I have an app that runs a function that could take a decent amount of time, so I need to add a callback method. How would I go about this?
Mainly, my question is what is the type that needs to be passed to the class constructor?
On C# (not only on WP7) you can call any function asynchronously by wrapping it in a delegate. On the delegate's BeginInvoke call you'd pass a callback which will be invoked when the operation is completed. See the example below:
int MyLongOperation(int x, int y) {
Thread.Sleep(10000);
return x+y;
}
void CallingLongOperation(){
int x = 4;
int y = 5;
Func<int, int, int> func = MyLongOperation;
func.BeginInvoke(x, y, OperationCallback, func);
}
void OperationCallback(IAsyncResult asyncResult) {
Func<int, int, int> func = (Func<int, int, int>) asyncResult.AsyncState;
int result = func.EndInvoke(asyncResult);
// do something with the result
}
If you need to pass some additional parameter in the asyncState / userState property, you can also use the AsyncDelegate property of the IAsyncResult parameter (which for delegate calls is always System.Runtime.Remoting.Messaging.AsyncResult) and retrieve the delegate from there as well, as shown below.
public int MyLongOperation(int x, int y)
{
Thread.Sleep(10000);
return x + y;
}
public void CallLongOperation()
{
Func<int, int, int> func = MyLongOperation;
func.BeginInvoke(5, 7, MyCallback, "Expected result: " + 12);
Console.WriteLine("Called BeginInvoke");
func.BeginInvoke(11, 22, MyCallback, "Expected result: " + 33);
Console.WriteLine("Press ENTER to continue");
Console.ReadLine();
}
void MyCallback(IAsyncResult asyncResult)
{
Func<int, int, int> func = (Func<int, int, int>)((System.Runtime.Remoting.Messaging.AsyncResult)asyncResult).AsyncDelegate;
string expectedResult = (string)asyncResult.AsyncState;
int result = func.EndInvoke(asyncResult);
Console.WriteLine("Result: {0} - {1}", result, expectedResult);
}
Related
There is a method that receives a ValueTuple and returns it after modifying it. In this case, can I specify the field name of the ValueTuple in the method parameter?
private static void Operation()
{
var tuple = (X: 10, Y: 20);
var changeTuple = ChangeTuple(tuple);
}
private static ValueTuple<int, int> ChangeTuple(ValueTuple<int, int> tuple)
{
tuple.Item1 = 100; // ex) tuple.X = 100;
tuple.Item2 = 200; // ex) tuple.Y = 200;
return tuple;
}
Yes, you can simply replace ValueTuple<int, int> to (int X, int Y):
private static (int X, int Y) ChangeTuple((int X, int Y) tuple)
{
tuple.X = 100;
tuple.Y = 200;
return tuple;
}
For reference: naming tuple's fields
Or you can use deconstruction:
private static (int X, int Y) ChangeTuple(ValueTuple<int, int> tuple)
{
var (X, Y) = tuple;
X = 100;
Y = 200;
return (X, Y);
}
Please note that in both cases you are not modifying the original ValueTuple rather create a new one:
In the first case it is because the ValueTuple is a struct, so it is passed by value
In the second case we explicitly create a new named ValueTuple
I want to pass a function as a parameter to another function (with actual values) and later execute it. Here is JavaScript code:
function functionToPass1 (x,y) {
console.log(x, y);
}
function functionToPass2 (x,y,z) {
console.log(x, y, z);
}
function mainFunction(param1, param2, functionToPass){
console.log(param1, param2);
functionToPass();
}
mainFunction("a", "b", function(){
functionToPass2(1, 2, 3) ;
});
How to write this in C# (or VB.Net)?
Func<> Delegates are what you are looking for to pass methods in another method.
