Is there a way to use optional arguments (default parameters) with lambda expressions in c#? I have read through the documentation but can find nothing to say one way or the other.
To illustrate, I can define simple method that uses an optional argument to supply a default value like so:
void MyMethod(string arg = "default-value")
{
Console.WriteLine(arg);
}
What I want to know is if I am able to do the same thing using a lambda expression.
// gives a syntax error
Action<string> MyMethod = (arg = "default") => Console.WriteLine(arg);
I can work in an optional parameter with a default value using a delegate, but this seems a bit clumsy.
delegate void MyDelegate(string arg = "default");
MyDelegate MyMethod = arg => Console.WriteLine(arg);
Alternatively I could check the parameter in the lambda body, something like...
Action<string> MyMethod = (arg) => Console.WriteLine(string.IsNullOrEmpty(arg) ?
"default" :
arg);
But again this seems a bit clumsy.
Is it possible to use optional parameters to set a default value in a lambda expression in c#?
No. The caller (the code invoking the delegate) doesn't "see" the lambda expression, so it doesn't make sense to specify the default parameter there. All the caller sees is the delegate. In your case, for example, the calling code only knows about Action<string> - how is the compiler meant to know to supply the default value that's specified by the lambda expression?
As an example of how things get tricky, imagine if this were viable. Then consider this code:
Action<string> action;
if (DateTime.Today.Day > 10)
{
action = (string arg = "boo") => Console.WriteLine(arg);
}
else
{
action = (string arg = "hiss") => Console.WriteLine(arg);
}
action(); // What would the compiler do here?
Bear in mind that the argument is provided by the compiler at the call site - so what should it do with the final line?
It's a bit like having an interface and an implementation - if you have a default parameter on an interface, that's fine; if you only have it on the implementation, then only callers who know the specific implementation will see it. In the case of lambda expressions, there's really no visible implementation for the caller to use: there's just the delegate signature.
The lambda will match whatever the signature of the delegate it's assigned to is; without being assigned to a delegate a lambda cannot compile.
If the delegate contains optional arguments then the use of that delegate can optionally supply arguments. If the delegate doesn't, then the use of that delegate cannot omit any arguments.
While the Action and Func delegates are very handy, and can represent most signatures, they can't represent any signature with optional arguments; you must use another delegate definition for that.
Remeber, Action and Func aren't particularly special, they're just two delegates that everyone uses so that they don't need to worry about creating their own for every little thing.
Related
I have a question regarding delegates. So in the Threading class, there is a method called public Thread(ThreadStart start) this method takes in a parameter of ThreadStart which is a void delegate defined by Microsoft. What I can't seem to wrap my head around is the fact if I pass a method inside the parameter of Thread(ThreadStart start) it doesn't give me an error which is great. But why is it when I try to pass a delegate inside this parameter that is not type ThreadStart it gives me an error. I understand the signature of this method takes a delegate of type ThreadStart but then why can I put any method in without an issue but forced to use a delegate of ThreadStart for it not to give me an error. But I can enter any method I want as long as it doesn't have a parameter. Sorry if this is a basic question when am doing the stuff I like to know why a certain thing does something otherwise it bugs me. My guess is that maybe when you define a parameter with a type of delegate. It will let you enter any method you want as long as it fits the delegate's signature. But when you pass a delegate inside a parameter that is a specific delegate it will want you to use the same delegate as the type?
In general you can turn a parameterized method into a non parameterized signature call
Just do something like:
var myThread = new Thread(() => MethodWithParemeters(Parameter));
doing () => Foo() uses linq (I think) to create a new method that calls Foo
When a delegate is defined it attaches the signature to the name. So for:
public delegate void ThreadStart();
So any ThreadStart delegate must have a matching signature. So a function with another signature can't be passed to anything expecting a ThreadStart delegate as the parameter.
Since you raised the issue. I suspect what your code is needing is the constructor:
Thread(ParameterizedThreadStart)
Where ParameterizedThreadStart is:
public delegate void ParameterizedThreadStart(object obj);
And the object that you pass is your own class or structure.
If I understand the question correctly you are asking why Case 1 succeed, but case 3 does not compile:
public delegate void MyDelegate1();
public delegate void MyDelegate2();
public void MyMethod() { }
public void AcceptDelegate(MyDelegate1 md) { }
public void Example()
{
AcceptDelegate(MyMethod); // Case 1: Works
AcceptDelegate(() => {}); // Case 2: Also Works
MyDelegate2 delegate2 = MyMethod;
AcceptDelegate(delegate2); // Case 3: Fails, cannot convert delegate
AcceptDelegate(new MyDelegate1(delegate2)); // Case 4: Works, since an explicit conversion was used
AcceptDelegate(() => delegate2());// Case 5: Works
}
The reason that case 3 fails is that the delegate types are different.
The reason that case 1 work is that the compiler uses the overload resolution rules to find a method with name MyMethod and have a compatible signature to MyDelegate. I'm not knowledgeable enough about the compiler to cite the exact rules that applies in this specific case. The important thing is that MyMethod is not e delegate, it is a 'method group' that can resolve down to a specific method that can be converted to a delegate.
