In C# I know you can do things like this:
int number = true ? 1 : 0;
This returns the left or right side depending on if the Boolean is true or not.
But is there a way to do the same thing but instead run a function instead of returning a value? Something like this:
This doesn't work (I get multiple syntax errors)
WPS.EDIT_MODE ? ExecuteEditMode() : ExecutePublicMode();
Thanks.
The conditional operator must return a value. Assuming the functions don't both return a meaningful value for you, you should use an if statement to do that:
if(someCondition)
doA();
else
doB();
Although technically you could use an anonymous function to do this if you really wanted:
int number = someCondition ?
new Func<int>(() => { doA(); return 0; })() :
new Func<int>(() => { doB(); return 1; })();
but that's not suggested; using an if/else is both easier and more readable for that case.
If both functions return an int, yes.
The ternary operator is about returning values. Not returning values doesn't make sense.
The conditional operator (?:) returns one of two values depending on
the value of a Boolean expression.
http://msdn.microsoft.com/en-us/library/ty67wk28(v=vs.80).aspx
Related
I'm trying to lear how to properly use lambda expressions in C# and I got into this problem. I have an array of booleans defined like this:
public bool[] worldState = new bool[25];
Now, after giving values to it, I want to cycle thorugh it and simply print "T" when the bool is true, and "F" when it's false. I know how to do it with loops, but I wanted to do it with a one-liner, so I came up with this:
Array.ForEach(worldState, x => x? Console.WriteLine("T"): Console.WriteLine("F") );
This, however, won't work. Why? What's the correct way to do this?
Try:
Array.ForEach(worldState, x => Console.WriteLine(x ? "T" : "F") );
The ternary function requires a value be returned, so in this case your T or F should be returned to the WriteLine method.
Personally, I think a foreach or for loop would be cleaner and more efficient.
foreach(bool bVal in Array) { Console.WriteLine(bVal ? "T" : "F"); }
or
for(i = 0; i < Array.Length; i ++) { Console.WriteLine(Array[i] ? "T" : "F"); }
It's because Console.WriteLine() doesn't return anything. From the Ternary operator documentation, emphasis mine:
The conditional operator (?:) returns one of two values
and Console.WriteLine() does not return a value.
Just use an if statement:
Array.ForEach(worldState, x => { if(x) Console.WriteLine("T"); else Console.WriteLine("F"); } );
Another possibility is just return the string you want using the ternary operator:
Array.ForEach(worldState, x => Console.WriteLine( x ? "T" : "F" ) );
The proper syntax for an if-else in ForEach would be:
Array.ForEach(worldState, x => { if(x) { Console.WriteLine("T"); } else { Console.WriteLine("F");}} );
The ternary operator ? : requires expressions that return compatible values as the arguments. Console.WriteLine is a void function and does not return a value.
Although I would note that a standard foreach loop is much easier to read and debug. I would not recommend using .ForEach() for this type of operation. In fact I wouldn't recommend it for any operation since it doesn't add any capability over the standard loop.
C# provides conditional operator (?:) that returns one of two values depending on the value of a Boolean expression. eg
condition ? first_expression : second_expression;
My question is can we use the same syntax to call a method when condition is true? and when condition is false then do nothing
public void Work(int? val)
{
var list = new List<int>();
//ofcourse line below doesn't work
//but is it possible to call method when condition is true and else do nothing
val.HasValue? list.Add(val.value) : else do nothing
}
the ?: has also been referred to as the ternary operator in the past. Ternary, for three. if this, then do this, else do this.
You have two expressions. If this, do this. This is exactly the point of an if statement. You are trying to fit your case into a construct that it isn't designed for. Don't do this.
Use the correct operation for the job:
if(val.HasValue)
{
list.Add(val.value)
}
The C# conditional operator is used to return a different value depending on the evaluation of a condition. It is not meant to be used to to be used the way you are trying to in your question. It should like be used this :
int test = i > 3 ? 0 : 1;
test will then equal 0 if i is less than (or equal to) 3, or test will equal 1 if 3 is greater than 3.
To do what you want you will have to use a regular if statement (which you can still write in one line by the way) :
if (val.HasValue) list.Add(val.value);
The conditional/ternary operator is supposed to return a value and that very specific value must be assigned back to somewhere.
So, in that case you can do that, yes. But, it would lead to bad design.
