I'm using C# 4.6.2, though could upgrade to 4.7.2 if it's possible there.
In many places in our code we have a loop with wait statements to check for a specific value when a function is called, wait and retry if it's not what we wanted until a maximum number of retries.
I'd like to abstract this out, but the only implementation I can think of requires you pass in a method with a variable number of arguments of variable types, which after much searching of Google appeared to not be possible about 5 years ago. There has been many improvements to C# since then, so
is it still not possible?
If it is now possible how do I do it?
If it isn't possible can you think of any other way I can achieve my goal?
The sort of thing I'm looking for is:
public bool GenericLoopWait(int maxWaitSeconds, int waitMsPerIteration,??? DoSomething,object expectedResult,...)
int maxRetries = maxWaitSeconds*1000/waitMsPerIteration;
SomeType result=null;
for(int i=0; i<maxRetries; i++){
result = DoSomething(...);
if(result==expectedResult) break;
Thread.Sleep(waitMsPerIteration);
}
return result==expectedResult
}
And then both of these would work:
GenericLoopWait(5,500,Browser.Webdriver.FindElements(selector).Any(),true);
GenericLoopWait(5,500,Api.GetSpecificObject(api,objectName),"expectedOutcome");
You could use generics and Func and wrap the actual calls parameters when you call through to the method.
public bool GenericLoopWait<T>(int maxWaitSeconds, int waitMsPerIteration, Func<T> DoSomething, T expectedResult = default(T))
{
int maxRetries = maxWaitSeconds * 1000 / waitMsPerIteration;
T result = default(T);
for (int i = 0; i < maxRetries; i++)
{
result = DoSomething();
if (expectedResult.Equals(result)) break;
Thread.Sleep(waitMsPerIteration);
}
return expectedResult.Equals(result);
}
Calling code:
GenericLoopWait(5, 500, () => Browser.Webdriver.FindElements(selector).Any(), true);
GenericLoopWait(5, 500, () => Api.GetSpecificObject(api,objectName), "expectedOutcome")
Working dotnetfiddle
The general pattern is to create a "Wrapper" method that accepts an Action or a Func as a parameter. Your wrapper can do it's own logic, and Invoke the parameter at the correct time.
As a simple generic example:
public void MethodWrapper(Action action)
{
Console.WriteLine("begin");
action.Invoke();
Console.WriteLine("end");
}
You could then do this:
void Main()
{
var a = 1;
var b = 2;
MethodWrapper(() => DoSomething(a));
MethodWrapper(() => DoSomethingElse(a,b));
}
public void DoSomething(int a)
{
Debug.WriteLine($"a={a}");
}
public void DoSomethingElse(int a, int b)
{
Debug.WriteLine($"a={a}, b={b}");
}
To generate this output:
begin
a=1
end
begin
a=1, b=2
end
For your specific case, your wrapper could take additional parameters specifying things like number of retries, time between calls, or acceptance criteria.
Related
Relative newcomer to c# here.
Let’s say I have 50 different methods a1(), a2(), … a50() and I want to call a random one. One way to do it is of course to generate a random int, nr, between 1 and 50 and then use a lot of if statements like if(nr == 1){
a1()
} and so on. Quite cumbersome - can I do something smarter?
Is it for example possible to do something along the lines of creating a string which is initially only “a” and then adding nr as a string and then calling that string as method? Like this:
Public void RandomMethod()
{
nr = Random.Range(1,51);
string = ‘a’ + nr.tostring();
string();
}
I know this doesn’t work, but something like this instead of my first idea would save me hundreds of lines of code
Any response is appreciated 😊
One option would be to put your functions into a collection, say a List for example. Then you could randomly index into that collection to get a random function to call. You would generate a random index between 0 and the length of the List minus 1. This could apply generally to any number of functions then (50 or otherwise).
To do exactly what you asked (and, I have no clue why you'd want to), consider something like this:
Create a delegate that matches the call signature of all of your methods (they all have to have the same call signature or ... I really can't imagine what you'd want to do if they didn't). You could use an Action or Func declaration, but I'm going to make it clear here:
public delegate void SomeMethod(int i);
Then write your 50 methods. All their call signatures will match the delegate:
public static void Method1(int i) { System.Console.WriteLine($"{nameof(Method1)}: {i}"); }
public static void Method2(int i) { System.Console.WriteLine($"{nameof(Method2)}: {i}"); }
public static void Method3(int i) { System.Console.WriteLine($"{nameof(Method3)}: {i}"); }
public static void Method4(int i) { System.Console.WriteLine($"{nameof(Method4)}: {i}"); }
// ...
public static void Method50(int i) { System.Console.WriteLine($"{nameof(Method50)}: {i}"); }
Then create an array of delegates:
public static SomeMethod[] Methods = new SomeMethod[]
{
Method1,
Method2,
Method3,
Method4,
//...
