I have a private method I would like to allow access to only to be passed as a delegate but not be executed otherwise.
for example:
class a
{
public delegate int myDelegate(int a);
public static int myMethod(int data, myDelegate action)
{
//my code
}
private static int methodA(int a){ //code}
private static int methodb(int a){ //code}
}
class b
{
public void anotherMethod()
{
var doAction = new myDelegate(methodA);
result = myMethod(8, doAction);
}
}
so in my example I would like methodA and MethodB to only be executed from within class a, but still enabling access to them so they can be used as delegates to be passed to methods from class a.
is it at all possible?
currently I am getting an error "methodA is inaccessible due to protection level"
What you are saying is not possible. Once a class has an instance of a delegate, no one can stop it from calling it. So if class B were able to pass a delegate to myMethod, it would also be able to call that delegate directly, unless methodA and methodB need special parameters, that only A knows about, to do anything useful.
One way of doing something similar is to create an enum called something like MethodOfA and declare MethodA and MethodB as the possible values of the enum. In class A declare a private Dictionary<MethodOfA, MyDelegate> methodDict that records what each enum value correspond to. Then declare another overload of myMethod that takes a MethodOfA like this:
public static int myMethod(int data, MethodOfA action)
=> myMethod(data, methodDict[action]);
Nevertheless, this looks like bad design to me. Maybe you don't need the overload of myMethod(int, MyDelegate) in the first place, and just check the enum to perform the corresponding action. The other thing that you might try is the strategy pattern.
If I'm understanding you correctly, you'd like to know how to use delegates (pass actions / functions) as parameters?
If so, then you should use Action<> and/or Func<> as they're delegates. Your code could look like this then:
class a
{
public int myMethod(int data, Func<int, int> func)
{
return func.Invoke(data);
}
}
class b
{
public void anotherMethod()
{
var classA = new a();
var result = classA.myMethod(8, Double);
}
private int Double(int i)
{
return i * 2;
}
}
Note: This question was asked at a time when C# did not yet support optional parameters (i.e. before C# 4).
We're building a web API that's programmatically generated from a C# class. The class has method GetFooBar(int a, int b) and the API has a method GetFooBar taking query params like &a=foo &b=bar.
The classes needs to support optional parameters, which isn't supported in C# the language. What's the best approach?
Surprised no one mentioned C# 4.0 optional parameters that work like this:
public void SomeMethod(int a, int b = 0)
{
//some code
}
Edit: I know that at the time the question was asked, C# 4.0 didn't exist. But this question still ranks #1 in Google for "C# optional arguments" so I thought - this answer worth being here. Sorry.
Another option is to use the params keyword
public void DoSomething(params object[] theObjects)
{
foreach(object o in theObjects)
{
// Something with the Objects…
}
}
Called like...
DoSomething(this, that, theOther);
In C#, I would normally use multiple forms of the method:
void GetFooBar(int a) { int defaultBValue; GetFooBar(a, defaultBValue); }
void GetFooBar(int a, int b)
{
// whatever here
}
UPDATE: This mentioned above WAS the way that I did default values with C# 2.0. The projects I'm working on now are using C# 4.0 which now directly supports optional parameters. Here is an example I just used in my own code:
public EDIDocument ApplyEDIEnvelop(EDIVanInfo sender,
EDIVanInfo receiver,
EDIDocumentInfo info,
EDIDocumentType type
= new EDIDocumentType(EDIDocTypes.X12_814),
bool Production = false)
{
// My code is here
}
From this site:
https://www.tek-tips.com/viewthread.cfm?qid=1500861
C# does allow the use of the [Optional] attribute (from VB, though not functional in C#). So you can have a method like this:
using System.Runtime.InteropServices;
public void Foo(int a, int b, [Optional] int c)
{
...
}
In our API wrapper, we detect optional parameters (ParameterInfo p.IsOptional) and set a default value. The goal is to mark parameters as optional without resorting to kludges like having "optional" in the parameter name.
You could use method overloading...
GetFooBar()
GetFooBar(int a)
GetFooBar(int a, int b)
It depends on the method signatures, the example I gave is missing the "int b" only method because it would have the same signature as the "int a" method.
You could use Nullable types...
GetFooBar(int? a, int? b)
You could then check, using a.HasValue, to see if a parameter has been set.
Another option would be to use a 'params' parameter.
GetFooBar(params object[] args)
If you wanted to go with named parameters would would need to create a type to handle them, although I think there is already something like this for web apps.
You can use optional parameters in C# 4.0 without any worries.
