Develop Reference

Can Boolean operators ever produce a non-Boolean result?

Please note, this question is not about the correctness of this style/convention, only about the use of Boolean operators. Just wanted to mention the style to give context.
Microsoft's C# Coding Conventions states that implicit typing is allowed "for local variables when the type of the variable is obvious from the right side of the assignment."
In the following code:
bool x = X();
bool y = Y();
var z = x && y;
Does the Boolean operator in the declaration of z make its type "obvious"? In other words, is there any situation in C# where && can be used with an x and y that are not Boolean, and produce a result that is not Boolean?

Are && and || guaranteed to return a boolean? No. Will they almost always result in a boolean? Most definitely.
By way of a counter example for && returning a non-boolean, consider the following:
public class Foo
{
public int Val { get; }
public Foo(int val) { Val = val; }
public static bool operator true(Foo val) {
return val.Val > 0;
}
public static bool operator false(Foo val) {
return val.Val <= 0;
}
public static Foo operator &(Foo left, Foo right) {
return new Foo(left.Val & right.Val);
}
public static Foo operator |(Foo left, Foo right) {
return new Foo(left.Val | right.Val);
}
}
Now we have a class that has indirectly overloaded the && and || operators, and they return type Foo.
The specification has this to say about user defined && and || operators:
When the operands of && or || are of types that declare an applicable user-defined operator & or operator |, both of the following must be true, where T is the type in which the selected operator is declared:
• The return type and the type of each parameter of the selected operator must be T. In other words, the operator must compute the logical AND or the logical OR of two operands of type T, and must return a result of type T.
• T must contain declarations of operator true and operator false.
Now, we evaluate the following:
var foo = new Foo(5) && new Foo(4);
foo is of Type Foo, with a Val of 4, as 5 & 4 is 4.
All of that said, I would not expect to see this kind of behavior outside of very specific circumstances.
Additionally, as the specification points out, the result of && or || must be the type of the parameters. In a bool && bool or bool || bool scenario, this should always be bool. So long as you're doing your logical operations on booleans (and not strange random non-boolean types) you should expect the result to be a bool.

Yes, it is possible for that to return something that is not a boolean as long as it can be evaluated as a boolean.
This requires you to overload the true and false operators, take this strange type:
public class MyStrangeType
{
public static readonly MyStrangeType Strange1 = new MyStrangeType(0);
public static readonly MyStrangeType Strange2 = new MyStrangeType(1);
private int myValue;
public int Value { get { return myValue; } }
private MyStrangeType(int value)
{
myValue = value;
}
public static bool operator true(MyStrangeType x) { return x.myValue == 1; }
public static bool operator false(MyStrangeType x) { return x.myValue != 1; }
public static MyStrangeType operator &(MyStrangeType x, MyStrangeType y)
{
return new MyStrangeType(3);
}
}
Now when you do something like this:
var result = MyStrangeType.Strange1 && MyStrangeType.Strange2;
result is not a boolean but a MyStrangeType.
Try it out!
public class Program
{
public static void Main()
{
var result = MyStrangeType.Strange1 && MyStrangeType.Strange2;
Console.WriteLine(result.GetType().FullName);
}
}
Outputs:
MyStrangeType

I could maybe misunderstood your question but this maybe could help to you or someone else.
There is "is" operator. You can check a veriable is a type of something.
For example;
var z = x is bool && y is bool;

You have two separate questions.
is there any situation in C# where && can be used with an x and y
that are not Boolean, and produce a result that is not Boolean?
This is shown by the other answers.
Does the Boolean operator in the declaration of z make its type "obvious"?
Yes, if x and y are bools. Since the && operator applied on boolean operands always returns a bool, which is defined in the specification. C# doesn't really allow variable return types for the same overload, except for the case of using the dynamic keyword. But then var is just inferred as dynamic as well.

C# Operator Overloading, rewriting string?