Method50,
};
And then a method that picks 1 or more from the list at random and runs them:
public void Run5RandomMethods()
{
Random random = new Random();
for(int i = 0; i < 5; i++)
{
var randNumber = random.Next(50);
var method = Methods[randNumber];
method.Invoke(i);
}
}
Note: this is untested, I'm not going to create 50 dummy methods for you. If you find an issue, comment below and I'll fix the code
By the way, what you show in your question (composing the name of the method by concatenating a string and the string representation of a number) is doable using a technology known as Reflection. Let me know if you really want to do that.
So, assuming we have fifty methods that all have a signature like
void SomeMethod()
{
...
}
You could declare an array like below, this is an array of Action delegates
var methods = new Action[]
{
SomeMethod,
SomeOtherMethod,
() => _ = SomeFunctionWithAHardcodedParameter("Wibble"),
...
}
Then you could call a random method by doing,
method[Random.Next(methods.Length)]();
First off, I just want to say something similar to what others have already said: you should readdress whether you need 50 methods named a1(), a2(), ..., a50(), and rethink what the problem you're trying to solve is (which you haven't provided enough information for us to help you with).
If that was hyperbole, try to avoid doing that; it may muddy the responses to solve a perceived problem ("why do you have 50 poorly-named methods?") instead of your actual problem ("can I execute a randomly selected method?" <- still a weird question, but who am I to judge...).
That out of the way, you can use something like Reflection. This can be "dangerous" and expensive when executing, so use with caution... or better yet don't use it, but be aware of it, because it can lead you to think Reflection is the answer to problems you don't actually have.
Anyway, you can:
// have an instance of an object
var obj = new ClassName();
// get all the methods of the object
var methodInfos = typeof(ClassName).GetMethods();
// filter them somehow
var filteredMethodInfos = methodInfos.Where(m => Regex.IsMatch(m.Name, #"\a[\d]{1,2}")).ToArray();
// get a random one and invoke it
var rnd = new Random();
filteredMethodInfos[rnd.Next(filteredMethodInfos.Length)].Invoke(obj, null);
I haven't tested this, but it should in theory work.
But again: don't use reflection if you don't have to. There's probably an issue with your root question (as Tim Schmelter said, this is an "XY-problem") if your answer is "randomly execute 1 of 50 methods".
I am curious as to what is the point of creating instances of delegates. I understand that they are function pointers but what is the purpose of the using delegate instances?
public delegate bool IsEven(int x);
class Program
{
static void Main(string[] args)
{
int[] nums = new int[] { 1, 3, 4, 5, 6,7, 8, 9 };
// creating an instance of delegate and making it point to 'CheckEven' function
IsEven isEven = new IsEven(CheckEven);
// using delegate for parameter to get sum of evens
int tot1 = Sums(nums, isEven);
// passing in the function directly as parameter and not delegate instance
int tot2 = Sums(nums, CheckEven);
// using lambda expressions
int tot3 = Sums(nums, x=> x%2==0);
ReadKey();
}
public static int Sums(int[] nums, IsEven isEvenOdd)
{
int sum = 0;
foreach(int x in nums)
{
if (isEvenOdd(x))
sum += x;
}
return sum;
}
public static bool CheckEven(int x)
{
if (x % 2 == 0)
return true;
else
return false;
}
}
At first thought opting to always use lambdas seems like the best idea if the functionality of the function you are going to pass does not have complex implementation, otherwise why wouldn't I just pass my function directly into the parameters? Also with Lambdas it is much easier to change it to compute the sum of odds as oppose to creating an entire new method as I would have to with delegates.
You already used delegates even if you passed the method directly, the Sums method could not receive the method name if it's not set its expectation by the delegate definition.
Consider changing the delegate name to public delegate bool EvenOddStatus(int x);
you can send either your CheckEven method or this method:
public static bool CheckOdd(int x)
{
if (x % 2 != 0) // here changed
return true;
else return false;
}
(of course you can use lambda here, but not in more complicated scenarios)
Now your Sums method can sum up either odd or even numbers based on the calling code (either pass CheckEven or CheckOdd).
Delegates are pointers to methods. you define how the code want the method to receive and return, and the calling code can pass a pre-defined implementation later, either a method or delegate.
What if your Main is another method that doesn't know what to sum, odds or evens? it will have a delegate from a calling code up the stack.
Let's consider this code:
public void DoSomething<T>(Func<T> MyFunc)
{
var NewLazyItem = new Lazy<T>(MyFunc);
// do stuff
// use the NewLazyItem
}
And let's say I have a method like this:
public int Add(int a, int b) { return a + b; }
What I would like to achieve is to be able to pass the Add method to the DoSomething method but with parameters along.