See How to convert delegate to identical delegate for more additional info.
I can't directly pass a lambda
form.Invoke(() => AMethod(form));
Because oddly enough a lambda is not a delegate type (per the compiler error). I seem to recall different information in Jon Skeet's .NET 4.0 book. And on MSDN it says:
A lambda expression is an anonymous function that you can use to
create delegates or expression tree types
public static void Main()
{
Form form = new Form();
form.Show();
form.Invoke(() => AMethod(form));
Application.Run();
Console.Read();
}
public static void AMethod(Form form)
{
form.SendToBack();
}
}
Yet, here we receive a compiler error:
Cannot convert lambda expression to type 'System.Delegate' because it
is not a delegate type.
Lambda's are supposed to be syntactic sugar for anonymous delegates. So what is the story here? What have I missed?
Lambda's themselves are not delegates. But, like your quoted text, they can be used to create delegates:
form.Invoke(new Action(() => AMethod(form)));
Yes, lambdas are used to create delegates, but they are not themselves delegates. The primary issue here is that lambdas don't have a signature, their parameter list and return type are determined based on the context they are placed in. When you place a lambda somewhere where a particular defined delegate instance is expected then the compiler knows what signature the lambda must match to be valid.
Invoke on the other hand doesn't take some particular delegate type, it takes an instance of Delegate which doesn't have any signature. You need to wrap your lambda in new SomeActualDelegateType(...) (Action is a common choice for some delegate here, but not the only choice) to give it an actual signature.
Try to create Action: form.Invoke(new Action(() => MyMethod(form)));
I see delegates in two forms:
A. Func<string, string> convertMethod = lambda
B. public delegate string convertMethod(string value);
I'm uncertain of what actually the difference between these two are. Are they both delegates? I believe the first one would use a lambda and the second would have to have a method to actually perform the work. I may be confused too.
First of all, your two examples are doing two totally separate things. The first is declaring a generic delegate variable and assigning a value to it, the second is just defining a delegate type. Your example, more completely, would be:
public static class Program
{
// you can define your own delegate for a nice meaningful name, but the
// generic delegates (Func, Action, Predicate) are all defined already
public delegate string ConvertedMethod(string value);
public static void Main()
{
// both work fine for taking methods, lambdas, etc.
Func<string, string> convertedMethod = s => s + ", Hello!";
ConvertedMethod convertedMethod2 = s => s + ", Hello!";
}
}
But more to the point, both Func<string,string> and delegate string convertMethod(string) would be capable of holding the same method definitions whether they be methods, anonymous methods, or lambda expressions.
As for which you should use, depends on the situation. If you want your delegate to be defined more by what it takes and returns, then the generic delegates are perfect. If you want the delegate to have some special name that gives more definition of what that delegate should do (beyond simple Action, Predicate, etc) then creating your own delegate is always an option.
The code sample you have is confusing things a bit so let me try and clear it up. The following 2 items are delegate declarations. These are easy to spot because they will always contain the delegate keyword
public delegate TReturn Func<TArg, TReturn>(Targ value);
public delegate string convertMethod(string value);
This line of code is assigning a value to a local which is typed to a delegate
Func<string, string> local = lambda;
The above code is not limited to using just lambdas though. The value could also be a compatible method group or another delegate value.
One other item to note is that even though Func<string, string> and convertMethod are both delegates with identical signatures their values are not convertible to each other. For example the following is illegal
Func<string, string> local1 = ...;
convertMethod local2 = local1; // Error!!!
From MSDN,
In versions of C# before 2.0, the only way to declare a delegate was
to use named methods. C# 2.0 introduced anonymous methods and in C#
3.0 and later, lambda expressions supersede anonymous methods as the
preferred way to write inline code.
and
There is one case in which an anonymous method provides functionality
not found in lambda expressions. Anonymous methods enable you to omit
the parameter list. This means that an anonymous method can be
converted to delegates with a variety of signatures.
You may also be interested in this SO answer on delegate keyword vs lambda expression.
Additionally, MSDN has a good article on Lambda Expressions:
delegate int del(int i);
static void Main(string[] args)
{
del myDelegate = x => x * x;
int j = myDelegate(5); //j = 25
}
In the previous example, notice that the delegate signature has one
implicitly-typed input parameter of type int, and returns an int. The
lambda expression can be converted to a delegate of that type because
it also has one input parameter (x) and a return value that the
compiler can implicitly convert to type int. (Type inference is
discussed in more detail in the following sections.) When the delegate
is invoked by using an input parameter of 5, it returns a result of
25.
A initializes an instance of a delegate (that can be called immediately). It's a variable of type Func< string, string >.
B specifies the definition of a delegate (its signature). It can be used to later define variables of type convertMethod.
Im having a little trouble understanding delegates.
I have a delegate that i will invoke when a y character is entered:
public delegate void respondToY(string msgToSend);
private respondToY yHandler;
i have a subscribe method in order that calling code can ask to be notified when the delegate is invoked:
public void Subscribe(respondToY methodName)
{
yHandler += methodName;
}
As far as i can see, to register with this delegate, i need to provide something of the type respondToY. Yet when calling the subscribe method, i can supply either a new instance of the delegate or simply the name of the method. Does this mean that any method matching the delegate signature can be used and will automatically be converted to the correct delegate type?
