I have a method that logs out data, and takes bunch of inputs using the params keyword
public static void LogData<T>(params object[] parameter)
{
// log out the data
}
I'd like to restrict the inputs to strings and numeric types, and my first thought was to use a where clause. The obvious issue with this are that you can't use concrete types in a where clause, but you can get around this by noticing that both string and numeric types are IComparable and IConvertible. I thought this might help:
public static void LogData<T>(params T[] parameter)
where T : IComparable, IConvertible
{
// log out the data
}
This doesn't work because then all the inputs have to be one type.
Is there a way to restrict the inputs to string and numeric types using the params keyword or some other syntax?
I'm not sure why you want to do that, when every object has a ToString method.
Anyway, you can declare a new class - NumberOrString that will support implicit conversions from a string and the numeric types you want to support. Then have your LogData method accept a params NumberOrString[] p
Consiidering you are talking about strings and numeric types, it's not possible. If condition would be or, you can define an optional parameter of type List<T> where T is any chosen numeric type.
I'd create multiple methods which are essentially overloads using the types I want to support.
might be too much code and repetitive but the basic "log out the data" routine would essentially be reused across all overloads.
Here's one potential solution:
public static void LogData(params IConvertible[] parameter)
{
// log out the data
}
Although I wonder if there's actually any reason you need the objects to be IComparable or IConvertible, or if you'd just like to restrict them to known types (i.e. strings or numbers). I think zmbq's answer might be the way to go. Or you might even really just want to see if the object implements a useful ToString method. Here's a way to find that out (not a compile-time restriction, as you might expect, but at runtime using reflection).
Related
What are some practical cases of why I would need something like this:
public void Show<FirstType, SecondType>(FirstType first, SecondType second)
{
//Do Something
}
And not just this:
public void Show(FirstType first, SecondType second)
{
//Do Something
}
Thanks so much
This example comes from the framework, but the LINQ extension methods are implemented this way. For example, the signature for .Where is:
IEnumerable.Where<TSource> (IEnumerable<TSource>, Func<TSource, Boolean>);
This doesn't have two type arguments like your example, but shows a good use case for having one. The "Where" is not type specific, and because the predicate given later uses the same type argument you get type safety when performing comparisons.
This far preferable to a .Where for every possible type, hence the use of generics.
Because you'll need to create another method for something like this, which might be duplication in some cases. To avoid duplicating logic, you use generics when the types have some behavior in common. Also, it's typesafe so no need to box/unbox when you use generics.
public void Show(ThirdType third, FourthType fourth)
{
//Do Something
}
I can give you a simple use of generics, for example, the method that compares two variables of the same type and returns true or false depending on the equality:
public static bool Equals<T>(T Variable1, T Variable2)
{
return Variable1.Equals(Variable2);
}
This method is now reusable for any parameter type. T stands for type and is usually written this way, but not necessarily. Using this class you can compare equality of any type you define in your main method without boxing/unboxing types inside the method. Also your equality compare method is safe from wrong type input once you use it in your main method. Example of using this method would be something like this:
public static void Main(string[] args)
{
if (VarCompare<int>(10, 10))
{
Console.WriteLine("Inputs are equal.");
}
else
{
Console.WriteLine("Inputs aren't equal.");
}
}
By simply changing your if condition to VarCompare<string>("A", "B") you could compare two string types instead of integers.
A good example of when you would want two generic parameters with different types is Dictionary<TKey, TValue>
http://msdn.microsoft.com/en-us/library/xfhwa508(v=vs.110).aspx
I created an Class which is only able to handle primitive (or ICloneable) Types
I want to know if it's possible to say something like:
public myobject(primitiv original){...}
or do I really need to create a constructor for each primitive type like:
public myobject(int original){...}
public myobject(bool original){...}
...
What I am trying to achieve is to create an object with 3 public properties Value, Original and IsDirty.