Ideally I could pass Add, 2 and 3 and when the NewLazy item gets used, Add(2, 3) gets called.
I tried various ideas, but I can't find a way to make it work.
Unless I'm misunderstanding the question, the simplest way is just to include the parameters as closures for a lambda expression:
var x = 2;
var y = 3;
DoSomething(() => Add(x, y));
I'm trying to port some AS3 code to C#(.NET) the majority of it has been done (90%) however I have run into a few problems in terms of Functions in Functions and functions being defined as Functions (I hope i'm understanding it correctly). I have done a lot of searching and the main thing that comes up is delegates and lambda's however trying to implement them is proving difficult for me to do. Seen as quiet a few sections are the same in layout ill just post a generic example of the AS3 code and hopefully can then apply any solution to the rest.
Here is the AS3 code:
static public function makeRadial(seed:int):Function {
var islandRandom:PM_PRNG = new PM_PRNG();
islandRandom.seed = seed;
var bumps:int = islandRandom.nextIntRange(1, 6);
var startAngle:Number = islandRandom.nextDoubleRange(0, 2*Math.PI);
var dipAngle:Number = islandRandom.nextDoubleRange(0, 2*Math.PI);
var dipWidth:Number = islandRandom.nextDoubleRange(0.2, 0.7);
function inside(q:Point):Boolean {
var angle:Number = Math.atan2(q.y, q.x);
var length:Number = 0.5 * (Math.max(Math.abs(q.x), Math.abs(q.y)) + q.length);
var r1:Number = 0.5 + 0.40*Math.sin(startAngle + bumps*angle + Math.cos((bumps+3)*angle));
var r2:Number = 0.7 - 0.20*Math.sin(startAngle + bumps*angle - Math.sin((bumps+2)*angle));
if (Math.abs(angle - dipAngle) < dipWidth
|| Math.abs(angle - dipAngle + 2*Math.PI) < dipWidth
|| Math.abs(angle - dipAngle - 2*Math.PI) < dipWidth) {
r1 = r2 = 0.2;
}
return (length < r1 || (length > r1*ISLAND_FACTOR && length < r2));
}
return inside;
}
In the AS3 code I don't understand the reasoning behind the ":Function" in the main function "static public function makeShape(seed:int):Function". I did search about it but was unable to find an example or explanation perhaps i'm not typing the correct meaning for it.
If anyone could help me with this problem by giving an example or pointing me closer in the direction I need to go I would be very grateful.
Thanks for your time.
The most direct translation would be to return a delegate. In this case, the generic Func<Point, bool> delegate would be sufficient. It's pretty easy to create these in C# using lambda expressions:
static public Func<Point, bool> makeShape(int seed) {
// initialization here
Func<Point, bool> inside = (Point q) => {
// some math here
return (myCondition);
}
return inside;
}
Although you can define your own delegate type if you prefer:
public delegate bool ShapeTester(Point point);
static public ShapeTester makeShape(int seed) {
// initialization here
ShapeTester inside = (Point q) => {
// some math here
return (myCondition);
}
return inside;
}
Another approach, but one which would require quite a bit more effort in refactoring, would be to encapsulate all the logic of what makes up 'shape' into a distinct type, for example:
public class Shape
{
public Shape(int seed)
{
// initialization here
}
public bool Test(Point q)
{
// some math here
return (myCondition);
}
}
And then return an instance of this type from your makeShape method:
static public Shape makeShape(int seed) {
return new Shape(seed);
}
And elsewhere you'd need to call the test method on the resulting object. Depending the specific you're developing, you may make more since if Shape is actually be an interface (IShape) or a struct. But in any case, using this approach, traditional OOP design principles (inheritance, polymorphism, etc.) should be followed.
Why can't you use a ref or out parameter in a lambda expression?
I came across the error today and found a workaround but I was still curious why this is a compile-time error.
CS1628: Cannot use in ref or out parameter 'parameter' inside an anonymous method, lambda expression, or query expression
Here's a simple example:
private void Foo()
{
int value;
Bar(out value);
}
private void Bar(out int value)
{
value = 3;
int[] array = { 1, 2, 3, 4, 5 };
int newValue = array.Where(a => a == value).First();
}
Lambdas have the appearance of changing the lifetime of variables that they capture. For instance, the following lambda expression causes the parameter p1 to live longer than the current method frame as its value can be accessed after the method frame is no longer on the stack
Func<int> Example(int p1) {
return () => p1;
}
Another property of captured variables is that changes to the variables are also visible outside the lambda expression. For example, the following code prints out 42
void Example2(int p1) {
Action del = () => { p1 = 42; };
del();
Console.WriteLine(p1);
}
These two properties produce a certain set of effects which fly in the face of a ref parameter in the following ways:
ref parameters may have a fixed lifetime. Consider passing a local variable as a ref parameter to a function.