In a regular case, one would do this:
int x = (a > b) ? a : b;
Now, lets assume AddToList() is your desired method when the condition renders to true and DoRest() is the method you want to invoke if the condition turns out to false.
In the aforementioned case, you'd end up doing something like this:
int result = val.HasValue? AddToList(val.value) : DoRest();
Now you have to rely on result for finding out which one has been called (if you ever need that) and it's not very expressive and doesn't point to proper code design.
If you get a tad more adventurous you'd end up with :
var actionToInvoke = val.HasValue ? (Action)AddToList: (Action)DoRest;
actionToInvoke();
In any case, none of these lead to very readable code.
So, sticking with a simple if(val.HasValue) would be the simplest way to go here.
The way null conditional operator works is you have to return a value for the variable you are assigning it to. So if you would like a string value or something else other than void you can call the method with out any problem. But to call a void method you can use a delegate.
delegate void DelMethod();
void Method() { }
void MethodTwo() { }
private void MyMethod()
{
DelMethod x;
x = condition == true ? (DelMethod)Method : (DelMethod)MethodTwo;
}
I want to use the ?: operator intstead of if else
e.g.
var directory = new DirectoryInfo(path);
if (!directory.Exist())
{
directory.Create();
}
else
{
// do nothing
}
I tried to use ?: like this:
var directory = new DirectoryInfo(path);
!directory.Exist() ? directory.Create() : void;
but it says "Invalid expression term void", null also isn't working.
Can anyone tell me a solution?
In you scenario, only having a if condition is better suited.
Just for your understanding, A ternary Operator (?:) needs to result into a value in the right side and also needs a variable in left side to assign the result value e.g.:
x = (y== null) ? 0: 1;
This means assign 0 to x when y is null otherwise assign 1.
So in your example/scenario, you may write something like this to result into a directory creation status as below:
var newDirectoryCreated = (!directory.Exist()) ? directory.Create() : false;
This way, if new directory is created then newDirectoryCreated will assigned with true otherwise false.
Simply keep the if statement and remove the else clase since you aren't doing anything in there.
You're trying to use the ternary operator which, by definition, must return a value.
From the documentation:
The conditional operator (?:) returns one of two values depending on the value of a Boolean expression.
If your just looking for brevity, you could try this instead:
if (!directory.Exist())
directory.Create();
Ternary operator are not designed to replace if/else They are to simplify assignment and declaration. It should be able to assign the result the statement to something like variable or function. There major use are in assignment or reading.
Something like
var status = !directory.Exist() ? directory.Create() : false;
The correct solution would be to stick to native if condition. The following is enough:
var directory = new DirectoryInfo(path);
if (!directory.Exist())
{
directory.Create();
}
The ?: operator's behavior is roughly this:
for x ? y : z, it will return y if x is true, and otherwise it will return z.
From this we can deduce a couple of things:
both y and z must return something (it won't work if either of them evaluate to void), and
y and z must evaluate to the same type. (Imagine you had something like this: var r = x ? y : z;. Which type is r? Is it the type of y or z? We don't know which of them will be returned, but we have to pick a type at compile-time. So they have to return the same type.
In your case, both evaluate to void, which doesn't work. (And if you changed the last part to null, as you said you'd tried, then they evaluate to different types, one of which is void, which breaks both rules)
The conditional operator (?:) musts return one of two values depending on the value of a Boolean expression. So you can stick with if clause instead, it is still simple to understand rather than conditional operator:
var directory = new DirectoryInfo(path);
if (!directory.Exist()) directory.Create();
The ?: operator is an extention of the if-then-else construction.
The extention is in the then-block and the else-block. Where the then-block and the else-block return void for the if-then-else construciton it must return a type for the ?: operator. An aditional constraint for the types in the ?: operator is that the two types must be identical. This constraint is softened a bit by the fact that automatic casting will be used by the compiler to make the two types identical.
Code using ?: operators is in general shorter but also harder to read. This is one point to consider when replacing the if-then-else construct with the ?: operator. Unless your then-block and else-block are one liners it seldom is worth replacing it with the ?: operator.
The if-then construction is a limited version of the if-then-else construction (or visa versa, the if-then-else construction is an extention of the if-then construction). Since the if-then construction has only one code block, the then-block, it is not possible to replace the if-then construction with the ?: operator. You first have to extend the if-then construct with an empty else-block.