I wrote a small example below, you can apply it according to your needs. Func<> is a generic delegate. It can be Func<int,string> Func<int,int,bool> and so on. The last one represents the return type and the others represent the input parameter types of method.
static void Main(string[] args)
{
MethodB(MethodA);
Console.ReadLine();
}
static string MethodA(string message) //Func<string,string>
{
return message;
}
static void MethodB(Func<string, string> method)
{
Console.WriteLine(method("I am MethodA"));
}
For more detailed information, you can check this link => https://www.tutorialsteacher.com/csharp/csharp-func-delegate
Thank you all for contributing, your help meant all to me. The only thing I needed to figure out that is an anonymous function to be passed. C# equivalent is like this:
string functionToPass1(int i, string x, string y)
{
return string.Join(',', new[] { i.ToString(), x, y });
}
string functionToPass2(int i, string x, string y, string z)
{
return string.Join(',', new[] { i.ToString(), x, y, z });
}
string mainFunction(string param1, string param2, Func<int,string>? functionToPass)
{
Console.WriteLine(param1);
Console.WriteLine(param2);
var res = "";
int i = 5;
if (functionToPass != null)
res = functionToPass(i);
return res;
}
var res = "";
res = mainFunction("a", "b", (x) =>
{
return functionToPass2(x, "1", "2", "3");
});
Console.WriteLine(res);
Console.WriteLine("---------------");
res = mainFunction("a", "b", (x) =>
{
return functionToPass1(x, "1", "2");
});
Console.WriteLine(res);
Console.WriteLine("---------------");
res = mainFunction("a", "b", null);
Console.WriteLine(res);
Console.WriteLine("---------------");
Console.ReadLine();
I didn't delete the first comment in case it might be useful to other people in the future. I made some changes for the code you refreshed. I tried to create a generic structure as much as possible. However, Javascript is much more flexible than C#. In other words, even if the structure is generic, at some points we have to break this genericity.
static void Main(string[] args)
{
MainFunction<string, string, int>("a", "b", FunctionToPass2<int, int, int>);
Console.ReadLine();
}
static void FunctionToPass1<T1, T2>(T1 x, T2 y)
{
Console.WriteLine(x + " " + y);
}
static void FunctionToPass2<T1, T2, T3>(T1 x, T2 y, T3 z)
{
Console.WriteLine(x + " " + y + " " + z);
}
static void MainFunction<T1, T2, T3>(T1 param1, T2 param2, Action<T3, T3, T3> functionToPass)
{
Console.WriteLine(param1 + " " + param2 + "\n");
FunctionToPass2(1, 2, 3);
}
If our parameter types are certain, I believe we will come up with a much simpler solution by not writing it generic. For example, I assume that T3 for Action<T3, T3, T3> delegate is int for this solution because your arguments are integer. Here we have to break the 'generic' structure a bit.
If I have function like this
double GetExpression(int x, int y, int z)
{
return x * y + z;
}
is it possible to modify it somehow, to not return result immediately only some object which is expression and then evaluate it this way:
var expression = GetExpression(1,2,3);
double result = expression.Execute() or expression.Evaluate()
other words just return function
?
Yes!
Func<double> GetExpression(int x, int y, int z)
{
return () => x * y + z;
}
use as:
var expression = GetExpression(1,2,3);
double result = expression();
Well, you could use delegates, and return a Func<double>:
Func<double> GetExpression(int x, int y, int z)
{
return () => x * y + z;
}
var expression = GetExpression(1,2,3);
double result = expression(); // Or expression.Invoke()
Is that what you were looking for?
Now each time you call expression() it will execute the code in the lambda expression. You can observe that if you use code which doesn't just return the same value each time. For example:
Func<int> GetExpression()
{
Random rng = new Random();
return () => rng.Next(10);
}
var expression = GetExpression();
for (int i = 0; i < 10; i++)
{
Console.WriteLine(expression());
}
That will print 10 random numbers in the range [0, 10). Note that this will only create one instance of Random (avoiding a common problem) - each time you call expression() it will call the Next() method on the same instance. The constructor for Random is called in the "normal" part of the method - it's not in the lambda expression. So that gets executed when you first call the GetExpression method (even if you never call expression() afterwards).
Note however that the arguments to your GetExpression were still passed by value though, and are captured by the lambda express. So consider this:
Func<int> GetExpression(int a, List<int> b)
{
return () => a + b.Count;
}
int x = 10;
List<int> list = new List<int> { 1, 2, 3 };
var expression = GetExpression(x, list);
Console.WriteLine(expression()); // 13
x = 20;
list.Add(100);
Console.WriteLine(expression()); // 14
Here the value of the list variable has been captured, so changes the the object that the value refers to are visible in the lambda expression, but changes to the x variable itself are not visible in the lambda expression. (Likewise if you had list = new List<int>(); that change wouldn't be visible...
