implicit operator bool(MyStruct ms)
{
Console.WriteLine("op bool on {0}", ms);
return ms._i > 0;
}
public override string ToString()
{
return this._i.ToString();
}
}

Answering your last question: "how can I make my type so that it can check if the two variables both are true?" - just use the & operator. The whole point of && is to short circuit, so that the second argument isn't checked when not necessary.
Check this out:
Console.WriteLine(b1 & b2); // outputs 1
Console.WriteLine(b2 & b1); // outputs 1
You are actually missing one important bit which will allow you to use MyStruct (with & and |) as a boolean - an implicit cast to bool:
public static implicit operator bool(MyStruct ms) {
return ms._i > 0;
}
This allows you to use MyStruct (so also the result of the operators) as such:
if (b1 & b2)
Console.WriteLine("foo");
As the last, possibly most important, note: the problem in your example comes from the fact that you want to do logical operations (check whether 2 instances of MyStruct are true), but your & operator is implemented incorrectly for such a purpose. It works in terms of binary arithmetic, yielding an instance of MyStruct with value 1 when called with arguments MyStruct(1) (true) and MyStruct(-1) (false). So it basically does (true & false) == true. This is why b1 && b2 gives a different result than b2 && b1 in your example. Any further logic based off of this operator will be broken and unpredictable. Behavior of &&, which is implemented in .NET in terms of false and &, confirms this.
EDIT: you want to be able to use MyStruct as a boolean. You implement operators true and false and expect && and || to work in terms of boolean logic. However, you implement & in terms of binary arithmetics (using & on int fields), which makes this implementation of & not compatible with the boolean logic you expect ((1 & -1) == 1, which means (true & false) == false in your interpretation of MyStruct boolean value). Now, consider that && in general is not a logical operator (it doesn't return a bool) - it is a short circuit implemented as T.false(x) ? x : T.&(x, y). Note that it returns MyStruct in your case, which you just interpret as true or false based on the value of its field. The bottom line: you expect && to do a logical test on both operands, but the .NET implementation of && uses your implementation of &, which is not compatible with the boolean logic you expect.

From Microsoft (http://msdn.microsoft.com/en-us/library/6292hy1k.aspx):
Prior to C# 2.0, the true and false operators were used to create user-defined
nullable value types that were compatible with types such as SqlBool. However,
the language now provides built-in support for nullable value types, and
whenever possible you should use those instead of overloading the true and
false operators.
If you're looking to just evaluate your object to a boolean, remove the operator true and operator false overloads and just use the bool overload.

The point of true/false operators is to provide boolean logic semantics without the need for implicit conversion to bool.
If you allow your type to implicitly cast into bool, the true/false operators are no longer needed. But if you want only explicit conversion, or no conversion, but still want to allow your type as a condition in if or while expresssion, you can use true and false operators.

What is the difference between & and && operators in C#

I am trying to understand the difference between & and &&operators in C#. I searched on the internet without success. Can somebody please explain with an example?

& is the bitwise AND operator. For operands of integer types, it'll calculate the bitwise-AND of the operands and the result will be an integer type. For boolean operands, it'll compute the logical-and of operands. && is the logical AND operator and doesn't work on integer types. For boolean types, where both of them can be applied, the difference is in the "short-circuiting" property of &&. If the first operand of && evaluates to false, the second is not evaluated at all. This is not the case for &:
bool f() {
Console.WriteLine("f called");
return false;
}
bool g() {
Console.WriteLine("g called");
return false;
}
static void Main() {
bool result = f() && g(); // prints "f called"
Console.WriteLine("------");
result = f() & g(); // prints "f called" and "g called"
}
|| is similar to && in this property; it'll only evaluate the second operand if the first evaluates to false.
Of course, user defined types can overload these operators making them do anything they want.

