Suppose I'm trying to write a wrapper method around an existing static method. Let's take the console for example.
public static void WriteWithPrefix(string prefix, ...)
{
Console.Write(prefix);
Console.Write(" - ");
// Console.Write(...)
}
The Console.Write method has something like 18 signatures, covering the gamut of primitive types as well as replicating the String.Format methods.
Is there a way to write this helper class such so that it can also accept any of the data types that Console.Write normally accepts?
Another example:
public static void WriteWithTimestamp(...)
{
Console.Write(DateTime.Now.ToString());
Console.Write(": ");
// Console.Write(...)
}
When I call the WriteWithTimestamp method I want to be able to pass it anything that I could also give to Console.Write...
To do this in a normal fashion, you would need to make an overload for each of the Console.Write overloaded functions, which means multiple overloads of your method.
However, you could use dynamic as an alternative approach to accomplish your goals:
public static void WriteWithTimestamp(dynamic value)
{
Console.Write(DateTime.Now.ToString());
Console.Write(": ");
Console.Write(value);
}
Using dynamic will cause it to evaluate and use the most appropriate overload at runtime. Since Console.Write has an overload which accepts object as a parameter, this should never cause an exception, as well.
You have to overload your method and write as many signatures as Console.Write() has with same types of parameters that are accepted by Console.Write()
Or..you can write one method that has all of Console.Write's parameters set to null or to some value like this:
public static void WriteWithTimestamp(string text = null, DateTime date = null, ...)
{
Console.Write(DateTime.Now.ToString());
Console.Write(": ");
// Console.Write(...)
}
Now you can choose which parameters to pass to your method without having to overload it.
Edit: Using dynamic is the better approach, had no idea it existed :-)
In the case of your specific example, I would just expose a WriteWithTimestamp(string) method, and leave it up to the caller to convert their parameters to string using ToString or String.Format calls.
However, assuming you meant to ask the question generally, I would suggest accepting a params object[] array, and passing it to an invocation of your static method resolved through reflection:
public static void Main(string[] args)
{
Console.WriteLine("Location is ({0},{1})", 4, 7);
WriteWithTimestamp("Location is ({0},{1})", 4, 7);
}
public static void WriteWithTimestamp(params object[] parameters)
{
Console.Write(DateTime.Now.ToString());
Console.Write(": ");
var parameterTypes = parameters.Select(p => p.GetType()).ToArray();
var writeLine = typeof(Console).GetMethod("WriteLine", parameterTypes);
writeLine.Invoke(null, parameters);
}
Related
REMARK after rethinking my issue.
Though the best solution for my exact example with WriteData method was proposed by #Dogu Arslan, the issue in question's title was actually solved by #InBetween and #Fabio. I. e. for my exact example with WriteData method it is better to move the conversion logic out of WriteData method, but to abstract the logic from unrelated types it is better to use proposed overloading. So all the three answers are helpful for me.
I already have read the following similar questions - 1, 2, and others - but not have come with something suitable solution in my case.
My case is: I have the following simple method. The important part is the line with commentary and the "source" parameter. The rest part is not so important and serves only to show that there is an "operation" from the question's title.
void WriteData(
int count,
int currentIndex,
List<byte> source,
StreamWriter sw,
string fileName)
{
var countToWrite = count - currentIndex;
if (countToWrite == 0)
return;
if (sw == null)
sw = new StreamWriter(GetFullPath(fileName));
//---------- Source's elements must be converted to string.
var dataToWrite =
source.GetRange(currentIndex, countToWrite)
.Select(x => Convert.ToString(x));
StringBuilder sb = new StringBuilder();
foreach (var item in dataToWrite)
sb.AppendLine(item);
sw.Write(sb.ToString());
}
For now I want the "source" parameter to be list with bytes, or doubles, or strings. Have I write three copies of WriteData method with only single change - the type of list in "source"? Or there is a better approach?
I tried type constraint, but to what type to constrain?