I'm very fresh to C#
Currently learning Operator overloading
i'm trying to do something like this:
string val = 500; (I can't implicitly)
and then
Number n1 = val;
I manages to get the Number n1 = someintvalue, for instance:
Number n1 = 500;
like this:
public struct Number
{
public int Value { get; set; }
public Number(int Val)
{
Value = Val;
}
public static implicit operator Number(int num)
{
return new Number(num);
}
}
However, when trying to make Number n1 = val; (when val is a string)
I simply cant since the first line cant compile:
string val = 500;
and the following wont work:
public static implicit operator string(int A)
{
return new string(A);
}
because of 1 error which i can not understand
1)User-defined conversion must convert to or from the enclosing type
by the way i get the idea of op overload
underthis specific case of: return new Number(num);
I simply init the ctor
still need some more fundemental understanding
thx ahead!

I presume the function you quote is within the Number class. You have added a conversion operator from an int to a string within that class, which is not legal. You can only add operators that convert to or from the type they're defined in, such as:
public static implicit operator string(Number A)
{
return new string(A.Value);
}
which will fail because string does not have a constructor that takes an int. You could do:
public static implicit operator string(Number A)
{
return A.ToString();
}
But the standard way to "convert" to a string is to overload the ToString method, which the compiler often calls automatically when a conversion to string is requested:
public override string ToString()
{
return Value.ToString();
}

How to make C# use int method overload for enum types?

I got a class in C# that has multiple overloads for different parameter types:
class Writer
{
public Writer Write(bool value)
{
// Do something with value
return this;
}
public Writer Write(double value)
{
// Do something with value
return this;
}
public Writer Write(int value)
{
// Do something with value
return this;
}
public Writer Write<T>(T value) where T : class, IInterface, new()
{
// Do something with value
return this;
}
}
class Reader
{
public Reader Read(out bool value)
{
// Retrieve value
return this;
}
public Reader Read(out double value)
{
// Retrieve value
return this;
}
public Reader Read(out int value)
{
// Retrieve value
return this;
}
public Reader Read<T>(out T value) where T : class, IInterface, new()
{
// value = new T() or null
return this;
}
}
Now I want to call Write and Read for multiple variables in a row, one of which is of an enum type. However, that enum type causes difficulties in the method resolving. (Btw: I am used to VB.NET, where Enum types are compatible to Integer parameters.)
enum MyEnum : int
{
Foo = 0, Bar = 1
}
class CallingClass
{
public void Call()
{
bool b;
double d;
int i;
IInterface o;
MyEnum e = MyEnum.Foo;
var w = new Writer();
// Unintuitive for Write
w
.Write(b)
.Write(d)
.Write(i)
.Write(o)
.Write((int) e);
// w.Write(e); // resolves to Writer.Write<T>(T)
// => Compile error: "MyEnum has to be reference type to match T"
// Even worse for Read, you need a temp variable
// and can't use fluent code anymore:
var r = new Reader();
r
.Read(out b)
.Read(out d)
.Read(out i)
.Read(out o);
int tmp;
r.Read(out tmp);
e = (MyEnum) tmp;
}
}
Is there any way I can modify Write/Read, Writer/Reader or MyEnum so that w.Write(e) will automatically resolve to Writer.Write(int) and more importantly r.Read(out e) to Reader.Read(int)?

Little late on the answer but since I was having the same issue, an overload like this should work
public void Foo(int a)
{
// do something
}
public void Foo(Enum #enum)
{
Foo(Convert.ToInt32(#enum));
}

with the where constraint:
public void Get<T>(out T value) where T : class, IInterface, new()
you are explicitly saying that T has to be a reference type (not a value type, as enums are). Try to delete class constraint.
[edit]
you can also try this, avoiding out param:
public T Get<T>() where T : new()
{
return default(T);
}
and call it as
c.Get<MyEnum>();
but again, if you add an IInterface constraint, no Enum can satisfy it.