Strongly recommend this article from Dotnet Mob : http://codaffection.com/csharp-article/short-circuit-evaluation-in-c/
-&& is short-circuit logical operator
For AND operations if any of the operand evaluated to false then total expression evaluated to false, so there is no need to evaluate remaining expressions, similarly in OR operation if any of the operand evaluated to true then remaining evaluation can be skipped
-& operator can be used as either unary or binary operator. that is, unary & can be used to get address of it’s operand in unsafe context.

& can be used on either integral types (like int, long etc.) or on bool.
When used on an integral type, it performs a bitwise AND and gives you the result of that. When used on a bool, it performs a logical AND on BOTH its operands and gives you the result.
&& is not used as a bitwise AND. It is used as a logical AND, but it does not necessarily check both its operands. If the left operand evaluates to false, then it doesn't check the right operand.
Example where this matters:
void Action()
{
string name = null;
if(name != null && name.EndsWith("ack"))
{
SomeOtherAction();
}
}
If name is null, then name.EndsWith("ack") will never get executed. It is smart enough to know if the left operand is false, then the right operand doesn't need to be evaluated (aka "short-circuiting"). That's good because calling a method on null will throw a NullReferenceException.
If you changed it into if(name != null & name.EndsWith("ack")), both sides would get evaluated and it would throw a NullReferenceException.
One detail: & can also be a unary operator (so it has one operand) in an unsafe context. It will give the address of a value or object. It's not important though, as most people don't ever have to touch this part of the language.

Below example and explanation's may help.
Example:
public static bool Condition1()
{
Console.WriteLine("Condition1 is evaluated.");
return false;
}
public static bool Condition2()
{
Console.WriteLine("Condition2 is evaluated.");
return true;
}
1. Logical Operator
& (ampersand) Logical AND operator
| (pipeline) Logical OR operator
Used for ensuring all operands are evaluated.
if(Condition1() & Condition2())
{
Console.WriteLine("This will not print");
//because if any one operand evaluated to false ,
//thus total expression evaluated to false , but both are operand are evaluated.
}
if (Condition2() | Condition1())
{
Console.WriteLine("This will print");
//because any one operand evaluated to true ,
//thus total expression evaluated to true , but both are operand are evaluated.
}
2. Conditional Short Circuit Operator
&& (double ampersand) Conditional AND operator
|| (double pipeline) Conditional OR operator
Used for Skipping the right side operands , Has Side effects so use carefully
if (Condition1() && Condition2())
{
Console.WriteLine("This will not print");
//because if any one operand evaluated to false,
//thus total expression evaluated to false ,
//and here the side effect is that second operand is skipped
//because first operand evaluates to false.
}
if (Condition2() || Condition1())
{
Console.WriteLine("This will print");
//because any one operand evaluated to true
//thus remaining operand evaluations can be skipped.
}
Note:
To get better understanding test it in console sample.
References
dotnetmob.com
wikipedia.org
stackoverflow.com

& is a bitwise operator and && is a logical operator that applies to bool operands.

Easy way to look it is logical & will evaluate both sides of the & regardless if the left side is false, whereas the short-circuit && will only evaluate the right side if the left is true.
d=0;
n=10;
// this throws divide by 0 exception because it evaluates the
// mod even though "d != 0" is false
if ( (d != 0) & (n % d) == 0 )
Console.Writeline( d + " is a factor of " + n);
// This will not evaluate the mod.
if ( (d != 0) && (n % d) == 0 )
Console.Writeline( d + " is a factor of " + n);

The first one is bitwise and the second one is boolean and.