If we have a method like:
int MyMetod(int param1, int param2, int param3=10, int param4=20){....}
when you call the method, you can skip parameters like this:
int variab = MyMethod(param3:50; param1:10);
C# 4.0 implements a feature called "named parameters", you can actually pass parameters by their names, and of course you can pass parameters in whatever order you want :)
An easy way which allows you to omit any parameters in any position, is taking advantage of nullable types as follows:
public void PrintValues(int? a = null, int? b = null, float? c = null, string s = "")
{
if(a.HasValue)
Console.Write(a);
else
Console.Write("-");
if(b.HasValue)
Console.Write(b);
else
Console.Write("-");
if(c.HasValue)
Console.Write(c);
else
Console.Write("-");
if(string.IsNullOrEmpty(s)) // Different check for strings
Console.Write(s);
else
Console.Write("-");
}
Strings are already nullable types so they don't need the ?.
Once you have this method, the following calls are all valid:
PrintValues (1, 2, 2.2f);
PrintValues (1, c: 1.2f);
PrintValues(b:100);
PrintValues (c: 1.2f, s: "hello");
PrintValues();
When you define a method that way you have the freedom to set just the parameters you want by naming them. See the following link for more information on named and optional parameters:
Named and Optional Arguments (C# Programming Guide) # MSDN
Hello Optional World
If you want the runtime to supply a default parameter value, you have to use reflection to make the call. Not as nice as the other suggestions for this question, but compatible with VB.NET.
using System;
using System.Runtime.InteropServices;
using System.Reflection;
namespace ConsoleApplication1
{
public class Class1
{
public static void SayHelloTo([Optional, DefaultParameterValue("world")] string whom)
{
Console.WriteLine("Hello " + whom);
}
[STAThread]
public static void Main(string[] args)
{
MethodInfo mi = typeof(Class1).GetMethod("sayHelloTo");
mi.Invoke(null, new Object[] { Missing.Value });
}
}
}
I agree with stephenbayer. But since it is a webservice, it is easier for end-user to use just one form of the webmethod, than using multiple versions of the same method. I think in this situation Nullable Types are perfect for optional parameters.
public void Foo(int a, int b, int? c)
{
if(c.HasValue)
{
// do something with a,b and c
}
else
{
// do something with a and b only
}
}
optional parameters are for methods. if you need optional arguments for a class and you are:
using c# 4.0: use optional arguments in the constructor of the class, a solution i prefer, since it's closer to what is done with methods, so easier to remember. here's an example:
class myClass
{
public myClass(int myInt = 1, string myString =
"wow, this is cool: i can have a default string")
{
// do something here if needed
}
}
using c# versions previous to c#4.0: you should use constructor chaining (using the :this keyword), where simpler constructors lead to a "master constructor".
example:
class myClass
{
public myClass()
{
// this is the default constructor
}
public myClass(int myInt)
: this(myInt, "whatever")
{
// do something here if needed
}
public myClass(string myString)
: this(0, myString)
{
// do something here if needed
}
public myClass(int myInt, string myString)
{
// do something here if needed - this is the master constructor
}
}
The typical way this is handled in C# as stephen mentioned is to overload the method. By creating multiple versions of the method with different parameters you effectively create optional parameters. In the forms with fewer parameters you would typically call the form of the method with all of the parameters setting your default values in the call to that method.
Using overloads or using C# 4.0 or above
private void GetVal(string sName, int sRoll)
{
if (sRoll > 0)
{
// do some work
}
}
private void GetVal(string sName)
{
GetVal("testing", 0);
}
You can overload your method. One method contains one parameter GetFooBar(int a) and the other contain both parameters, GetFooBar(int a, int b)
You can use default.
public void OptionalParameters(int requerid, int optinal = default){}
For a larger number of optional parameters, a single parameter of Dictionary<string,Object> could be used with the ContainsKey method. I like this approach because it allows me to pass a List<T> or a T individually without having to create a whole other method (nice if parameters are to be used as filters, for example).
Example (new Dictionary<string,Object>() would be passed if no optional parameters are desired):
public bool Method(string ParamA, Dictionary<string,Object> AddlParams) {
if(ParamA == "Alpha" && (AddlParams.ContainsKey("foo") || AddlParams.ContainsKey("bar"))) {
return true;
} else {
return false;
}
}
Instead of default parameters, why not just construct a dictionary class from the querystring passed .. an implementation that is almost identical to the way asp.net forms work with querystrings.
i.e. Request.QueryString["a"]
This will decouple the leaf class from the factory / boilerplate code.
You also might want to check out Web Services with ASP.NET. Web services are a web api generated automatically via attributes on C# classes.