Examples:
// initialising an integer with an if-then construct.
int x = 0;
if (some_condition)
{
x = 1;
}
Think of this as if the then-blockreturns an integer.
It is not possible to use the ?: operator strait away.
// initialising an integer with an if-then-else construct.
int y;
if (some_condition)
{
y = 1;
}
else
{
y = 0;
}
Extended the if-then construct to a if-then-else construct and think of the then-block and else-block as returning a integer for witch the types coincidently ;-) match with each other.
It is possible to use the ?: operator in this case.
// initialising an integer with a ?: operator.
int z = (some_condition) ? 1 : 0;
About your code:
var directory = new DirectoryInfo(path);
if (!directory.Exist())
{
directory.Create();
}
In this case I do not see a sensible way to make the then-block returning a value. That makes using the ?: operator impossible or highly complicated with ugly code as a result. My advise, stick to the if-then construct in this case.
It seems the compiler is not going let this syntax fly.
void main()
{
foo(false?0:"");
}
void foo(int i) {return;}
void foo(string s) {return;}
The only other way I can see of fixing this is something as follows:
void bar(object o)
{
if (o is string){//do this}
else{//im an int, do this}
}
Anyone have any better ideas?
You cannot use a method with a void return type in a ternary expression in this way. End of story.
To understand why this is, remember what the ternary operator actually does -- it evaluates to the following:
(condition ? [true value] : [false value])
What this implies is that the following code:
int x = a ? b : c;
Must be rewritable to:
int x;
if (a)
{
x = b;
}
else
{
x = c;
}
The two above are logically identical.
So how would this work with a method with void as its return type?
// Does this make sense?
int x = condition ? foo(s) : foo(i);
// Or this?
if (condition)
{
x = foo(s);
}
else
{
x = foo(i);
}
Clearly, the above is not legal.
That said, others' suggestions would otherwise be valid if only your foo overloads returned a value.
In other words, if your signatures looked like this:
object foo(string s);
object foo(int i);
Then you could do this (you're throwing away the return value, but at least it'll compile):
object o = condition ? foo(0) : foo("");
Anyway, the ol' if/else is your best bet, in this case.
The method call of foo is determined at compile time, so it cannot call a different method (or overload) based on the result of evaluating the condition. Instead, try something like this:
condition ? foo(0) : foo("")
This way, the compiler will succeed in performing overload resolution and will resolve the first call to foo(int) and the second call to foo(string).
EDIT: As noted by other, you cannot use the ?: operator as a statement, nor can you use methods which return void in it. If your actual methods return compatible types, you could do something like:
int result = condition ? foo(0) : foo("");
If not, you must use an if:
if (condition)
foo(0);
else
foo("");
You're example doesn't make a whole lot of sense (the second example doesn't relate to the first).
I think the first example would be fine as:
void main()
{
foo("");
}
Since 0 will never be passed anyway (false is always false) and you can't use the inline conditional operator without an assignment somewhere (which your example is lacking).
As for the second way, that is probably how I would prefer to see it:
void bar(object o)
{
if(o is string) foo(o as string);
else foo((int)o);
}
I wouldn't pass in an object as a parameter. The int will be boxed, which is a little less efficient. Let the compiler figure out which method to call.
If you wrote:
foo(0);
foo("");
The appropriate method would be called. You could also write:
if (condition) {
foo(0);
} else {
foo("");
}
Depending on what you're trying to do (your example is lacking in a little detail).
If you use Inline if expressions in C#, both parts before and after the ":" have to be of the same type. What you are intending would never work.
Even if you like to do something like this:
DateTime? theTime = true ? DateTime.Now : null;
The compiler is not satisfied. In this case you will have to use:
DateTime? theTime = true ? DateTime.Now : default(DateTime?);
The conditional operator needs the true and false part to be of the same type. Which is why it's not compiling.
var x = condition ? 0 : "";
What type should the compiler choose for x? If you really want it to choose object make a cast or you could force it to choose dynamic in which case method overload would still work but you loose type safety. Both are however strong smells.
Having to test the runtime type is usually a design error but with the limited code (that will always have the same result) it's hard to help with a different approach that would require testing on runtime types
This:
foo(false?0:"")
Could be this:
false ? foo(0) : foo("")
Both results of the conditional operator must of the same type (or be implicitly convertible). So foo(false ? 0 : "") won't work because it is trying to return an Int32 and a String. Here's more information on the conditional operator.
