With linq i can create a query like this
XElement.Elements("...").Select(x=> useX(x));
now as x creates only a wrapper Action and useX's parameter is XElement you can use it like this aswell:
XElement.Elements("...").Select(useX);
However when i have a type that has a constructor with a matching type i.e. MyClass(XElement element) i have to use:
XElement.Elements("...").Select(x=> new MyClass(x));
My question: Is there any way to shorten the construction of an object in a way like above but with a constructor? I imagined something like this:
XElement.Elements("...").Select(MyClass);
Is there any way to shorten the construction of an object in a way like above but with a constructor
Short answer: No, there is not.
Longer "answer" (not actually an answer, more a workaround if you want it):
You can add a static construction method to MyClass:
public class MyClass
{
public MyClass(XElement elem)
{
// your constructor logic
}
public static MyClass Create(XElement elem)
{
return new MyClass(elem);
}
}
and use that
XElement.Elements("...").Select(MyClass.Create);
But whether it's worth it is up to you! (Personal opinion: It's not worth it, just stick with new MyClass(x))
No, you can't shorten a call to a constructor inside a lambda expression. The problem is that a constructor is called from an operation with new new operator.
You are stuck with .Select(x=> new MyClass(x)). Of course, you can make a factory method to call instead, but that is more like a workaround.
Actually the syntax that accepts a method-group only works, if the actually delegate to be used can clearly be inferred from the usage. E.g. a method returning an int and expecting one, that has no further overloads, can also be used like this:
Select(myMethod);
However if there are overloads, you can´t do this, as it´s unclear which of them you mean.
For constructors it´s a bit different. There´s no way on referencing it using a method-group. MyClass isn´t a valid method-group for new MyClass().
So in short: you can´t reference a constructor using a method-group.
Related
For a test, I want to create a generic "helper" method which will take take two arguments, the first argument is a function (or a reference to the function) and the 2nd argument is a list of objects for that function that are to be called as its parameters.
The following does this perfectly:
CallMyFunctionWithParamsPlease(new Func<int, int>(MyMethod), new object[] {1});
public static int CallMyFunctionWithParamsPlease(Delegate func, params object[] args)
{
func.DynamicInvoke(args);
return 3;
}
The thing is, this doesn't look very nice when calling it and I wish to abstract it into another method to act as syntatic sugar.
Ideally I want it to be called like this:
CallMyFunctionWithParamsPlease(myMethod, new Object[] {1});
From what I can gather, there is no elegant solution to do this in C# since I cannot pass myMethod by itself as a reference anywhere, instead I must pass it by declaring a new Func along with the return type of the method. Since I'm not using this return type anywhere, I'm not sure why it's necessary to input this information. My limited understanding is that because C# is statically typed, the compiler must know everything and things like this just aren't possible.
Is this true or not? How would I create syntatic sugar to simply pass a method to another method which can be called there without needing to invoke "new Func"? I would have thought simply passing the function as a reference pointer would allow me to do this, but I'm having difficultly doing this too. I looked into delegates, using "unsafe" with pointers, and a few other options. None of them seem to make this possible, or if they do, they didn't explain it in a manner that I could understand.
I simply want to pass a method to another method, and invoke it with a variable list of object params with variable length whereby I don't need to specify this whilst invoking it. I'm not sure if I'm trying to force C# to do something it's not meant to do here, and instead I'd be better off using a dynamically typed language to do this. The problem is I really enjoy the intellisense that the static typing of C# offers, along with the performance improvements over a language like Python. I'd just like a way to syntactically abstract away the boilerplate with my own helper methods for things like this.
UPDATE: Thanks to the comments here it seems I can do this with a lambda expression nice and elegantly. The signature can be simply changed to public static long CallMyFunctionWithParamsPlease<T>(Func<T> func)
If deferred execution is what you want simply pass a Func<TReturnType> to your method (or class). The calling method doesn't need to know how many parameters are involved.
e.g. Assuming MyMethod has a signature int MyMethod(int arg):
CallMyFunctionWithParamsPlease(() => MyMethod(1));
public static int CallMyFunctionWithParamsPlease(Func<int> func)
{
return func();
}
If MyMethod takes two parameters, it's the same call:
CallMyFunctionWithParamsPlease(() => MyMethod(1, 2));
I would like to create an extension method that will work for classes only.
