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Is it safe to assume if-and statements will always break if the first condition is false? [duplicate]

This question already has answers here:
Usage of '&' versus '&&'
(4 answers)
Closed 1 year ago.
If I have
if (false && true)
...
can I be sure that every computer/compiler/whatever will do the shortcut that ignores the second condition? In my implementation, the second condition assumes that the first is true, otherwise it will cause a fatal error. For example:
if (Foobar is BoolContainer && Foobar.BoolVar)
...
where BoolContainer is an example class with a boolean property BoolVar.

MSDN states(https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/boolean-logical-operators):
The conditional logical AND operator &&, also known as the "short-circuiting" logical AND operator, computes the logical AND of its operands. The result of x && y is true if both x and y evaluate to true. Otherwise, the result is false. If x evaluates to false, y is not evaluated.
So your assumption is correct. Though you should be careful when you use &:
The & operator computes the logical AND of its operands. The result of x & y is true if both x and y evaluate to true. Otherwise, the result is false.
The & operator evaluates both operands even if the left-hand operand evaluates to false, so that the operation result is false regardless of the value of the right-hand operand.
You might get unwanted evaluation when you use &.

There is an extreme edge case where && won't necessarily do what you expect. You won't experience this in normal code. ;)
In the below code both booleans will output as True, but if you && them the result will (on some versions of .NET, but not all) not be True (because true2 is an "unusual" true).
Again, this is not something you will ever experience in real life.
namespace MyNamespace
{
using System;
using System.Runtime.InteropServices;
namespace Work
{
[StructLayout(LayoutKind.Explicit)]
public struct HackedBoolean
{
[FieldOffset(0)] public int value;
[FieldOffset(0)] public bool boolean;
public bool GetBool(int seed)
{
value = seed;
return boolean;
}
}
public class MyClassCS
{
public static void Main(string[] args)
{
var hacked = new HackedBoolean();
var true1 = hacked.GetBool(1);
var true2 = hacked.GetBool(2);
Console.WriteLine(true1);
Console.WriteLine(true2);
Console.WriteLine(true1 && true2);
Console.WriteLine(true1 == true2);
Console.ReadLine();
}
}
}
}

Yes it is safe.
If you want both sides to be evaluated you can use & instead of &&.
Otherwise it will skip second condition in this case.

As per the documentation of Conditional Logical ANN operator && on MSDN condition using && operator evaluates to true only when all the conditions evaluates to true.
If any of the conditions evaluates to false, the entire evaluations results as false.
while evaluating the conditions from left to right, next condition is not evaluated if the previous one evaluates to false.

Yes, it is guaranteed by the language specification:
The operation x && y corresponds to the operation x & y, except that y is evaluated only if x is not false.
In my experience it is a very common idiom for the second part of a short-circuiting conditional expression like this to rely on this behavior.
You might be wondering about why it says "not false" instead of "true". For a bool type, these two are equivalent. At first I thought it was because bool? had a lifted operator to deal with null, but this is not the case. But you can do something similar with a type that overloads the true, false, &, and | operators, and in fact the spec provides the DBBool example struct which does just that.

Short method to check if integer is 0 and return a boolean

Can this be done in a better way?
public bool AreMoreNeeded() => EntitiesNeededCount == 0 ? true : false;
I am using this to check if an integer is 0 and return a boolean based on the operation result .

yes, remove the true and false:
public bool AreMoreNeeded() => EntitiesNeededCount == 0;
the equality operator :
The equality operator == returns true if its operands are equal, false otherwise.
EDIT:
You can make it even a little more shorter and make it a property instead of a method:
public bool AreMoreNeeded => EntitiesNeededCount == 0;

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.

How does operator overloading of true and false work?

You can overload operator true and false i looked at examples and found this http://msdn.microsoft.com/en-us/library/aa691312%28v=vs.71%29.aspx
I completely dont understand how they work. I know if i write if(obj) and true returns true then the if is executed. It doesnt matter what false returns. However how does false work? in that doc it is suggected that the && operator uses it. I wrote the code below. I dont know how to get && to compile. || gives me a compile error too. How do i get false to be called? and how do i get the && and || to work?
var ts= new MyTimeSpan();// ts += ts;
if (ts){ Console.WriteLine("Is true"); }
else { Console.WriteLine("not true"); }
//Console.WriteLine(ts && ts ? "Y" : "N");
class MyTimeSpan
{
public static MyTimeSpan operator +(MyTimeSpan t, MyTimeSpan t2) { return new MyTimeSpan(); }
public static bool operator true(MyTimeSpan t) { return true; }
public static bool operator false(MyTimeSpan t) { return false; }
}

The defining property of a short circuiting operator is that it doesn't need to evaluate the right side if the left side already determines the result. For or if the left side is true the result will be true in any case and it's unnecessary to evaluate the right side. For and it's the same except that if the left side is false the result will be false.
You need to overload both | and true to get ||. And & and false to get &&.
a||b corresponds to something like op_true(a)?a:(a|b). So if the true operator returns true it does not need to evaluate the expression of b.
a&&b corresponds to something like op_false(a)?a:(a&b). So if the false operator returns true it does not need to evaluate the expression of b.
Overloading the short circuiting operators is useful when creating a custom boolean type, such as nullable bools(See DBBool)

When overloading the true and false operators they don't just return true and false, they are used to determine if a value of your type is considered to be true or false.
If for example a zero value in your class represents false, then a non-zero value represents true:
public static bool operator true(MyTimeSpan t) { return t.Value != 0; }
public static bool operator false(MyTimeSpan t) { return t.Value == 0; }
As the operators are each others inverse, the compiler doesn't need to use both to determine if a value is true or false. If you write if(obj) the compiler will use the true operator to determine if the value is true.

You will need to override & operator for && and | or ||.
Somthing like this: (Just a dummy code)
public static MyTimeSpan operator &(MyTimeSpan t, MyTimeSpan s) { return t; }
public static MyTimeSpan operator |(MyTimeSpan t, MyTimeSpan s) { return t; }

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.