I am new to C#. I have read that the use of the keyword dynamic is somewhat controversial, and that other methods may be preferred depending on the use case. I want to make sure that my use of dynamic is appropriate for the language and the case. I'm working with this code:
public void myMethod(Action<T> myFunc) {
dynamic arg = "example"; // a string
myFunc(arg);
}
I don't know what the type T will be until runtime, and so I thought that dynamic would be useful. In the event that T is a string, I want to invoke myFunc with that argument. Is using dynamic the best way to do this in C#?
edit: To give more context, if T is string, then I want to pass arg to myFunc. If I don't use dynamic, I get an error that "cannot convert from 'string' to 'T'". Using dynamic solves this problem, I'm just not sure if it's the best way to solve it.
No. That is not an appropriate usage. You code will fail at runtime if T is anything other than string. That is the thing with dynamic, it turns of compiler checks, it does not mean your code will run, only that compiler errors turn into runtime errors. Dynamic is intended for interoperability with dynamic languages, or COM, where you are forced to cast objects at runtime anyway, so dynamic just makes it easier, without loosing any actual safety.
If you want to give myFunc a string, declare it as Action<string>. If you want to create a new object and give it to myFunc, add a new() restriction, i.e.
public void myMethod(Action<T> myFunc) where T : new(){
myFunc(new T());
}
If you don't know how to construct T, let the caller give it to you:
public void myMethod(Action<T> myFunc, T value) {
myFunc(value);
}
You can also use Func<T> to give your method a delegate that constructs values on demand.
Also Action<T> would be called a "generic delegate". Even if c++ templates and c# generics are used for a similar purpose, they work in a very different way. In effect c# generics is more restrictive, but also makes compiling much faster and easier since you do not have to generate code for each specialization until it is jitted.
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));
Question
How does a delegate store a reference to a function? The source code appears to refer to it as an Object, and the manner in which it invokes the method seems redacted from the source code. Can anyone explain how C# is handling this?
Original Post
It seems I'm constantly fighting the abstractions C# imposes on its programmers. One that's been irking me is the obfuscation of Functions/Methods. As I understand it, all methods are in fact anonymous methods assigned to properties of a class. This is the reason why no function is prefixed by a datatype. For example...
void foo() { ... }
... would be written in Javascript as...
Function foo = function():void { ... };
In my experience, Anonymous functions are typically bad form, but here it's replete throughout the language standard. Because you cannot define a function with its datatype (and apparently the implication/handling is assumed by the compiler), how does one store a reference to a method if the type is never declared?
I'm trying very hard to avoid Delegates and its variants (Action & Func), both because...
it is another abstraction from what's actually happening
the unnecessary overhead required to instantiate these classes (which in turn carry their own pointers to the methods being called).
Looking at the source code for the Delegate.cs, it appears to refer to the reference of a function as simply Object (see lines 23-25).
If these really are objects, how are we calling them? According to the delegate.cs trail, it dead-ends on the following path:
Delegate.cs:DynamicInvoke() > DynamicInvokeImpl() > methodinfo.cs:UnsafeInvoke() > UnsafeInvokeInternal() > RuntimeMethodHandle.InvokeMethod() > runtimehandles.cs:InvokeMethod()
internal extern static object InvokeMethod(object target, object[] arguments, Signature sig, bool constructor);
This really doesn't explain how its invoked if indeed the method is an object. It feels as though this is not code at all, and the actual code called has been redacted from source repository.
Your help is appreciated.
Response to Previous Comments
#Amy: I gave an example immediately after that statement to explain what I meant. If a function were prefixed by a datatype, you could write a true anonymous function, and store it as a property to an Object such as:
private Dictionary<string, Function> ops = new Dictionary<string, Function> {
{"foo", int (int a, int b) { return a + b } }
};
As it stands, C# doesn't allow you to write true anonymous functions, and walls that functionality off behind Delegates and Lambda expressions.
#500 Internal server error: I already explained what I was trying to do. I even bolded it. You assume there's any ulterior motive here; I'm simply trying to understand how C# stores a reference to a method. I even provided links to the source code so that others could read the code for themselves and help answer the question.
#Dialecticus: Obviously if I already found the typical answer on Google, the only other place to find the answer I'm looking for would be here. I realize this is outside the knowledge of most C# developers, and that's why I've provided the source code links. You don't have to reply if you don't know the answer.
While I'm not fully understanding your insights about "true anonymous functions", "not prefixed by a data type" etc, I can explain you how applications written in C# call methods.
First of all, there is no such a thing "function" in C#. Each and every executable entity in C# is in fact a method, that means, it belongs to a class. Even if you define lambdas or anonymous functions like this:
collection.Where(item => item > 0);
the C# compiler creates a compiler-generated class behind the scenes and puts the lambda body return item > 0 into a compiler-generated method.
