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something C# writing multiplying code in static

first of all, i apologize if this question has already been asked. This is my first time asking something on stackoverflow.
so, i am currently learning C# and decided to write a piece of code for practise.
this monstrocity is the result:
static class Calculator
{
public static int multiply;
public static int Sum(int times1, int times2)
{
multiply= times1 * times2;
return multiply;
}
public new static string ToString()
{
return $"this is the result of a calculation/multiplication: {multiply}";
}
}
pretty neat right? its supposed to return a simple multiplication and print it in the main string using an override ToString method (but since that cant be done in static, I made a new one and used that).
class Program
{
public static void Main(string[] args)
{
Console.WriteLine(Calculator.ToString());
}
}
this is where the problem comes into play, because the console simply states:
this is the result of a calculation/multiplication: 0
even when i assign values to 'times1' and 'times2' the result is the same.
I cant figure out what I am doing wrong here, can anybody help me out?

Multiply doesn't contain a value. First call Sum function with whatever numbers you want to multiply and then you can write the result.
class Program
{
public static void Main(string[] args)
{
Calculator.Sum(2, 4);
Console.WriteLine(Calculator.ToString());
}
}

it's due to the static keyword. So, basically, your new static method ToString()(is not the best name) knows nothing about multiply returned after Sum execution,
Do not use static at all
Perform calculation directly in ToString()(suggest to rename =))

Call Method based on Type of Parameter

I have an object that can be of type AudioRequest or VideoRequest. Both classes inherit from Request. I have this class:
public static DoThings
{
public static void HandleRequest(AudioRequest r)
{
// Do things.
}
public static void HandleRequest(VideoRequest r)
{
// Do things.
}
}
I want to be able to call DoThings.HandleRequest(r) where r can be either a VideoRequest or AudioRequest and have it call the correct one. Is that possible? I have no control over the *Request classes, so I can't do anything to them. I do have control of the DoThings class and the code that calls HandleRequest. This is the code that calls it, it is WebAPI:
public Response Post(Request input)
{
return DoThings.HandleRequest(input);
}
The code above gives the error Argument 1: cannot convert from 'Request' to 'AudioRequest'.
The original code that I was cleaning up had this:
if (input.GetType() == typeof(AudioRequest))
{
var audioRequest = (AudioRequest)input;
DoThings.HandleRequest(audioRequest);
}
else if (input.GetType() == typeof(VideoRequest))
{
var videoRequest = (VideoRequest)input;
DoThings.HandleRequest(videoRequest);
}
But I figured there was a cleaner way to do this.

Based on the information you've provided so far, your question appears to be a duplicate of How to call a function dynamically based on an object type. I agree with the answer, that the fact that you want to do this suggests you should rethink the design. But, you can use dynamic to accomplish what you want.
Here's a simple console program that demonstrates the basic idea:
class Program
{
static void Main(string[] args)
{
A b = new B(), c = new C();
M(b);
M(c);
}
static void M(A a)
{
WriteLine("M(A)");
M((dynamic)a);
}
static void M(B b)
{
WriteLine("M(B)");
}
static void M(C c)
{
WriteLine("M(C)");
}
}
class A { }
class B : A { }
class C : A { }
The output is:
M(A)
M(B)
M(A)
M(C)
As you can see, in each case the M(A) method is called first, and then the appropriate M(B) or M(C) overload is called from M(A).
In your own example, this could look something like this:
public static DoThings
{
public static void HandleRequest(Request r)
{
// Dynamic dispatch to actual method:
HandleRequest((dynamic)r);
}
public static void HandleRequest(AudioRequest r)
{
// Do things.
}
public static void HandleRequest(VideoRequest r)
{
// Do things.
}
}
Note that dynamic does incur a run-time cost, particularly the first time a method is called with a given run-time type. But depending on the frequency and complexity of these "requests", using dynamic could be the cleanest way out of the current situation.