The fix I would do is change that line to false ? foo(0) : foo("").
EDIT: Derp, can't use a conditional operator just in the open like that. They can only be used for assignments. You'll have to use a if/else block. Not in one line, but it'll do in a pinch.
I have the following code snippet:
// Notify the source (the other control).
if (operation != DropOperation.Reorder) {
e = new DroppedEventArgs()
{
Operation = operation == DropOperation.MoveToHere ? DropOperation.MoveFromHere : DropOperation.CopyFromHere,
Source = src,
Target = this,
DroppedItems = srcItems
};
src.OnDropped(e);
}
I do not understand the
Operation = operation == DropOperation.MoveToHere ? DropOperation.MoveFromHere : DropOperation.CopyFromHere line.
Can someone explain it? For the record...dropOperation is an enum.
Can you give vb syntactical equivalent is all I need.
Seth
The reason it's hard to understand is due to the fact that you're unfamiliar with the ternary operator ?:. Basically what it does is evaluate an expression, and return one of two value depending on whether the evaluation returned true or false.
For example, the following expression will return "true" if the boolean is true, and "false" elsewise:
bool test = false;
string testString = test ? "true" : "false";
It does in fact exist in VB.NET as well - expressed a bit differently though. These two statements in respectively C# and VB.NET are in fact the same
Dim s As String = If(True, "kek", "lol")
string s = true ? "kek" : "lol";
The difference between IIf and the tenary operator is that IIf will always evaluate both the second and third parameter because IIf is a function instead of an operator. For this reason the tenary operator is much to prefer.
Note: The tenary operator was added in VB 9, so if you're using previous versions you'll have to rely on the IIF function for this functionality.
If (operation = DropOperation.MoveToHere) Then
Operation = DropOperation.MoveFromHere
Else
Operation = DropOperation.CopyFromHere
End If
Obligatory wikipedia link. I gave up on mentioning this link in a comment, so here it is in an answer. You can replace uses of the ? operator with calls to the IIF function:
Operation = IIF(operation = DropOperation.MoveToHere, DropOperation.MoveFromHere, DropOperation.CopyFromHere)
Note that they are not strictly equivalent, since the IIF function evaluates both the true and the false case, whereas the ? operator only evaluates the case it returns.
It is sort of equivalent of the IIf function in VB.NET (see Brian's comment):
Operation = IIf(operation = DropOperation.MoveToHere, _
DropOperation.MoveFromHere, _
DropOperation.CopyFromHere)
In C# this is called the conditional operator, and is a sort of shortcut for a simple if/else statement.
This is the conditional operator, it is very similar to VB's IIf function:
Returns one of two objects, depending on the evaluation of an expression.
Public Function IIf( _
ByVal Expression As Boolean, _
ByVal TruePart As Object, _
ByVal FalsePart As Object _
) As Object
In this particular example the IIf function would be written like this:
Operation = IIF((operation = DropOperation.MoveToHere), _
DropOperation.MoveFromHere, _
DropOperation.CopyFromHere)
This is using the ? operator for conditional assignment. This line is basically syntactic sugar for:
// C# expanded example
if (operation == DropOperation.MoveToHere)
{
Operation = DropOperation.MoveFromHere;
}
else
{
Operation = DropOperation.CopyFromHere;
}
Which, in VB, would be equivalent to:
If operation = DropOperation.MoveToHere Then
Operation = DropOperation.MoveFromHere
Else
Operation = DropOperation.CopyFromHere
End If
operation == DropOperation.MoveToHere ? DropOperation.MoveFromHere : DropOperation.CopyFromHere
This is called the ternary operator. It's basically a short way of writing:
if (operation == DropOperation.MoveToHere)
return DropOperation.MoveToHere;
else
return DropOperation.CopyFromHere;
The ?: construct is the ternary operator, basically an inline if (x) y else x. The benefit of the inline is seen here in that it is assigned immediately to a variable. You can't do that with an if statement.
C# Bloggers use the "?" a lot. Look this code:
int Foo(int x, int y){
return x==y? 10: 11;
}
Is equal to:
int Foo(int x, int y){
if (x==y)
return 10;
else
return 11;
}
Just read the well explained Donut's answer!!
("VB-er" I like the term)
It's called the ternary operator. I don't think it exists in VB but it's basically just a shorthand for an if/else.