** Edit **
So on this assumption it would also be valid to supply only a method name to things like click event handlers for buttons (provided the method took the sender, and the relevant event object), it would be converted to the required delegate?
This is a method group conversion. It converts a method group (basically the name of a method or overloaded methods) to an instance of a delegate type with a compatible signature.
Yes, any compatible method can be used. Note that you can provide a target too - for example:
string text = "Hello there";
Func<int, int, string> func = text.Substring;
Console.WriteLine(func(2, 3)); // Prints "llo", which is text.Substring(2, 3)
There must be a specific delegate type involve though. You can't just use:
Delegate x = methodName;
... the compiler doesn't know the kind of delegate to create.
For more information, see section 6.6 of the C# 4 language specification.
Note that a method group conversion always creates a new instance of the delegate in question - it isn't cached (and can't be without violating the specification.)
as far as i know ... yes, the delegate type just makes sure that the signatures match
I am curious why C# allows me to ignore delegate parameters in some cases but not others.
For instance this is permitted:
Action<int> action = delegate { Console.WriteLine("delegate"); };
but this is not:
Action<int> action = () => Console.WriteLine("lambda");
Is there a way to initialize a delegate and ignore the parameters using a lambda? I know that I can add a single parameter to the lambda and fix the previous line but this is more of an academic question pertaining to the compiler and why or how this works.
I believe that your first sample actually creates an anonymous function that is able to take on many different signatures whose body is the single statement Console.WriteLine.... Because it can match different signatures, it does not cause a problem. In the second sample, the lambda syntax itself defines a function that takes no parameters with the same body. Obviously the latter is not consistent with the defined Action so you get the error.
C# Anonymous Method Reference
There is one case in which an
anonymous method provides
functionality not found in lambda
expressions. Anonymous methods enable
you to omit the parameter list, and
this means that an anonymous method
can be converted to delegates with a
variety of signatures. This is not
possible with lambda expressions.
To elaborate on tvanfosson's answer; this behavior is described in the C# 3.0 language specification (§7.14):
The behavior of lambda-expressions and
anonymous-method-expressions is the
same except for the following points:
• anonymous-method-expressions permit
the parameter list to be omitted
entirely, yielding convertibility to
delegate types of any list of value
parameters.
• lambda-expressions permit parameter
types to be omitted and inferred
whereas anonymous-method-expressions
require parameter types to be
explicitly stated.
• The body of a lambda-expression can
be an expression or a statement block
whereas the body of an
anonymous-method-expression must be a
statement block.
• Since only lambda-expressions can
have an expression body, no
anonymous-method-expression can be
successfully converted to an
expression tree type (§4.6).
I think:
Action<int> action = () => Console.WriteLine("lambda");
is the equivalent of:
Action<int> action = delegate() { Console.WriteLine("delegate"); };
which wouldn't compile either. As Daniel Plaisted says () is explicitly saying there aren't any parameters.
If there were an equivalent of delegate{} it might be:
Action<int> action = => Console.WriteLine("lambda")
Which isn't very pretty and I suspect it suspect isn't in the spirit of lambda expressions.
As others said, no, you can't skip declaring the parameters to a lambda. But, for cleanliness, I suggest giving them a name such as _. For example
foo.Click += (_,__) => { ... }
You aren't ignoring them per-se, but you're indicating you don't care what they are and will not use them.
The () => ... syntax explicitly specifies that the lambda takes no parameters. Perhaps the language could be modified so that () => really meant "Infer the parameters of this lambda for me" in the same way the delegate syntax does, but that would make the language more complicated. When designing new language features, you start at minus 100, and I don't think this one passes the test.
There may also be more technical reasons why this would be difficult to implement (which is probably more in line with what you were asking for, but I doubt the technical reasons drove this decision if it ever came up).
I'd say it's to have a forced use of the parameters of the lambda expression.
Take your first example, how would you interact with the passed in value, there's no local representation of it.
What about this?
Func<int> lamdapointer = () => TwoArgMethodThatReturnsInt(10,20); // the same method cannot be called with the delegate "NoArgmethodThatReturnsInt"
lamdapointer();
Delegate int NoArgmethodThatReturnsInt();
NoArgmethodThatReturnsInt del = NoArgmethodThatReturnsInt; // only this is possible with delegates
public int TwoArgMethodThatReturnsInt(int x,int y)
{
return x + y;
}
public int NoArgmethodThatReturnsInt()
{
return 20;
}
Actually, delegate {} does not specify any parameters and fits any delegate method signature - therefore it is permitted in your first construcion.
The Lambda expression () => ...; specifically states parameterless delegate, which contradicts the signature required by Action - a delegate with single parameter.
You may want to use one of the following options.
If you need the action to have a parameter, you can do it the next way ("_" is a legal character for identifier name).
Action<int> action = _ => Console.WriteLine("lambda");
Or you may want to use parameterless Action as follows:
Action action = () => Console.WriteLine("lambda");