According to - C# 4.0 The Complete Reference by Herbert Schildt
& - Logical AND Operator
| - Logical OR Operator
&& - Short Circuited AND Operator
|| - Short Circuited OR Operator
A simple example can help understand the phenomena
using System;
class SCops {
static void Main() {
int n, d;
n = 10;
d = 2;
if(d != 0 && (n % d) == 0)
Console.WriteLine(d + " is a factor of " + n);
d = 0; // now, set d to zero
// Since d is zero, the second operand is not evaluated.
if(d != 0 && (n % d) == 0)
Console.WriteLine(d + " is a factor of " + n);
// Now, try the same thing without the short-circuit operator.
// This will cause a divide-by-zero error.
if(d != 0 & (n % d) == 0)
Console.WriteLine(d + " is a factor of " + n);
}
}
Here the & operator checks each and every operand and && checks only the first operand.
As you might notice for 'AND' operations any operand which is false will evaluate the whole expression to false irrespective of any other operands value in the expression.
This short circuited form helps evaluate the first part and is smart enough to know if the second part will be necessary.
Running the program will throw a divide-by-zero error for the last if condition where both the operands are checked for & operator and no exception handling is done to tackle the fact that 'd' can be 0 at any point of time.
The same case applies to '|' in C#.
This is slightly different than C or C++ where '&' and '|' were bitwise AND and OR operators. However C# also applies the bitwise nature of & and | for int variables only.

Hai Friend,
The Operator &&(Logical Operator) is used in conditional statements.
For Instance
if(firstName == 'Tilsan' && lastName == 'Fighter')
{
Response.Write("Welcome Tilsan The Fighter!");
}
the Response.Write statement will run only if both variables firstName and lastName match to their condition.
Whereas & (Bitwise Operator)operator is used for Binary AND operations, i.e., if we write:
bool a, b, c;
a = true;
b = false;
c = a & b;
Response.Write(c); // 'False' will be written to the web page
Here first Binary And operation will be performed on variables a and b, and the resultant value will be stored in variable c.

OR, AND Operator

Newbie question.
How to calculate the value of the formula A f B, where f - the binary function OR or AND?

There is a distinction between the conditional operators && and || and the boolean operators & and |. Mainly it is a difference of precendence (which operators get evaluated first) and also the && and || are 'escaping'. This means that is a sequence such as...
cond1 && cond2 && cond3
If cond1 is false, neither cond2 or cond3 are evaluated as the code rightly assumes that no matter what their value, the expression cannot be true. Likewise...
cond1 || cond2 || cond3
If cond1 is true, neither cond2 or cond3 are evaluated as the expression must be true no matter what their value is.
The bitwise counterparts, & and | are not escaping.
Hope that helps.

Logical OR is ||, logical AND is &&.
If you need the negation NOT, prefix your expression with !.
Example:
X = (A && B) || C || !D;
Then X will be true when either A and B are true or if C is true or if D is not true (i.e. false).
If you wanted bit-wise AND/OR/NOT, you would use &, | and ~. But if you are dealing with boolean/truth values, you do not want to use those. They do not provide short-circuit evaluation for example due to the way a bitwise operation works.

if(A == "haha" && B == "hihi") {
//hahahihi?
}
if(A == "haha" || B != "hihi") {
//hahahihi!?
}

I'm not sure if this answers your question, but for example:
if (A || B)
{
Console.WriteLine("Or");
}
if (A && B)
{
Console.WriteLine("And");
}

Use '&&' for AND and use '||' for OR, for example:
bool A;
bool B;
bool resultOfAnd = A && B; // Returns the result of an AND
bool resultOfOr = A || B; // Returns the result of an OR

If what interests you is bitwise operations look here for a brief tutorial : http://weblogs.asp.net/alessandro/archive/2007/10/02/bitwise-operators-in-c-or-xor-and-amp-amp-not.aspx .bitwise operation perform the same operations like the ones exemplified above they just work with binary representation (the operation applies to each individual bit of the value)
If you want logical operation answers are already given.

&& it's operation return true only if both operand it's true which implies
bool and(bool b1, bool b2)]
{
if(b1==true)
{
if(b2==true)
return true;
}
return false;
}
|| it's operation return true if one or both operand it's true which implies
bool or(bool b1,bool b2)
{
if(b1==true)
return true;
if(b2==true)
return true;
return false;
}
if You write
y=45&&34//45 binary 101101, 35 binary 100010
in result you have
y=32// in binary 100000
Therefore, the which I wrote above is used with respect to every pair of bits

many answers above, i will try a different way:
if you are looking for bitwise operations use only one of the marks like:
3 & 1 //==1 - and
4 | 1 //==5 - or