I tried to check the type and to throw exceptions if it is not in my type list (byte, double, string). But exceptions work only at run-time, and I want it to work in compile-time.
For now I have to restrict myself with run-time type checking as temporary solution, but, as I mentioned before, it is not suitable in perspective.
Why not using overload methods with one "hided" actual solution
public void WriteData(List<byte> source) { WriteData<byte>(source); }
public void WriteData(List<double> source) { WriteData<double>(source); }
public void WriteData(List<string> source) { WriteData<string>(source); }
private void WriteData<T>(List<T> source)
{
// Your actual implementation
}
In private method you can check types and throw exception when wrong type is given, in case you want "protect" from making private method public by "mistake".
The best solution is to publicly expose the needed overloads and then delegate implementation to a private generic method:
public void WriteData( , , List<string> source, , ) { WriteData<string>(...); }
public void WriteData( , , List<byte> source, , ) { WriteData<byte>(...); }
public void WriteData( , , List<double> source, , ) { WriteData<double>(...); }
private void WriteData<T>( , , List<T> source, , )
{
Debug.Assert(typeof(T).Equals(typeof(string)) ||
typeof(T).Equals(typeof(byte)) ||
typeof(T).Equals(typeof(double)));
...
}
One thing you could do is to remove the string conversion responsibility to another class altogether.ie. StringConverter class which would have overloaded constructors that would take these different input types to convert them to string and provide a method that returns a string. That way even if you extend in future the types you support you would not need to change this method here in your example, all of that would be transparent behind you string converter class. The checks will also be compile time because string converter class would take the input through its constructor. Yes this would mean everytime you want to call WriteData method you would need to instantiate a new StringConverter object but C# does not provide type constraints on the level you ask out of the shelf.
I came across an example similar to this:
public Dictionary<string, object> generate(
string elementId,
Dictionary<string, object> additionalAttributes = null)
{
.... method body
}
Why would the dictionary passed as parameter be initiated to null? I haven't seen such construct. Does it have to do something with an optional parameter?
I can't speak to your first question, but the answer to your second question is yes. This is an optional parameter. C# only allows optional reference-type parameters to take a default value of null, except for string, which can take any constant string value, I believe.
Ref: MSDN
I use that to save time writing functions overloading. For example, instead of overloading two functions:
void SameFunctionName(Parameter1){ .. }
void SameFunctionName(Parameter1, Parameter2){ .. }
// maybe additional function body with three parameters .. etc
I just write one using this case:
void MyFunction(Parameter1, Parameter2 = null){ .. }
So, a small if statement inside my function would check if Parameter2 is null or not, to then make decisions. All in one function body.
and the function call for this case would work in both cases:
MyFunction(Parameter1); // This is a valid syntax
MyFunction(Parameter1, Parameter2); // This is a valid syntax
Optional parameter such as the one you use in your example can only be set to constant values, this means that you can't use any reference values, which is what Dictionary is, as such null is the only allowed value you can initialise an optional variable of type Dictionary to, if the method was using a value type like int or string then a value could be initialised for the optional parameter otherwise it has to be null
Yes it saves time if you are using Function Overloading For example this
can be avoided
Void Main()
{
fun1(11);
fun1(12,13);
}
public fun1(int i )
{
Print(i);
}
public fun1(int i ,int j)
{
Print(i+j);
}
This can be avoided by Code below and it also saves time and space
Void Main()
{
fun1(12);
}
public fun1(int i ,int j = NULL)
{
if(j==NULL)
Print(i);
else
Print(i+j);
}
This question already has answers here:
Should you declare methods using overloads or optional parameters in C# 4.0?
(13 answers)
Closed 9 years ago.
which one is better? at a glance optional parameter seems better (less code, less XML documentation, etc), but why do most MSDN library classes use overloading instead of optional parameters?
Is there any special thing you have to take note when you choose to use optional parameter (or overloading)?