C# will call the appropriate function that matches the arguments and their types.
That being said, both of your functions accept AudioRequest, I believe one of those should accept a VideoRequest.
public static DoThings
{
public static void HandleRequest(AudioRequest r)
{
// Do things.
}
public static void HandleRequest(VideoRequest r)
{
// Do things.
}
}

If for some reason you must have two different functions that take only AudioRequest you can differentiate between two function with an extra parameter
public static class DoThings
{
public static void HandleRequest(AudioRequest r)
{
// Do things.
}
public static void HandleRequest(AudioRequest r, bool UseAlternativeMethod)
{
// Do other things.
}
}
Simply having a second parameter will call the second method regardless of it's value.
This isn't a best practices solution as you'd rather discriminate between them by accurately renaming the method name to be accurate but in practice you don't always have a choice.

grouping static classes with the same behavior

I have groups of logic that consist of static classes such as:
static class A {
static int mutate(int i) { /**implementation*/ };
static double prop(double a, double b) { /**implementation*/ };
}
static class B {
static int mutate(int i) { /**implementation*/ };
static double prop(double a, double b) { /**implementation*/ };
}
In this case, A and B are static classes that implement the same behavior via a group of functions (e.g. mutate). I would like to use something like an interface for this pattern, however since static classes cannot implement interfaces I am not sure what to do. What is the best way to implement this type of behavior cleanly?
EDIT:
Here is an example of what I am currently doing. The classes have no state so normally I would make them static.
Interface IMutator {
int mutate(int i);
}
class A : IMutator {
int mutate(int i) { /**implementation*/ };
}
class B : IMutator {
int mutate(int i) { /**implementation*/ };
}
class C {
public List<IMutator> Mutators;
public C(List<IMutator> mutators) {
Mutators = mutators;
}
}
//Somewhere else...
//The new keyword for A and B is what really bothers me in this case.
var Cinstance = new C(new List<IMutator>() {new A(), new B() /**...*/});

The stateless class doesn't have to be static.
Moreover, static dependencies isn't a good choice, when you want to write unit tests, or when you want to extract some common interface (as in your case).
It's OK to have non-static classes, containing logic only. E.g., people build ASP .NET applications using stateless controllers.
So, just throw away static and extract an interface.

Apart from #Dennis answer (which I have +1'ed, and it's indeed the way to go), other approach that may work, is having a set of functions (Func<>) and/or actions (Action<>) and resolve them using reflection. The code would not be specially elegant nor performant, but it works.
I've made a quick example on dotnetfiddle

C# caching with delegates

I'm working on a simple cache class for my application.
using System;
namespace Program {
public class Cache {
public delegate int CacheMethodInt();
public static int Get(CacheMethodInt method) {
//todo: generate cachekey here
return method.Invoke();
}
}
public class Calculator {
public int Add(int x, int y) {
return x + y;
}
}
class Program {
static void Main(string[] args) {
Calculator c = new Calculator();
Console.WriteLine(Cache.Get(() => c.Add(1, 2)));
}
}
}
In Cache:Get I need to check whether the return value is already cached and if so return it without invoking the method. The problem is that I can't figure out how to generate a good cachekey. In this case I would like something like this:
Calculator:Add:1(int):2(int)
Is it possible to get this info in Cache:Get? I'm using .NET 2.0.

It's possible using reflection.
As alternative, on a project I used Postsharp for the same purposes. As benefit more generic and common approach
And do not forget about cache invalidation or expiration.
Related question shows how to get MethodInfo and method name:
Using MethodInfo.GetCurrentMethod() in anonymous methods
When you have MethodInfo than you can get all you need

C#: Creating an instance of an abstract class without defining new class

I know it can be done in Java, as I have used this technique quite extensively in the past. An example in Java would be shown below. (Additional question. What is this technique called? It's hard to find an example of this without a name.)
public abstract class Example {
public abstract void doStuff();
}
public class StartHere{
public static void main(string[] args){
Example x = new Example(){
public void doStuff(){
System.out.println("Did stuff");
}
};
x.doStuff();
}
}
Now, my main question would be, can this also be done in C#, and if so, how?

The Java technique is called "Anonymous inner class", and there is no equivalent in C#.