A little late to the party, but I was looking for the answer to this question and ultimately figured out yet another way to do this. Declare the data types for the optional args of your web method to be type XmlNode. If the optional arg is omitted this will be set to null, and if it's present you can get is string value by calling arg.Value, i.e.,
[WebMethod]
public string Foo(string arg1, XmlNode optarg2)
{
string arg2 = "";
if (optarg2 != null)
{
arg2 = optarg2.Value;
}
... etc
}
What's also decent about this approach is the .NET generated home page for the ws still shows the argument list (though you do lose the handy text entry boxes for testing).
I have a web service to write that takes 7 parameters. Each is an optional query attribute to a sql statement wrapped by this web service. So two workarounds to non-optional params come to mind... both pretty poor:
method1(param1, param2, param 3, param 4, param 5, param 6, param7)
method1(param1, param2, param3, param 4, param5, param 6)
method 1(param1, param2, param3, param4, param5, param7)... start to see the picture. This way lies madness. Way too many combinations.
Now for a simpler way that looks awkward but should work:
method1(param1, bool useParam1, param2, bool useParam2, etc...)
That's one method call, values for all parameters are required, and it will handle each case inside it. It's also clear how to use it from the interface.
It's a hack, but it will work.
I had to do this in a VB.Net 2.0 Web Service. I ended up specifying the parameters as strings, then converting them to whatever I needed. An optional parameter was specified with an empty string. Not the cleanest solution, but it worked. Just be careful that you catch all the exceptions that can occur.
For just in case if someone wants to pass a callback (or delegate) as an optional parameter, can do it this way.
Optional Callback parameter:
public static bool IsOnlyOneElement(this IList lst, Action callbackOnTrue = (Action)((null)), Action callbackOnFalse = (Action)((null)))
{
var isOnlyOne = lst.Count == 1;
if (isOnlyOne && callbackOnTrue != null) callbackOnTrue();
if (!isOnlyOne && callbackOnFalse != null) callbackOnFalse();
return isOnlyOne;
}
optional parameters are nothing but default parameters!
i suggest you give both of them default parameters.
GetFooBar(int a=0, int b=0) if you don't have any overloaded method, will result in a=0, b=0 if you don't pass any values,if you pass 1 value, will result in, passed value for a, 0 and if you pass 2 values 1st will be assigned to a and second to b.
hope that answers your question.
In the case when default values aren't available the way to add an optional parameter is to use .NET OptionalAttribute class - https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.optionalattribute?view=netframework-4.8
Example of the code is below:
namespace OptionalParameterWithOptionalAttribute
{
class Program
{
static void Main(string[] args)
{
//Calling the helper method Hello only with required parameters
Hello("Vardenis", "Pavardenis");
//Calling the helper method Hello with required and optional parameters
Hello("Vardenis", "Pavardenis", "Palanga");
}
public static void Hello(string firstName, string secondName,
[System.Runtime.InteropServices.OptionalAttribute] string fromCity)
{
string result = firstName + " " + secondName;
if (fromCity != null)
{
result += " from " + fromCity;
}
Console.WriteLine("Hello " + result);
}
}
}
You can try this too
Type 1
public void YourMethod(int a=0, int b = 0)
{
//some code
}
Type 2
public void YourMethod(int? a, int? b)
{
//some code
}
I would like to avoid declaring the function signature twice for a delegate when I already have the function sitting there in the code. I feel like I'm missing something obvious but I can't seem to find how to do this by googling.
public class SomeLibraryV1
{
public int DoSomething(int x, int y);
}
public class SomeLibraryV2
{
public int DoSomething(int x, int y);
}
public class SomeConsumer
{
// Create the delegate by manually declaring the signature again :-(
private delegate int DoSthDelegate(int x, int y);
public void Run(DoSthDelegate aDelegate)
{
aDelegate(1, 2);
}
}
So what I'd like to do is just avoid the second declaration of int fn(int, int). Something like
public class SomeConsumer
{
// Create the delegate from the function signature we already know :-)
public delegate DoSthDelegate(SomeLibrary.DoSomething);
public void Run(DoSthDelegate aDelegate)
{
aDelegate(1, 2);
}
}
This would not only save me typing every single declaration twice but would save me having to maintain things in two places should the signature of DoSomething change.