To expand on Jon Skeet's answer, about capturing variables, if you didn't want to, you could do something like this:
double GetExpression(int x, int y, int z)
{
return x * y + z;
}
Func<int, int, int, double> expression = GetExpression;
for (int i = 0; i < 10; i++)
Console.WriteLine("{0}", expression(i, i+1, i+2));
Which avoids modifying the existing code (in case you wanted to point to a function you didn't have control over), and avoids capturing values outside of the lamda.
I'm just wondering if there is a C# equivilent for this python code.
I want to store the names of methods in some sort of collection and call them later on. I have searched, but I really don't know what to look for.
For example in python I could do:
def add_one(x):
return x + 1
def double_it(x):
return x*2
maths_rules = [add_one, double_it]
def do_maths(maths_rules, x):
for func in maths_rules:
x = func(x)
return x
print do_maths(maths_rules, 9)
# >>> 20
This is a silly example, but you should get the idea.
You are looking for delegates.
A delegate is a type that defines a method signature. When you instantiate a delegate, you can associate its instance with any method with a compatible signature. You can invoke
(or call) the method through the delegate instance.
Your example in C#, using the Func<T, TResult> Delegate:
int add_one(int x) { return x + 1; }
int double_it(int x) { return x * 2; }
var maths_rules = new List<Func<int,int>> { add_one, double_it };
int do_maths(IEnumerable<Func<int,int>> maths_rules, int x)
{
foreach (var func in maths_rules)
{
x = func(x);
}
return x;
}
Console.WriteLine(do_maths(maths_rules, 9));
// prints "20"
Yes, you can use delegates. For this one use Func<int, int>.
like:
int addone(int x)
{
return x + 1;
}
and in main:
Func<int, int> myFunc = new Func<int, int>(addone);
myFunc(5); // to use it, you can pass it as you like
example on your code:
static int add_one(int x)
{
return x + 1;
}
static int double_it(int x)
{
return x * 2;
}
static int do_maths(List<Func<int, int>> math_rules, int x)
{
foreach(var func in math_rules)
x = func(x);
return x;
}
static void Main(string[] Args)
{
List<Func<int, int>> math_rules = new List<Func<int, int>>();
math_rules.Add(new Func<int, int>(add_one));
math_rules.Add(new Func<int, int>(double_it));
Console.WriteLine(do_maths(math_rules, 9)); // 20
}
or use lambdas as suggested in comment:
static int do_maths(List<Func<int, int>> math_rules, int x)
{
foreach(var func in math_rules)
x = func(x);
return x;
}
static void Main(string[] Args)
{
List<Func<int, int>> math_rules = new List<Func<int, int>>();
math_rules.Add(new Func<int, int>((x) => (x + 1)));
math_rules.Add(new Func<int, int>((x) => (x * 2)));
Console.WriteLine(do_maths(math_rules, 9)); // 20
}
Lets say I have the following code:
delegate int MyDel (int n); // my delegate
static int myMethod( MyDel lambda, int n) {
n *= n;
n = lambda(n);
return n; // returns modified n
}
This way, having different lambda expression I can tune the output of the Method.
myMethod ( x => x + 1, 5);
myMethod ( x => x - 1, 5);
Now, if I don't want to do any aritmethic in lambda expression, I could use:
myMethod ( x => x, 5); // and lambda will simply return x
My question is, is there a way to use the lambda expresion with 'params' optional properties? Maybe somehow embedding my delegate in array?
static int myMethod (int n, params MyDel lambda) {
Does this work?
EDIT
Sorry, was doing this with one eye, let me rephrase that.
static int myMethod (int n, params MyDel[] lambdas) {
Yes you can.
delegate int MyDelegate(int n);
static void MyMethod(int n, params MyDelegate[] handlers)
{
for (int i = 0; i < handlers.Length; i++)
{
if (handlers[i] == null)
throw new ArgumentNullException("handlers");
Console.WriteLine(handlers[i](n));
}
}
static void Main(string[] args)
{
MyMethod(1, x => x, x => x + 1);
Console.Read();
}
Output:
1
2