So assuming you have this code:
class Example
{
public static void StaticMethod() { }
public void InstanceMethod() { }
public Action Property { get; } = () => { };
}
static class Program
{
static void Main()
{
Example.StaticMethod();
var ex = new Example();
ex.InstanceMethod();
ex.Property();
}
}
The C# compiler will create an IL code out of that. The IL code is not executable right away, it needs to be run in a virtual machine.
The IL code will contain a class Example with two methods (actually, four - a default constructor and the property getter method will be automatically generated) and a compiler-generated class containing a method whose body is the body of the lambda expression.
The IL code of Main will look like this (simplified):
call void Example::StaticMethod()
newobj instance void Example::.ctor()
callvirt instance void Example::InstanceMethod()
callvirt instance class [mscorlib]System.Action Example::get_Prop()
callvirt instance void [mscorlib]System.Action::Invoke()
Notice those call and callvirt instructions: these are method calls.
To actually execute the called methods, their IL code needs to be compiled into machine code (CPU instructions). This occurs in the virtual machine called .NET Runtime. There are several of them like .NET Framework, .NET Core, Mono etc.
A .NET Runtime contains a JIT (just-in-time) compiler. It converts the IL code to the actually executable code during the execution of your program.
When the .NET Runtime first encounters the IL code "call method StaticMethod from class Example", it first looks in the internal cache of already compiled methods. When there are no matches (which means this is the first call of that method), the Runtime asks the JIT compiler to create such a compiled-and-ready-to-run method using the IL code. The IL code is converted into a sequence of CPU operations and stored in the process' memory. A pointer to that compiled code is stored in the cache for future reuse.
This all will happen behind the call or callvirt IL instructions (again, simplified).
Once this happened, the Runtime is ready to execute the method. The CPU gets the compiled code's first operation address as the next operation to execute and goes on until the code returns. Then, the Runtime takes over again and proceeds with next IL instructions.
The DynamicInvoke method of the delegates does the same thing: it instructs the Runtime to call a method (after some additional arguments checks etc). The "dead end" you mention RuntimeMethodHandle.InvokeMethod is an intrinsic call to the Runtime directly. The parameters of this method are:
object target - the object on which the delegate invokes the instance method (this parameter).
object[] arguments - the arguments to pass to the method.
Signature sig - the actual method to call, Signature is an internal class that provides the connection between the managed IL code and native executable code.
bool constructor - true if this is a constructor call.
So in summary, methods are not represented as objects in C# (while you of course can have a delegate instance that is an object, but it doesn't represent the executable method, it rather provides an invokable reference to it).
Methods are called by the Runtime, the JIT compiler makes the methods executable.
You cannot define a global "function" outside of classes in C#. You could get a direct native pointer to the compiled (jitted) method code and probably even call it manually by directly manipulating own process' memory. But why?
You clearly misunderstand main differences between script languages, C/C++ and C#.
I guess the main difficulty is that there is no such thing as a function in C#. At all.
C#7 introduced the new feature "a local function", but that is not what a function in JS is.
All pieces of code are methods.
That name is intentionally different from function or a procedure to emphasize the fact that all executable code in C# belongs to a class.
Anonymous methods and lambdas are just a syntax sugar.
A compiler will generate a real method in the same (or a nested) class, where the method with anonymous method declaration belongs to.
This simple article explains it. You can take the examples, compile them and check the generated IL code yourself.
So all the methods (anonymous or not) do belong to a class. It's impossible to answer your updated question, besides saying It does not store a reference to a function, as there is no such thing in C#.
How does one store a reference to a method?
Depending on what you mean by reference, it can be either
An instance of MethodInfo class, used to reference reflection information for a method,
RuntimeMethodHandle (obtainable via RuntimeMethodInfo.MethodHandle) stores a real memory pointer to a JITed method code
A delegate, that is very different from just a memory pointer, but logically could be used to "pass a method reference to another method" .
I believe you are looking for the MethodInfo option, it has a MethodInfo.Invoke method which is very much alike Function..apply function in JS. You have already seen in the Delegate source code how that class is used.
If by "reference" you mean the C-style function pointer, it is in RuntimeMethodHandle struct. You should never use it without solid understanding how a particular .Net platform implementation and a C# compiler work.
Hopefully it clarifies things a bit.
A delegate is simply a pointer(memory location to jump to) to a method with the specified parameters and return type. Any Method that matches the signature(Parameters and return type) is eligible to fulfill the role, irrespective of the defined object. Anonymous simply means the delegate is not named.
Most times the type is implied(if it is not you will get a compiler error):
C# is a strongly typed language. That means every expression (including delegates) MUST have a return type(including void) as well as strongly typed parameters(if any). Generics were created to permit explicit types to be used within general contexts, such as Lists.
To put it another way, delegates are the type-safe managed version of C++ callbacks.
Delegates are helpful in eliminating switch statements by allowing the code to jump to the proper handler without testing any conditions.
A delegate is similar to a Closure in Javascript terminology.