I can write an extension method for type object, like this:
public static void SomeMethod(this object obj)
{
// some code
}
However this will work for primitives too, I would like to disable to do something like this:
int number = 2;
number.SomeMethod();
Any ideas how can I do this, if it's possible at all?
I think you need something like this, but I will not recommend you do that only if it's not for serialization or something like that.
public static void SomeMethod<T>(this T obj) where T : class
{
}
You are going to be fighting boxing behaviour here...
Essentially you limit your type that it must be an object, but C# adds some sugar and says, thats a primitive, I can box it into an object for you and treat it like an object too!
If you have some level of subclass you can target, that might stop the behaviour you are after (as in don't target all objects but perhaps something that is IObservable, IEnumerable etc.)
The other way to defensively code this is to add an explicit type check and exception throw on your method
if (myobject.GetTypeCode != TypeCode.Object) { throw new ArgumentException() }
Don't do it. If you have a method you want to call on a whole variety of objects, then centralize it in a function somewhere. That way some poor programmer who comes along 10 years after you will not be wondering where that weird method is located.
I want to implement a method that will find stuff in my custom class. It should work like generic collections work - i pass a pointer to a function, and the method will iterate through all it has to look in, apply this function, and if it returns true return the found item.
I'd like to pass function pointer as a parameter, but i dont want to declare delegate types.
I know i can do something like:
delegate bool Foo(MyClass)
MyClass MyMethod(Foo x)
{...}
And i know i can do something like this:
MyClass MyMethod(Func<MyClass,bool> x)
But can i do it without declaring a delegate type and without using built in stuff like Func<> which has limits on how many parameters i can have (in case of Func, one...)
You can just use delegate if you want, although it's a bit old school :)
public void TestInvokeDelegate()
{
InvokeDelegate( new TestDelegate(ShowMessage), "hello" );
}
public void InvokeDelegate(TestDelegate del, string message)
{
del(message);
}
public delegate void TestDelegate(string message);
public void ShowMessage(string message)
{
Debug.WriteLine(message);
}
You can allways pass in a Delegate and call DynamicInvoke on it:
MyClass MyMethod(Delegate x) {
// ...
x.DynamicInvoke(....);
// ...
}
It looks like you are trying to implement the Visitor pattern. In this case visiting methods usually have only one parameter - the instance to visit. Having additional arguments passed around conceals the use of the pattern and makes it harder to reason about. This article shows you one way to implement it in C#.
The key is to create a visitor class that will encapsulate all the parameters that affect the visiting process. This way you don't need to pass anythnig other than an object in question in the visiting method - everything else lives in instance fields.
However, if you really want to pass some additional parameters in the method and don't know what type they can have, there are ways to do that. More or less standard approach in .NET world is to use a delegate without return value and with single parameter of type object, the example would be ParameterizedThreadStart delegate:
public delegate void ParameterizedThreadStart(
Object obj
)
This way you get to pass only one parameter in the delegate, but it could be anything - an instance of a class, an array or null, if you end up not needing additional arguments after all. The downside of this approach is that it requires type casting which can lead to runtime errors.
I recently wrote the following statement by accident.
MyCollection myCollection = new MyCollection();
SomeMethod(myCollection.SomeVoidOperation());
Stupidly it took me some time to work out why it didn't work (Had a brainfart) but then it got me thinking; why is such a statement actually invalid in any general C type syntax context?
I understand that I can accomplish the same functionality with method chaining but what I don't get is why such a feature isn't (or perhaps can't) be implemented? To me the intent seems clear and I've tried but I can't see any ambiguity it might create. I'm sure there are some good reasons (or something that I'm missing) and hopefully someone can point it out to me.