One good use case for 'Optional parameters' in conjunction with 'Named Parameters' in C# 4.0 is that it presents us with an elegant alternative to method overloading where you overload method based on the number of parameters.
For example say you want a method foo to be be called/used like so, foo(), foo(1), foo(1,2), foo(1,2, "hello"). With method overloading you would implement the solution like this,
///Base foo method
public void DoFoo(int a, long b, string c)
{
//Do something
}
/// Foo with 2 params only
public void DoFoo(int a, long b)
{
/// ....
DoFoo(a, b, "Hello");
}
public void DoFoo(int a)
{
///....
DoFoo(a, 23, "Hello");
}
.....
With optional parameters in C# 4.0 you would implement the use case like the following,
public void DoFoo(int a = 10, long b = 23, string c = "Hello")
Then you could use the method like so - Note the use of named parameter -
DoFoo(c:"Hello There, John Doe")
This call takes parameter a value as 10 and parameter b as 23.
Another variant of this call - notice you don't need to set the parameter values in the order as they appear in the method signature, the named parameter makes the value explicit.
DoFoo(c:"hello again", a:100)
Another benefit of using named parameter is that it greatly enhances readability and thus code maintenance of optional parameter methods.
Note how one method pretty much makes redundant having to define 3 or more methods in method overloading. This I have found is a good use case for using optional parameter in conjunction with named parameters.
Optional Parameters provide issues when you expose them publicly as API. A rename of a parameter can lead to issues. Changing the default value leads to issues (See e.g. here for some info: Caveats of C# 4.0 optional parameters)
Also, optional params can only be used for compile-time constants. Compare this:
public static void Foo(IEnumerable<string> items = new List<string>()) {}
// Default parameter value for 'items' must be a compile-time constant
to this
public static void Foo() { Foo(new List<string>());}
public static void Foo(IEnumerable<string> items) {}
//all good
Update
Here's some additional reading material when a constructor with default parameters does not play nicely with Reflection.
I believe they serve different purposes. Optional parameters are for when you can use a default value for a parameter, and the underlying code will be the same:
public CreditScore CheckCredit(
bool useHistoricalData = false,
bool useStrongHeuristics = true) {
// ...
}
Method overloads are for when you have mutually-exclusive (subsets of) parameters. That normally means that you need to preprocess some parameters, or that you have different code altogether for the different "versions" of your method (note that even in this case, some parameters can be shared, that's why I mentioned "subsets" above):
public void SendSurvey(IList<Customer> customers, int surveyKey) {
// will loop and call the other one
}
public void SendSurvey(Customer customer, int surveyKey) {
...
}
(I wrote about this some time ago here)
This one almost goes without saying, but:
Not all languages support optional parameters. If you want your libraries to be friendly to those languages, you have to use overloads.
Granted, this isn't even an issue for most shops. But you can bet it's why Microsoft doesn't use optional parameters in the Base Class Library.
Neither is definitively "better" than the other. They both have their place in writing good code. Optional parameters should be used if the parameters can have a default value. Method overloading should be used when the difference in signature goes beyond not defining parameters that could have default values (such as that the behavior differs depending on which parameters are passed, and which are left to the default).
// this is a good candidate for optional parameters
public void DoSomething(int requiredThing, int nextThing = 12, int lastThing = 0)
// this is not, because it should be one or the other, but not both
public void DoSomething(Stream streamData = null, string stringData = null)
// these are good candidates for overloading
public void DoSomething(Stream data)
public void DoSomething(string data)
// these are no longer good candidates for overloading
public void DoSomething(int firstThing)
{
DoSomething(firstThing, 12);
}
public void DoSomething(int firstThing, int nextThing)
{
DoSomething(firstThing, nextThing, 0);
}
public void DoSomething(int firstThing, int nextThing, int lastThing)
{
...