Side effects in the lambda would need to be visible on the ref parameter itself. Both within the method and in the caller.
These are somewhat incompatible properties and are one of the reasons they are disallowed in lambda expressions.
Under the hood, the anonymous method is implemented by hoisting captured variables (which is what your question body is all about) and storing them as fields of a compiler generated class. There is no way to store a ref or out parameter as a field. Eric Lippert discussed it in a blog entry. Note that there is a difference between captured variables and lambda parameters. You can have "formal parameters" like the following as they are not captured variables:
delegate void TestDelegate (out int x);
static void Main(string[] args)
{
TestDelegate testDel = (out int x) => { x = 10; };
int p;
testDel(out p);
Console.WriteLine(p);
}
You can but you must explicitly define all the types so
(a, b, c, ref d) => {...}
Is invalid, however
(int a, int b, int c, ref int d) => {...}
Is valid
As this is one of the top results for "C# lambda ref" on Google; I feel I need to expand on the above answers. The older (C# 2.0) anonymous delegate syntax works and it does support more complex signatures (as well closures). Lambda's and anonymous delegates at the very least have shared perceived implementation in the compiler backend (if they are not identical) - and most importantly, they support closures.
What I was trying to do when I did the search, to demonstrate the syntax:
public static ScanOperation<TToken> CreateScanOperation(
PrattTokenDefinition<TNode, TToken, TParser, TSelf> tokenDefinition)
{
var oldScanOperation = tokenDefinition.ScanOperation; // Closures still work.
return delegate(string text, ref int position, ref PositionInformation currentPosition)
{
var token = oldScanOperation(text, ref position, ref currentPosition);
if (token == null)
return null;
if (tokenDefinition.LeftDenotation != null)
token._led = tokenDefinition.LeftDenotation(token);
if (tokenDefinition.NullDenotation != null)
token._nud = tokenDefinition.NullDenotation(token);
token.Identifier = tokenDefinition.Identifier;
token.LeftBindingPower = tokenDefinition.LeftBindingPower;
token.OnInitialize();
return token;
};
}
Just keep in mind that Lambdas are procedurally and mathematically safer (because of the ref value promotion mentioned earlier): you might open a can of worms. Think carefully when using this syntax.
And maybe this?
private void Foo()
{
int value;
Bar(out value);
}
private void Bar(out int value)
{
value = 3;
int[] array = { 1, 2, 3, 4, 5 };
var val = value;
int newValue = array.Where(a => a == val).First();
}
You can not use an out parameter directly in a lambda expression. The reason why you can not do that is explained in the other answers.
Workaround
But you can use a local temporary variable with for the inner function and, after the inner function has been executed, assign the out value from the inner function to the out value of the outer function:
private static int OuterFunc (int i_param1, out int o_param2)
{
int param2 = 0;
var del = () => InnerFunc (i_param1, out param2);
int result = del ();
o_param2 = param2;
return result;
}
private static int InnerFunc (int i_param1, out int o_param2)
{
o_param2 = i_param1;
return i_param1;
}
private static void Main (string[] args)
{
int result = OuterFunc (123, out int param2);
Console.WriteLine (result); // prints '123'
Console.WriteLine (param2); // prints '123'
}
Please note
The question was created in 2009. My answer was created in 2023 using C#10 and .NET 6. I don't know whether this answer had also worked back in 2009, which means, the code here might depend on enhancements to C# and .NET that might have been made in the meantime.
I will give you another example.
Description
The code below will throw out this error. Because the change brought by the lambda expression (i)=>{...} only works in the function test.
static void test(out System.Drawing.Image[] bitmaps)
{
int count = 10;
bitmaps = new System.Drawing.Image[count];
Parallel.For(0, count, (i) =>
{
bitmaps[i] = System.Drawing.Image.FromFile("2.bmp");
});
}
Solution
So, if you remove out of the parameter, it works.
static void test(System.Drawing.Image[] bitmaps)
{
int count = 10;
bitmaps = new System.Drawing.Image[count];
Parallel.For(0, count, (i) =>
{
bitmaps[i] = System.Drawing.Image.FromFile("2.bmp");
});
}
If you need out really, don't change the parameter in the lambda expression directly. Instead, use a temporary variable please.
static void test(out System.Drawing.Image[] bitmaps)
{
int count = 10;
System.Drawing.Image[] bitmapsTemp = new System.Drawing.Image[count];
Parallel.For(0, count, (i) =>
{
bitmapsTemp[i] = System.Drawing.Image.FromFile("2.bmp");
});
bitmaps = bitmapsTemp;
}