So, why can’t an operation be
performed as part of an assignment?
UPDATE:
I understand why this doesn't work. IE: I have a method that expects some parameter and I am calling a method that doesn't return anything - but you are missing my point.... I see that code as two statements one is myCollection (IE: An instance) and the second is "invoke this method".
Here is a more complete example:
public class Stock
{
public Guid ID { get; set; }
public string Description { get; set; }
}
public class StockList : List<Stock>
{
public void SomeSortOperation() { }
}
public void SomeMethod(StockList stockList)
{
}
StockList myList = new StockList();
SomeMethod(myList.SomeSortOperation());
It looks like you're trying to use the result of SomeVoidOperation as a method argument (presumably to a method with no arguments) - but presumably you're not, because it's a void method.
It's never a good idea for code to look like it's doing one thing, when it's actually doing another.
EDIT: Okay, no, having seen the edit, I still don't think this is a good idea. You're basically saying that if you try to use a void expression as a method argument, it should actually use some different value, based on the expression. That expression would have to be very carefully defined... for example, would:
Foo(x.MethodReturningBar().MethodReturningVoid());
consider the argument to be the type of x, or Bar (the return type of the intermediate method call)?
Again, you're writing code which looks like it's doing one thing (using the value of the whole expression as an argument) when it's actually doing something else (using the value of part of the expression as an argument). That's simply a bad idea.
If you mean by SomeVoidOperation() that the operation returns no value (void):
You can't pass nothing to a method that expects to get something. You can't cast a nothing to the type that SomeMethod() expects to get.
I believe that you should backtrack a little and try to understand the void type first. See this answer C# void type- safety. I believe this will give more insight in why what you're trying to do doesn't work.
Is there a way via System.Reflection, System.Diagnostics or other to get a reference to the actual instance that is calling a static method without passing it in to the method itself?
For example, something along these lines
class A
{
public void DoSomething()
{
StaticClass.ExecuteMethod();
}
}
class B
{
public void DoSomething()
{
SomeOtherClass.ExecuteMethod();
}
}
public class SomeOtherClass
{
public static void ExecuteMethod()
{
// Returns an instance of A if called from class A
// or an instance of B if called from class B.
object caller = getCallingInstance();
}
}
I can get the type using System.Diagnostics.StackTrace.GetFrames, but is there a way to get a reference to the actual instance?
I am aware of the issues with reflection and performance, as well as static to static calls, and that this is generally, perhaps even almost univerally, not the right way to approach this. Part of the reason of this question is I was curious if it was doable; we are currently passing the instance in.
ExecuteMethod(instance)
And I just wondered if this was possible and still being able to access the instance.
ExecuteMethod()
#Steve Cooper:
I hadn't considered extension methods. Some variation of that might work.
Consider making the method an extension method. Define it as:
public static StaticExecute(this object instance)
{
// Reference to 'instance'
}
It is called like:
this.StaticExecute();
I can't think of a way to do what you want to do directly, but I can only suggest that if you find something, you watch out for static methods, which won't have one, and anonymous methods, which will have instances of auto-generated classes, which will be a little odd.
I do wonder whether you should just pass the invoking object in as a proper parameter. After all, a static is a hint that this method doesn't depend on anything other than its input parameters. Also note that this method may be a bitch to test, as any test code you write will not have the same invoking object as the running system.
I do not believe you can. Even the StackTrace and StackFrame classes just give you naming information, not access to instances.
I'm not sure exactly why you'd want to do this, but know that even if you could do it it would likely be very slow.
A better solution would be to push the instance to a thread local context before calling ExecuteMethod that you can retrieve within it or just pass the instance.
In the case of a static method calling your static method, there is no calling instance.
Find a different way to accomplish whatever you are trying to do.
Just have ExecuteMethod take an object. Then you have the instance no matter what.
I feel like I'm missing something, here. The static method can be called from literally anywhere. There's no guarantee that a class A or class B instance will appear anywhere in the call stack.
There's got to be a better way to accomplish whatever you're trying to do.