In your response to Amy, you are attempting to equate a loosely typed language like JS, and a strongly typed language C#. In C# it is not possible to pass an arbitrary(loosely-typed) function anywhere. Lambdas and delegates are the only way to guarantee type safety.
I would recommend trying F#, if you are looking to pass functions around.
EDIT:
If you are trying to mimic the behavior of Javascipt, I would try looking at using inheritance through Interfaces. I can mimic multiple inheritance, and be type safe at the same time. But, be aware that it cannot fully supplant Javascript's dependency injection model.
As you probably found out C# doesn't have the concept of a function as in your JavaScript example.
C# is a statically typed language and the only way you can use function pointers is by using the built in types (Func,Action) or custom delegates.(I'm talking about safe,strongly typed pointers)
Javascript is a dynamic language that's why you can do what you describe
If you are willing to lose type safety, you can use the "dynamic" features of C# or refection to achieve what you want like in the following examples (Don't do this,use Func/Action)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
namespace ConsoleApp1
{
class Program
{
private static Dictionary<string, Func<int, int, int>> FuncOps = new Dictionary<string, Func<int, int, int>>
{
{"add", (a, b) => a + b},
{"subtract", (a, b) => a - b}
};
//There are no anonymous delegates
//private static Dictionary<string, delegate> DelecateOps = new Dictionary<string, delegate>
//{
// {"add", delegate {} }
//};
private static Dictionary<string, dynamic> DynamicOps = new Dictionary<string, dynamic>
{
{"add", new Func<int, int, int>((a, b) => a + b)},
{"subtract", new Func<int, int, int>((a, b) => a - b)},
{"inverse", new Func<int, int>((a) => -a )} //Can't do this with Func
};
private static Dictionary<string, MethodInfo> ReflectionOps = new Dictionary<string, MethodInfo>
{
{"abs", typeof(Math).GetMethods().Single(m => m.Name == "Abs" && m.ReturnParameter.ParameterType == typeof(int))}
};
static void Main(string[] args)
{
Console.WriteLine(FuncOps["add"](3, 2));//5
Console.WriteLine(FuncOps["subtract"](3, 2));//1
Console.WriteLine(DynamicOps["add"](3, 2));//5
Console.WriteLine(DynamicOps["subtract"](3, 2));//1
Console.WriteLine(DynamicOps["inverse"](3));//-3
Console.WriteLine(ReflectionOps["abs"].Invoke(null, new object[] { -1 }));//1
Console.ReadLine();
}
}
}
one more example that you shouldn't use
delegate object CustomFunc(params object[] paramaters);
private static Dictionary<string, CustomFunc> CustomParamsOps = new Dictionary<string, CustomFunc>
{
{"add", parameters => (int) parameters[0] + (int) parameters[1]},
{"subtract", parameters => (int) parameters[0] - (int) parameters[1]},
{"inverse", parameters => -((int) parameters[0])}
};
Console.WriteLine(CustomParamsOps["add"](3, 2)); //5
Console.WriteLine(CustomParamsOps["subtract"](3, 2)); //1
Console.WriteLine(CustomParamsOps["inverse"](3)); //-3
I will provide a really short and simplified answer compared to the others. Everything in C# (classes, variables, properties, structs, etc) has a backed with tons of things your programs can hook into. This network of backend stuff slightly lowers the speed of C# when compared to "deeper" languages like C++, but also gives programmers a lot more tools to work with and makes the language easier to use. In this backend is included things like "garbage collection," which is a feature that automatically deletes objects from memory when there are no variables left that reference them. Speaking of reference, the whole system of passing objects by reference, which is default in C#, is also managed in the backend. In C#, Delegates are possible because of features in this backend that allow for something called "reflection."
From Wikipedia:
Reflection is the ability of a computer program to examine,
introspect, and modify its own structure and behavior at runtime.
So when C# compiles and it finds a Delegate, it is just going to make a function, and then store a reflective reference to that function in the variable, allowing you to pass it around and do all sorts of cool stuff with it. You aren't actually storing the function itself in the variable though, you are storing a reference, which is kinda like an address that points you to where the function is stored in RAM.
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.
I define a Delegate type like :
delegate void DrawShape(Brush aBrush,Rectangle aRect);
Would you tell me why the below below methods of creating delegate object are all correct:
DrawShape DrawRectangleMethod = CreateGraphics().FillRectangle;
DrawShape AnotherDrawRectangleMethod = new DrawShape(CreateGraphics().FillRectangle);
Why can the method without "New" work correctly?
DrawShape DrawRectangleMethod = CreateGraphics().FillRectangle;
is possible thanks to C#2's implicit method group conversions which are spec'd here.
C# is smart enough to handle method groups for you. A method group is the method name without the parentheses; there may be many overloads of the method with different signatures, but the "base" name without parentheses is called the method group. The compiler will insert the constructor call for you, allowing you to write a more concise line of code.
Cause the compiler will automatically insert it for you. It's just syntactic sugar.
Simply take a look into Jons Bluffer's Guide at chapter Delegates.