The Value will be an deep Clone of Original so the Original needs to be primitve or ICloneable
Primitive types in C# are defined as structs (implemented generally as ValueType in the .NET CLR). I believe you have two options:
As has been said already: Receive any type, check it against every acceptable type, throw an exception if it doesn't match.
Make your class generic, make the constructor generic with a constraint of where T : struct (with T being the type parameter). This will catch all structs, not just the primitive types, but I think that's the best you can hope for without manual checking and with compile-time checking. You can mix this constraint with other ones, of course.
And you can combine the two options above to have some of the checking be done at compile-time and some of it be done at run-time.
If you want to do that to force whomever is using your API to use such types (through compile time errors should they use the wrong types), I'm afraid it can't be done.
You could, however, receive an object in the constructor, evaluate its type, and throw an ArgumentException in case the parameter is neither one of the "primitive" types nor implements ICloneable.
Edit: This might be useful. You can determine whether a variable belongs to a primitive type with the following code:
Type t = foo.GetType();
t.IsPrimitive; // so you don't have to do an evaluation for each primitive type.
It is not exactly what you asked, but you can have 2 constructors, one for structs and one for ICloneable:
public myobject(System.ValueType original){...}
public myobject(ICloneable original){...}
How about generics instead of reflection?
public class MyObject<T>
where T: IComparable
{
public MyObject(T original)
{
// do runtime check
}
}
var c1 = new MyObject<int>(1);
// or
var c2 = new MyObject<Int32>(2);
I'm building a HTTP-API wrapper for .NET, which has a bunch of methods to set data in an object, and then it serializes the data and sends it to my server. There are 6 datatypes allowed:
string
int
long
float
double
DateTime
My data attributes use generics:
SetAttribute<T>(string key, T value)
So there is only one generic method to set data. Since I cannot constrain the data types to the 6 mentioned, I use run-time checks and throw an exception when the wrong data type is used.
Now for my problem: I have two versions of SetAttribute, one that takes a single value (of type T) and one that takes multiple values (of type IEnumerable<T>). The problem is that when a programmer uses this wrapper and does not specify the type parameter, the runtime guesses which method to use, for instance:
SetAttribute("testkey","thing,anotherthing,athirdthing".Split(','))
This defaults to the single value method and T is String[] which of course makes my method cast an exception because String[] is not a valid type. If you specify:
SetAttribute<string>("testkey","thing,anotherThing,aThirdThing".Split(','))
The runtime chooses the correct method (multi-value) and no exception is cast because T is then string.
My question: how can I label my methods so that the type parameter is mandatory and must be explicitly defined? Or do I have to detect this at runtime and redirect to the multi-method myself?
Ok, this was originally a comment above since it doesn't necessarily answer your original question but suggests an alternate approach;
I would say using a public generic SetAttribute in this case isn't necessarily a good idea.
Since the types are so constrained, you should probably just write the overloads and move the errors from runtime to compile time. It would also allow you to take IEnumerable<string> etc. with another 6 overloads and eliminate the problem you're having entirely.
You can always implement SetAttribute with a private generic and just call that from each overload, that will remove some duplication.
It will also more or less eliminate the need for runtime checks, since the types are already constrained by the compiler.
Given a parameter type, the compiler finds a best match from your overloads. If you cast your string[] to an IEnumerable<string> you will probably find it works as expected because the best match is a method that has exactly those parameters. But you have no method that takes a string[], so given one as a parameter, the compiler makes the best guess it can.
I would have two separately named methods rather than overloads otherwise it is too easy to run into this problem. Or have 6 separate overloads, as #Joachim suggests.
I would suggest a better solution would be to test whether the value passed in is IEnumerable after it fails everything else and treat it as such if it is. (I imagine that you're handling IEnumerable as a seventh case already).
