I am currently using SWIG to generate C# bindings for a third party C++ codebase. I've mostly gotten things working, but there is one thing I'm struggling with. In the codebase, there is a "clone()" method in many of their classes which returns a copy of the calling object. The method uses covariant return types in the C++ code to return an object of the given class's type, but, due to C# not supporting covariant return types, the return types of the clone methods in the generated C# code are all that of the base class.
Is there a method within SWIG to change the return type of all "clone()" methods to match that of the class in which they reside?
I tried using typemap with
%typemap(cstype) *::clone "$csclassname"
and similar variations, but received errors when trying to generate the bindings.
The only "working" method I've found is to use
%ignore *::clone;
then use typemap(cscode) to redefine the clone method with the correct type for all classes that use it, but that is extremely tedious.
I think I just don't completely understand how SWIG typemaps work/if they can work with specific methods like this. If anyone has suggestions for a better way to do this and/or an explanation that could help clarify things for me, it would be greatly appreciated. Thank you.
Edit: Adjusted description to identify the problem as C# not supporting covariant return types.
Edit 2: To clarify, C# 9.0 supports covariant return types, but earlier versions do not. I am using an earlier version for compatibility with a Unity project. I may be able to upgrade the project and my C# version for covariant return type compatibility, though I'm not sure if SWIG's C# tool is compatible with C# 9.0. I wanted to check if there is a workable SWIG solution for my current C# and Unity versions before pursuing that potential option.
Sample Code to Reproduce Issue:
Parent.h
#ifndef PARENT_H
#define PARENT_H
class Parent
{
public:
int val1;
Parent::Parent() = default;
Parent::Parent(const Parent &copiedObject);
virtual Parent* clone() const;
};
#endif
Parent.cpp
#include "Parent.h"
Parent::Parent(const Parent &copiedObject)
{
val1 = copiedObject.val1;
}
Parent* Parent::clone() const
{
Parent* clone = new Parent(*this);
return clone;
}
Child.h
#ifndef CHILD_H
#define CHILD_H
#include "Parent.h"
class Child : public Parent
{
public:
int val2;
Child::Child() = default;
Child::Child(const Child &copiedObject);
Child* clone() const override;
};
#endif
Child.cpp
#include "Child.h"
Child::Child(const Child &copiedObject) : Parent(copiedObject)
{
val1 = copiedObject.val1;
val2 = copiedObject.val2;
}
Child* Child::clone() const
{
Child* clone = new Child(*this);
return clone;
}
CSharpModule.i
%module CSharpModule
%{
#include "Parent.h"
#include "Child.h"
%}
%newobject *::clone;
%include "Parent.h"
%include "Child.h"
clone() method in generated Child.cs
public override Parent clone() {
global::System.IntPtr cPtr = CSharpModulePINVOKE.Child_clone(swigCPtr);
Child ret = (cPtr == global::System.IntPtr.Zero) ? null : new Child(cPtr, true);
return ret;
}
Due to lack of support for covariant return types in C#, the clone() method would need to change to "new" instead of "override (SWIG's %csmethodmodifiers directive) in the derived class:
Desired clone() method
public new Child clone() {
global::System.IntPtr cPtr = CSharpModulePINVOKE.Child_clone(swigCPtr);
Child ret = (cPtr == global::System.IntPtr.Zero) ? null : new Child(cPtr, true);
return ret;
}
I don't know where you get your information, but your entire premise is wrong. C# definitely supports covariant return types:
abstract class B
{
public abstract B Build();
}
class D : B
{
public override D Build() => new();
}
That said, this feature in either language adds essentially nothing, since any "correct" OOP program will use the base class' (or interface's) methods. If you find yourself constantly down-casting to get the more specific version of the function, you should probably rethink your design.
After all, the code generated is identical with or without this feature. This is a compile-time only feature.
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
}
Note that this might be a duplicate of this question, I'm not entirely sure.
My problem is that I have a class library project which has a reference to a third-party type library (COM). I want to put contracts into the methods in the class library, like so:
public class foo
{
public static int divide(TypeFromTypeLib tftl, int a, int b)
{
Contract.Requires<ArgumentException>(b != 0);
return a / b;
}
}
And then have a client project make use of this method, e.g.
var n = foo.divide(null, 4, 2);
But I'd also like the client project also use contracts in some of its methods. So, I set the Code Contracts properties on both projects to 'Perform Runtime Contract Checking' (without which you get the runtime assert telling you that it needs this setting).