From the comments to my question and Gian Paolo's answer, it has become clear that - opposed to VB.NET - C# does not support implicit type conversion of enum to int or vice versa, even when using tricks.
Hence, my desired "one-method handles all enum types" solution is impossible to achieve.
If you cannot (project hierarchy) or do not want to add overloads per enum type to the Writer/Reader class itself, you can create extension methods for the enum types.

C# - Replace Struct with Class and subclasses

I want to replace the struct in the following code with a parent class with no data members and four subclasses, each subclass adds a different field. e.g. The SMValueFlt subclass adds a field named fltValue, and so on.
I am very new to C# and my Java is very rusty, so this is proving harder than I thought. And beyond actually setting up the class and subclasses i'm not sure how to proceed. Any help would be appreciated.
public class Interpreter {
enum TypeCode { None, IntType, FloatType, StringType };
struct SMValue {
public TypeCode t;
public int intValue;
public float fltValue;
public string strValue;
public SMValue( int i ) {
t = TypeCode.IntType; intValue = i; fltValue = 0.0F; strValue = null; }
public SMValue( float f ) {
t = TypeCode.FloatType; fltValue = f; intValue = 0; strValue = null; }
public SMValue( string s ) {
t = TypeCode.StringType; strValue = s; intValue = 0; fltValue = 0.0F; }
public override string ToString() {
if (t == TypeCode.IntType) return String.Format("{0}", intValue);
if (t == TypeCode.FloatType) return String.Format("{0}", fltValue);
if (t == TypeCode.StringType)
return strValue==null? "--null--" : strValue;
return "???";
}
}
}

I kept your TypeCode around in the first example, but it's not really necessary. You can inspect the type of a variable at runtime. For example,
var x = new SMFltValue() // (x.GetType() == typeof(SMFltValue)) = true, x is SMFltValue = true
Without using generics:
public enum TypeCode { IntType, FloatType, StringType };
public abstract class SMValue {
public TypeCode t;
public SMValue(TypeCode typeCode) {
t = typeCode;
}
public abstract string ToString();
}
public class SMFltValue : SMValue {
public float fltValue;
public SMFltValue(float f) : base(TypeCode.FloatType)
{
fltValue = f;
}
public override string ToString()
{
return String.Format("{0}", fltValue);
return String.Format("{0}", intValue);
return strValue==null ? "--null--" : strValue;
}
}
public class SMIntValue : SMValue {
public int intValue;
public SMIntValue(int i) : base(TypeCode.IntType)
{
intValue = i;
}
public override string ToString()
{
return String.Format("{0}", intValue);
}
}
public class SMStrValue : SMValue {
public string strValue;
public SMStrValue(string s) : base(TypeCode.StringType)
{
strValue = s;
}
public override string ToString()
{
return strValue==null ? "--null--" : strValue;
}
}
But generics would make it much nicer.
public class SMValue<T> {
public T value;
public SMValue(T value) {
this.value = value;
}
public string ToString() {
if (value == null)
{
return "--null--";
}
else
{
return string.Format("{0}", value);
}
}
}
Then you could use it as.
int i = 3;
float f = 5.0f;
string s = null;
new SMValue<int>(i).ToString() ==> 3
new SMValue<float>(f).ToString() ==> 5.0
new SMValue<string>(s).ToString() ==> "--null--"
The <int>, <float>, <string> aren't actually necessary because the compiler can infer the type from the variable being passed to the constructor.

The semantics of a struct with exposed fields are fundamentally different from those of a class. Fundamentally, each structure-type variable holds a bunch of fields stuck together with duct tape, while a class-type variable holds a not-necessarily-unique reference to a class object. If a structure type has two int fields, and one has two variables of that type, one has four integers which may be written independently. By contrast, if a class type has two int fields and one has two variables of that type, it's possible that the variables may at any given time reference different instances (in which case they would encapsulate a total of four independently-writable integers), or they may identify the same instance (in which case both variables would identify the same pair of integers, and so writing the first number in one pair would also write the first number in the other).
Some people think all types should behave like class objects, and regard as "evil" any types that don't. In reality, there are situations where it's useful to stick a bunch of variables together with duct tape (so they may be passed around as a unit when convenient), but guarantee that every bunch of variables is distinct. Class types can be used to mimic this behavior, awkwardly, but structures naturally work that way.
Without knowing exactly how you intend to use your type, it's hard to say whether a class will be able to fulfill your needs without having to rework all your client code. It's important to note, however, that any class used to replace a struct must almost always be immutable. If you can't easily convert your struct to a mutable class, you'll probably have to keep it a struct.