}
Optional parameters has to be last. So you can not add an extra parameter to that method unless its also optional. Ex:
void MyMethod(int value, int otherValue = 0);
If you want to add a new parameter to this method without overloading it has to be optional. Like this
void MyMethod(int value, int otherValue = 0, int newParam = 0);
If it can't be optional, then you have to use overloading and remove the optional value for 'otherValue'. Like this:
void MyMethod(int value, int otherValue = 0);
void MyMethod(int value, int otherValue, int newParam);
I assume that you want to keep the ordering of the parameters the same.
So using optional parameters reduces the number of methods you need to have in your class, but is limited in that they need to be last.
Update
When calling methods with optional parameters, you can used named parameters like this:
void MyMethod(int value, int otherValue = 0, int newValue = 0);
MyMethod(10, newValue: 10); // Here I omitted the otherValue parameter that defaults to 0
So optional parameters gives the caller more possibilities.
One last thing. If you use method overloading with one implementation, like this:
void MyMethod(int value, int otherValue)
{
// Do the work
}
void MyMethod(int value)
{
MyMethod(value, 0); // Do the defaulting by method overloading
}
Then when calling 'MyMethod' like this:
MyMethod(100);
Will result in 2 method calls. But if you use optional parameters there is only one implementation of 'MyMethod' and hence, only one method call.
What about a 3rd option: pass an instance of a class with properties corresponding to various "optional parameters".
This provides the same benefit as named and optional parameters, but I feel that this is often much clearer. It gives you an opportunity to logically group parameters if necessary (i.e. with composition) and encapsulate some basic validation as well.
Also, if you expect clients that consume your methods to do any kind of metaprogramming (such as building linq expressions involving your methods), I think that keeping the method signature simple has its advantages.
A good place to use optional parameter is WCF since it does not support method overloading.
This is not really an answer to the original question, but rather a comment on #NileshGule's answer, but:
a) I don't have enough reputation points to comment
b) Multiple lines of code is quite hard to read in comments
Nilesh Gule wrote:
One benefit of using optional parameters is that you need not have to do a conditional check in your methods like if a string was null or empty if one of the input parameter was a string. As there would be a default value assigned to the optional parameter, the defensive coding will be reduced to a great extent.
This is actually incorrect, you still have to check for nulls:
void DoSomething(string value = "") // Unfortunately string.Empty is not a compile-time constant and cannot be used as default value
{
if(value == null)
throw new ArgumentNullException();
}
DoSomething(); // OK, will use default value of ""
DoSomething(null); // Will throw
If you supply a null string reference, it will not be replaced by the default value. So you still need to check the input parameters for nulls.
To address your first question,
why do most MSDN library classes use
overloading instead of optional
parameters?
It is for backward compatibility.
When you open a C# 2, 3.0 or 3.5 project in VS2010, it is automatically upgraded.
Just imagine the inconvenience it would create if each of the overloads used in the project had to be converted to match the corresponding optional parameter declaration.
Besides, as the saying goes, "why fix what is not broken?". It is not necessary to replace overloads that already work with new implementations.
One benefit of using optional parameters is that you need not have to do a conditional check in your methods like if a string was null or empty if one of the input parameter was a string. As there would be a default value assigned to the optional parameter, the defensive coding will be reduced to a great extent.
Named parameters give the flexibility of passing parameter values in any order.
I am wondering how I can fetch the incoming arguments of my method in an array. Or just retrieve the values of my arguments dynamically.
Meaning, a call like:
MyMethod(10, "eleven");
For method:
void MyMethod(int Test, str Test2) {}
Would resolve in an array like:
{{"Test" => 10}, {"Test2", "eleven"}}
Would be even better if I could achieve this with reflection.. Ex. somehow with StackTrace.
I think, what you are looking for does not exist. The closest you can have is params:
MyMethod(params object[] args)
{
// if you have to do this, it's quite bad:
int intArg = (int)args[0];
string stringArg = (string)arg[1]:
}
// call with any number (and type) of argument
MyMethod(7, "tr");
There is no compile time type checking, and therefore it is not an all-purpose way to handle arguments. But if your arguments are dynamic, it's probably a solution.