One solution would be to break your original method into 6 non-generic overloads, and add another generic overload for collections:
void SetAttribute(string key, int value);
void SetAttribute(string key, string value);
// etc
// abd this takes care of collections:
void SetAttribute<T>(string key, IEnumerable<T> value);
Is it in anyway possible ( preferably without using any third party libs), to create a function whose type is determined at runtime in C#?
e.g
public static void myfunc(var x)
{
System.Windows.Forms.MessageBox.Show(x); //just an example
}
NOTE: I want the runtime to determine the type of the parameter and do not want to later cast the parameter to another type, as would be necessary if I use generics. e.g I don't want:
myfunc<T>(T x)
// and then :
MessageBox.Show((string)m);
UPDATE:
I am actually making a function parser for my programming language, which translates to C# code. In my language, I wanted the parameter types to be determined at runtime always. I was looking for some good C# feature for easy translation.
e.g
in my language syntax:
function msg << x
MessageBox.Show x
end
needed to be translated to something that didn't ask for a type at compile time, but would need one at runtime.
e.g
public static void msg(var x)
{
System.Windows.Forms.MessageBox.Show(x);
}
The keyword introduced for runtime binding in C# 4 is dynamic.
public static void myfunc(dynamic x)
This allows you to make assumptions about x that are unchecked at compile time but will fail at runtime if those assumptions prove invalid.
public static void MakeTheDuckQuack(dynamic duck)
{
Console.WriteLine(duck.Quack());
}
The assumption made here is that the parameter will have a method named Quack that accepts no arguments and returns a value that can then be used as the argument to Console.WriteLine. If any of those assumptions are invalid, you will get a runtime failure.
Given classes defined as
class Duck
{
public string Quack()
{
return "Quack!";
}
}
class FakeDuck
{
public string Quack()
{
return "Moo!";
}
}
And method calls
MakeTheDuckQuack(new Duck());
MakeTheDuckQuack(new FakeDuck());
MakeTheDuckQuack(42);
The first two succeed, as runtime binding succeeds, and the third results in an exception, as System.Int32 does not have a method named Quack.
Generally speaking, you would want to avoid this if possible, as you're essentially stipulating that an argument fulfill an interface of some sort without strictly defining it. If you are working in an interop scenario, then perhaps this is what you have to do. If you are working with types that you control, then you would be better served trying to achieve compile time safety via interfaces and/or base classes. You can even use different strategies (such as the Adapter Pattern) to make types you do not control (or cannot change) conform to a given interface.
If you need to know the type... then you need to know the type. You can't have your cake and eat it too.
First off, the cast in your example is unnecessary as all objects implement ToString(). Instead of telling us what you think you need, tell us what problem you are trying to solve. There is almost certainly a solution either via generics or the use of the dynamic keyword (though dynamic is rarely needed), but we need more info. If you add more I'll update this answer.
You could use a type of object or, if you don't know how many items are available, you could use a params object array, i.e. params object[] cParams.
hey guys, I've removed some of the complexities of my needs to the core of what I need to know.
I want to send a collection of Values to a method, and inside that method I want to test the Value against, say, a property of an Entity. The property will always be of the same Type as the Value.
I also want to test if the value is null, or the default value, obviously depending on whether the value type is a reference type, or a value type.
Now, if all the values sent to the method are of the same type, then I could do this using generics, quite easily, like this:
public static void testGenerics<TValueType>(List<TValueType> Values) {
//test null/default
foreach (TValueType v in Values) {
if (EqualityComparer<TValueType>.Default.Equals(v, default(TValueType))) {
//value is null or default for its type
} else {
//comapre against another value of the same Type
if (EqualityComparer<TValueType>.Default.Equals(v, SomeOtherValueOfTValueType)) {
//value equals
} else {
//value doesn't equal
}
}
}
}
My questions is, how would I carry out the same function, if my Collection contained values of different Types.
My main concerns are successfully identifying null or default values, and successfully identifying if each value passed in, equals some other value of the same type.
Can I achieve this by simply passing the type object? I also can't really use the EqualityComparers as I can't use generics, because I'm passing in an unknown number of different Types.
is there a solution?
thanks
UPDATE
ok, searching around, could I use the following code to test for null/default successfully in my scenario (taken from this SO answer):
object defaultValue = type.IsValueType ? Activator.CreateInstance(type) : null;
I reckon this might work.