With lamba expressions and class initializers you can get the same behaviour with a bit of effort.
public class Example {
public Action DoStuff;
public Action<int> DoStuffWithParameter;
public Func<int> DoStuffWithReturnValue;
}
class Program {
static void Main(string[] args) {
var x = new Example() {
DoStuff = () => {
Console.WriteLine("Did Stuff");
},
DoStuffWithParameter = (p) => {
Console.WriteLine("Did Stuff with parameter " + p);
},
DoStuffWithReturnValue = () => { return 99; }
};
x.DoStuff();
x.DoStuffWithParameter(10);
int value = x.DoStuffWithReturnValue();
Console.WriteLine("Return value " + value);
Console.ReadLine();
}
}
One problem with this solution that I just realized is that if you were to create fields in the Example class, the lambda expressions would not be able to access those fields.
However, there is no reason that you could not pass the instance of Example to the lambda expressions which would give them access to any public state that example might hold. AFAIK that would be functionally equivalent to the Java Anonymous Inner Class.
P.S. If you are going to vote an answer down, do us all a favour and add a comment as to why you disagree :-)

Typically, problems that are solved with anonymous inner classes in Java are solved in a much cleaner fashion using delegates in .Net. Your example is a little too simplistic to determine your intent. If your intent by using the abstract class is to pass around a "behavior" think about just using an Action delegate instead.
public class StartHere{
public static void main(string[] args){
Action doStuff = () => Console.WriteLine("Did stuff");
executeSomething(doStuff);
}
public static void executeSomething(Action action)
{
action();
}
}

That can't be done in C#; you need to declare a new class type. The closest you can get in C# is probably a named nested class:
public class StartHere{
private class Foo : Example {
public override void doStuff()
{
Console.WriteLine("did stuff");
}
}
public static void Main(string[] args){
Example x = new Foo();
x.doStuff();
}
}

This is not supported in C#, and if it were up to me it shouldn't be so either.
The proliferation of inner classes in java is mainly due to the lack of delegates or lambdas, which C# has. So while this type of functionality currently is "your only hope" in java, you can usually use other mechanisms in C# to achieve the same ends. Java feels like playing the piano with one hand in this regard.
(Admittedly a lot of us have gotten quite good at this one-handed playing; and now it seems like we have to wait at least until java 8 for closures...)

Since your class represents only an action, you can use a delegate in your case, there is an existing delegate :
public delegate void Action();
This is the exact equivalent of your class.
And the déclaration of your anonymous class is even cleaner :
Action action = () => Console.WriteLine("Hello world");
action(); // invoke
you can even use closure :
public void Hello(string name)
{
Action action = () => Console.WriteLine("Hello " + name);
action(); // will call the above lambda !
}

While all good answers, most of the work arounds suggested rely on C# 3.0
So, for the sake of completeness, I'll add another solution that uses neither lambdas nor Func type (Granted that, as Matt Olenik mentioned in the comments, one could generalize the below delegates to work the same way.). For those, like me who may still be working with C# 2.0. Maybe not the best solution, but it works.
public class Example
{
public delegate void DoStuffDelecate();
public DoStuffDelecate DoStuff;
public delegate void DoStuffWithDelecate(int n);
public DoStuffWithDelecate DoStuffWithParameter;
public delegate int DoStuffWithReturnDelecate();
public DoStuffWithReturnDelecate DoStuffWithReturnValue;
}
class Program
{
static int MethodWithReturnValue()
{
return 99;
}
static void MethodForDelecate()
{
Console.WriteLine("Did Stuff");
}
static void MethodForDelecate(int n)
{
Console.WriteLine("Did Stuff with parameter " + n);
}
static void Main(string[] args)
{
var x = new Example();
x.DoStuff = MethodForDelecate;
x.DoStuffWithParameter = MethodForDelecate;
x.DoStuffWithReturnValue = MethodWithReturnValue;
x.DoStuff();
x.DoStuffWithParameter(10);
int value = x.DoStuffWithReturnValue();
Console.WriteLine("Return value " + value);
Console.ReadLine();
}
}

You are able to accomplish this with Mocking in .NET. However there is no in-language support for this feature, I think it will be available in C# 4.0. There are a number of libraries out there for Mocking, including:
Moq
RhinoMock

In short no, you have to define it as separate sub class. I think this feature is coming C# 4.0 though?
Edit: No it's not coming C# 4.0 I made that up.