Edit: had another idea:
You need to put all argument manually into a list / dictionary. You can write a helper class to allow the following:
MyMethod(int arg1, string arg2)
{
Arguments.Add(() => arg1);
Arguments.Add(() => arg2);
//
}
The helper looks like this
public static void Add<T>(Expression<Func<T>> expr)
{
// run the expression to get the argument value
object value = expr.Compile()();
// get the argument name from the expression
string argumentName = ((MemberExpression)expr.Body).Member.Name;
// add it to some list:
argumentsDic.Add(argumentName, value);
}
Your best option is to do it with an anonymous type, as shown in this example.
Nice Question (+1). I think this is what you need -
MethodBase mb = MethodBase.GetCurrentMethod();
ParameterInfo[] pi = mb.GetParameters();
One way I know (not sure if it's the only way nowadays, but it used to be) is to use aspect-oriented programming (AOP), and in particular interception. It is a bit painful to roll it by hand, but there are excellent tools that come to the rescue. One such tool is PostSharp: http://www.postsharp.org/.
Since the method is using named parameters, why can't you just explicitly populate a dictionary with their names and values? There is little point in using reflection to get their names since you already know them.
As already stated, the params keyword can be used to define a method with a variable number of parameters, but by definition those are nameless.
I'm not sure what you are asking, in the way you've explained it, makes any sense. Perhaps you could elaborate further?
Maybe this won't be exactly what you were looking for, but I found I could get a reasonable compromise for my situation using a variation on the method Matt Hamilton suggested and making use of the implicit naming of anonymous type parameters:
public void MyMethod(string arg1, bool arg2, int arg3, int arg4)
{
var dictionary = new PropertyDictionary(new
{
arg1, arg2, arg3, arg4
});
}
public class PropertyDictionary : Dictionary<string, object>
{
public PropertyDictionary(object values)
{
if(values == null)
return;
foreach(PropertyDescriptor property in TypeDescriptor.GetProperties(values))
Add(property.Name, property.GetValue(values);
}
}
As I said, it may not be helpful in your situation, but in mine (unit testing a method which processes XML) it was very useful.
If you go right now and type string.Format into your IDE, you'll see that there are 4 different overloads: one taking a string and object, another taking a string and two objects, then one taking three objects, and finally one that uses params. According to this answer, this is because params generates 'overhead', and some other languages may not support it.
My question is, why can't a method call like this:
void Foo()
{
Bar(1, 2, 3);
}
void Bar(params int[] args)
{
// use args...
}
Be essentially transformed at compile time to
void Foo()
{
Bar(new[] { 1, 2, 3 });
}
void Bar(int[] args)
{
// use args...
}
? Then it wouldn't create any overhead except for the array creation (which was necessary anyway), and would be fully compatible with other languages.
The number of arguments is already known at compile-time, so what's preventing the C# compiler from doing some kind of string substitution and making the first scenario essentially syntactic sugar for the second? Why did we have to implement hidden language features specifically to support variadic arguments?
The title makes an incorrect assumption.
Both a params and a non-params methods take an array; the difference is the compiler will emit the IL to create an array implicitly when making a params method call. An array is passed to both methods, as a single argument.
This can be seen in this .NET Fiddle (view "Tidy Up -> View IL").
using System;
public class Program
{
public static void Main()
{
var a1 = 1;
var a2 = 2;
var a3 = 3;
with_params(a1,a2,a3);
no_params(new [] {a1,a2,a3});
}
public static void with_params(params int[] x) {}
public static void no_params(int[] x) {}
}
In both cases the IL is identical; a new array is created, it is populated, and the array is supplied to the invoked method.
There is an "exception" to this identical IL generation in that the compiler can move out constant-valued arrays when used in the non-parameter form and use 'dup' initialization, as seen here. However, a new array is supplied as the argument in both cases.