Now, how can I successfully compare two values of the same Type, without knowing their types successfully and reliably?
There is Object.Equals(object left, object right) static method, it internally relies on Equals(object) implementation available at one of provided arguments. Why do you avoid using it?
The rules of implementing equality members are nearly the following:
Required: Override Equals(object) and GetHashCode() methods
Optional: Implement IEquatable<T> for your type (this is what EqualityComparer.Default relies on)
Optional: Implement == and != operators
So as you see, if you'll rely on object.Equals(object left, object right), this will be the best solution relying on strongly required part of equality implementation pattern.
Moreover, it will be the fastest option, since it relies just on virtual methods. Otherwise you'll anyway involve some reflection.
public static void TestGenerics(IList values) {
foreach (object v in values) {
if (ReferenceEquals(null,v)) {
// v is null reference
}
else {
var type = v.GetType();
if (type.IsValueType && Equals(v, Activator.CreateInstance(type))) {
// v is default value of its value type
}
else {
// v is non-null value of some reference type
}
}
}
}
The short answer is "yes", but the longer answer is that it's possible but will take a non-trivial amount of effort on your part and some assumptions in order to make it work. Your issue really comes when you have values that would be considered "equal" when compared in strongly-typed code, but do not have reference equality. Your biggest offenders will be value types, as a boxed int with a value of 1 won't have referential equality to another boxed int of the same value.
Given that, you have to go down the road of using things like the IComparable interface. If your types will always specifically match, then this is likely sufficient. If either of your values implements IComparable then you can cast to that interface and compare to the other instance to determine equality (==0). If neither implements it then you'll likely have to rely on referential equality. For reference types this will work unless there is custom comparison logic (an overloaded == operator on the type, for example).
Just bear in mind that the types would have to match EXACTLY. In other words, an int and an short won't necessarily compare like this, nor would an int and a double.
You could also go down the path of using reflection to dynamically invoke the Default property on the generic type determined at runtime by the supplied Type variable, but I wouldn't want to do that if I didn't have to for performance and compile-time safety (or lack thereof) reasons.
Is the list of types you need to test a pre-determined list? If so, you can use the Visitor Pattern (and maybe even if not since we have Generics). Create a method on your Entities (can be done using partial classes) that takes in an interface. Your class then calls a method on that interface passing itself. The interface method can be generic, or you can create an overload for each type you want to test.
Battery about to die otherwise would give example.
Fifteen seconds after hitting "Save" the machine went into hibernate.
After thinking about it, the Visitor pattern might not solve your specific problem. I thought you were trying to compare entities, but it appears you are testing values (so potentially ints and strings).
But for the sake of completion, and because the visitor pattern is kind of cool once you realize what it does, here's an explanation.
The Visitor pattern allows you to handle multiple types without needing to figure out how to cast to the specific type (you decouple the type from the item using that type). It works by having two interfaces - the visitor and the acceptor:
interface IAcceptor
{
void Accept(IVisitor visitor);
}
interface IVisitor
{
void Visit(Type1 type1);
void Visit(Type2 type2);
.. etc ..
}
You can optionally use a generic method there:
interface IVisitor
{
void Visit<T>(T instance);
}
The basic implementation of the accept method is:
void Accept(IVisitor visitor)
{
visitor.Visit(this);
}
Because the type implementing Accept() knows what type it is, the correct overload (or generic type) is used. You could achieve the same thing with reflection and a lookup table (or select statement), but this is much cleaner. Also, you don't have to duplicate the lookup among different implementations -- various classes can implement IVisitor to create type-specific functionality.
The Visitor pattern is one way of doing "Double Dispatch". The answer to this question is another way and you might be able to morph it into something that works for your specific case.
Basically, a long-winded non-answer to your problem, sorry. :) The problem intrigues me, though -- like how do you know what property on the entity you should test against?