You´re missing an assential - not to say the important point - on delegates. You can register any method with the same signature. Thus it´s not really helpful to bind your delegate to just one fix method. If you´re only invoking the same method, than you shouldn´t use a delegate at all. Simply call your DoSomething-method directly in your Run-method instead of invoking the delegate:
Run()
{
var retVal = DoSomething(3, 5)
}
In fact a delegate is nothing but a signature which may be satisified by multiple methods as well. Moreover that´s the basis for events where you can register multiple event-handlers via the same delegate-definition.
So all a delegate does it to indicate that there is a method that expects two int-values and returns an int. However it doesn´t make any assumption on where this method is defined - it might also be an anonymous one, e.g.:
Run((x, y) => x + y);
Of course this is some duplicated typing. However it ensures that you can´t invoke a method that doesn´t fit the delegates signature. So the only thing you can do is to use the Func-delegate instead of creating your own one:
public TheFunc Func<int, int, int> { get; set; }
You can set this via this statement:
Run(instanceOfMyLibrary.DoSomething);
or also:
Func<int, int, int> f = instanceOfyLibrary.DoSomething;
Run(f);
NB: Your delegate must be public in order to provide it as parameter to your Run-method.
Is there a way to "tell" a method with optional parameters to use the default values, beyond omitting the parameter? I imagine the syntax would look something like this:
DoSomething(myParam: default) //calling the method
public void DoSomething(int myParam = 5) { ... } //method definition
The reason for doing this is to avoid ambiguity between an overload of this method that takes no parameters. In other words, I don't want this method to be called:
public void DoSomething() { ... }
Thanks in advance.
If you don't want the user to call the DoSomething method that takes no parameters, just make it private like so:
private void DoSomething();
It seems like in this particular case you just shouldn't be using optional parameters at all. They are designed to make your life easy. When they stop doing that it's probably time to just stop using them.
You can accomplish the same general goal of optional parameters with various overloads using this general approach:
public void Foo(int param1, int param2, int param3){ /* actual Foo code goes here */ }
public void Foo(int param1, int param2) { Foo(param1, param2, 5); }
public void Foo(int param1) { Foo(param1, 0, 5); }
public void Foo() { Foo(42, 0, 5); }
This is what optional parameters were designed to avoid, but sometimes going back to this boilerplate code can be simpler, even if it bloats the code a tad.
Say I have 2 classes, class A and class B. Class A creates an instance of Class B. Class A has a function that I would like to pass into a method from Class B.
class A {
void Main(string[] args) {
B classB=new B();
DelegateCaller(new delFunction(classB.TheFunction()); // <-- Won't compile (method name expected)
DelegateCaller(new delFunction(B.TheFunction()); // <-- Won't compile (object reference is req'd)
}
public delegate string delFunction();
public DelegateCaller(delFunction func) {
System.Console.WriteLine(func());
}
}
class B {
public string TheFunction() {
return "I'm Printing!!!";
}
}
I'm not sure if it a syntax issue or it's just something I can't do. Maybe I need to define the delegate in B, but reference it in A? What about B's this pointer?
It's just a syntax issue; get rid of the parentheses after classB.TheFunction - they indicate that you wish to invoke the method.
DelegateCaller(new delFunction(classB.TheFunction));
Do note that there is an implicit conversion available from a method-group, so you can just do:
DelegateCaller(classB.TheFunction);
Also note that creating your own delegate-type in this case is unnecessary; you could just use the in-built Func<string> type.
EDIT: As Darin Dimitrov points out, there is also the unrelated issue of calling an instance method as though it were a static method.
Try like this:
class A
{
static void Main()
{
B classB = new B();
DelegateCaller(classB.TheFunction);
}
public delegate string delFunction();
public static void DelegateCaller(delFunction func)
{
Console.WriteLine(func());
}
}
class B
{
public string TheFunction()
{
return "I'm Printing!!!";
}
}
Let me elaborate about the different changes I've made to your initial code:
TheFunction in class B needs to be public so that you can access it from class A
The DelegateCaller method in class A should be static and not necessarily return a value (declare it as void) if you want to call it from the static Main method.
The definition of the delFunction delegate should return a string.
Take the parenthesis off the end of TheFunction. You want the method, not the result of a call to the method.
If you want to capture an instance method for usage in a general purpose fashion you should use Delegate.CreateDelegate(Type,MethodInfo). This is nice as it allows you to create an "open delegate" meaning it isn't bound to an instance and can take any instance that is a ClassB. It makes reflection quite fast if you know the type information, as this method will perform much faster than the equivalent statement using MethodInfo.Invoke.
DelegateCaller(new delFunction(B.TheFunction());
Should be
DelegateCaller(new delFunction(B.TheFunction);
To use classB.TheFunction you would need to make TheFunction static. You pass in the function with no parens.