Why is an implicit operator static?

I notice that an implicit operator is required to be called static but it actually is not really static at all... How come the implicit operator cannot be accessed statically but it can be accessed through an instance. This is the complete opposite of static. Suppose I wanted a static static implicit operator so that I could implicitly convert the static states of a class.
For example
a = new b(); // implicitly converts b to a.
a = b(); // implicitly convert the static states of b to a.
So for example b is a non-static class because it has a form but for all intents and purposes it is static and all instances are sharing the same information so I want to implicitly convert the class' static internals.
I will try to go into more detail of my example since Jeff thinks it makes no sense.
class b displays a dialog form, but it saves all the information entered into static variables. This is because the instances of b are only to display the dialog form and the data entered is one logical block of data(there is only one logical instance of the data entered). All the static variables fit directly into class a and so I can seamlessly convert the static variables in b to an instance of a class, however I would like to use implicit operator for this task instead of having a separate method. But it laments me that I cannot have an actual static implicit operator. Perhaps I am misunderstanding the word static and I am only using it in terms of how it works with methods and classes.

If an operator was not static, it could not handle null operands.
This applies equally to the case of an implicit conversion operator:
public class MyClass
{
public MyClass(int x)
{
this.X = x;
}
public int X { get; private set; }
public static implicit operator int(MyClass operand)
{
if (operand == null)
{
return 0;
}
return operand.X;
}
}
internal class Program
{
internal static void Main(string[] args)
{
MyClass x = null;
int y = x; // What instance would a non-static operator use here?
}
}

In C#, all operator definitions are static, see for instance http://msdn.microsoft.com/en-us/library/aa288467(v=vs.71).aspx
-Binary operator definitions, of course, because it would be arbitrary to declare one or the other to be this.
-Unary operator definitions, to match the theme of binary operator definitions being static.
It is just convention that it is done this way.

No operator works on the 'static state' of a class (except possibly typeof). In other words it's not possible to do anything like this:
var result = System.Int32 + System.Int32;
Operators only work on instances of a class.
int a = ..., b = ...;
var result = a + b;
All operators are necessarily static, so there's no need to disambiguate between 'normal static' operators and 'static static' operators.
You might consider using a singleton pattern. Something like this:
public class Foo
{
public int Member { get; set; }
private static Foo instance = new Foo();
public static Foo Instance { get { return instance; } }
private Foo()
{
}
public static implicit operator int(Foo foo)
{
return foo.Member;
}
}
Then you can use it as:
int a = Foo.Instance;

Here's a second answer from me, trying to guess at your underlying problem (instead of answering the question in the title, as I did in my first answer):
From the editted question and comments, it looks like you are trying to:
ensure that there is only a single instance of an InputBox class.
Convert data in the input box class to a string.
I'd make sure that you really need to enforce only a single instance of the InputBox class, but if that requirement is well founded then you could do something like this, using the Singleton pattern, and overriding the ToString instance method.:
class InputBox
{
static InputBox Instance = new InputBox();
int someNumber;
string someText;
private InputBox()
{
}
// ...
public override string ToString()
{
return someNumber.ToString() + " " + someText;
}
}
Usage:
string result = InputBox.Instance.ToString();
Addendum:
If it's really all about the syntax you want to use, you could add a delegate like this:
// Please don't really do this ;)
Func<string> InputBox = () => MyNamespace.InputBox.Instance.ToString();
And then use it like this:
string result = InputBox();
But anyone reading your code would cry ;)