Now, when I then try to compile the client, I get the following error:
Could not resolve member reference: my_class_lib.foo::divide.
ccrewrite : error : Rewrite aborted due to metadata errors.
Which seems unavoidable - any time a method is called which has a type from the third party type library this happens. Remove the type from the method's signature and it's fine.
Can anyone explain why this happens? Is this a clue that the structure of my code is fundamentally flawed (if so, why?), or is it a quirk of code contracts? Is there a recommended fix for this problem?
To be honest I don't know why ccrewrite has a problem with interop types but I can give you 3 workarounds:
Solution 1
This one is the simplest:
Go to list of references for a project.
Find third-party type library.
Right click.
From the context menu select Properties.
Change Embed Interop Types from True to False.
You have to do that for both projects. The drawback of this solution is that after a build you will get an additional interop assembly in the bin folder.
Solution 2
Another solution might be to remove types from a third-party type library from a public interface i.e.:
public class foo
{
public static int divide(int a, int b)
{
return divide(null, a, b);
}
private static int divide(TypeFromTypeLib tftl, int a, int b)
{
Contract.Requires<ArgumentException>(b != 0);
return a / b;
}
}
Of course you can do that only if you do not need to use TypeFromTypeLib in your client.
Solution 3
If you need to use TypeFromTypeLib in your client you may write a wrapper for this class i.e.:
public class MyTypeFromTypeLib
{
//...
}
public class foo
{
public static int divide(MyTypeFromTypeLib mytftl, int a, int b)
{
var tftl = new TypeFromTypeLib();
//Map MyTypeFromTypeLib to TypeFromTypeLib
//...
return divide(tftl , a, b);
}
private static int divide(TypeFromTypeLib tftl, int a, int b)
{
Contract.Requires<ArgumentException>(b != 0);
return a / b;
}
}
However, this solution is cumbersome because additional classes are needed.
I have an native C++ dll, some header files and the import library. Is there a way how to instantiate an object within C# that is defined in the dll?
The two ways I'm aware of are:
to wrap the C++ code into COM
to use DLLImport and external C functions
C++/CLI is your friend for this. You'll run into one problem though: it is not possible to store standard C++ objects inside C++/CLI ref or value classes (the ones for .NET). So you'll have to resort to the following class (that you can modify) that I use in production code:
#pragma once
#include <boost/shared_ptr.hpp>
template <typename T>
ref class Handle
{
boost::shared_ptr<T>* t;
!Handle()
{
if (t != nullptr)
{
delete t;
t = nullptr;
}
}
~Handle() { this->!Handle(); }
public:
Handle() : t(new boost::shared_ptr<T>((T*)0)) {}
Handle% operator=(T* p)
{
if (p != t->get()) t->reset(p);
return *this;
}
static T* operator&(Handle% h) { return h.t->get(); }
static boost::shared_ptr<T> operator->(Handle% h) { return *h.t; }
T& reference() { return *t->get(); }
T const& const_reference() { return *t->get(); }
};
Usage: Handle<MyCppClass>^ handle; inside a C++/CLI class. You then implement stub methods, forwarding them to the handle member. Garbage collected objects will call destructors of the C++ class instance iff there is no more pointer to it:
public ref class Foo
{
void bar() { handle->bar(); }
internal:
Handle<CppNamespace::Foo>^ handle;
};
I think that your option is only build a C++/CLI class wrapper (so you can reference it like a c# class), otherwise you can't instantiate a c++ class (unmanaged) from c# code.
Alternative could be the "ugly" way: instantiate the c++ class through a c function, but you'll treat the class as a void pointer inside the c# code, so you basically will not do anything with it (except if you create other functions to interact with this class; only C functions)
C# understands C, if you want make it understand C++ you have to use C++/CLI
P.S.
C# has some basic understanding of C++ classes, but it's only about converting class data (I mean bytes: fields) into some usable data in C# (there are some Attributes), but will not allow to work with methods and things like that (avoid totally is my suggestion).