I'm trying to determine with Roslyn if two method signatures are equivalent (ignoring parameter order). It becomes somewhat non-trivial when the methods contain generic parameters.
I would like to detect AGenericMethod and AnotherGenericMethod below as having equivalent signatures:
public void AGenericMethod<X>(X someX, int someInt) where X : IEnumerable<int>
{
//...
}
public void AnotherGenericMethod<Y>(Y someY, int someInt) where Y : IEnumerable<int>
{
//...
}
It seems like .Equals() will return false for the ITypeParameterSymbols corresponding to X and Y.
Is there any existing functionality in Roslyn for classing ITypeParameterSymbols as 'equivalent' (in the sense of having equivalent constraints)? If not, what would be the best way to implement this (considering that the constraints may themselves be type parameters)?
Related
I came across this problematic quite often: I like to overload some method with same parameters for different return types, but .NET refuses generic constraints to sealed classes/primitives. I'll refer to this pattern as phantom generics.
I know an ugly workaround: Putting every single interface the type implements behind the where statement.
My Question: Is there any way to use explicit types in generics to illustrate the return type and keep methods distinct?
Here is my code:
public static class Reinterpret {
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static unsafe float Cast<T>(int value) where T : float { //'float' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter.
return *((float*)&value); //reinterpret the bytes of 'value' to a float
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static unsafe float Cast<T>(uint value) where T : float { //'float' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter.
return *((float*)&value);
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static unsafe double Cast<T>(long value) where T : double { //'double' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter.
return *((double*)&value);
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static unsafe double Cast<T>(ulong value) where T : double { //'double' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter.
return *((double*)&value);
}
}
Here's one slightly different way of approach it:
// Constraints just to be vaguely reasonable.
public static class Reinterpret<T> where T : struct, IComparable<T>
{
public T Cast(int value) { ... }
public T Cast(uint value) { ... }
public T Cast(float value) { ... }
public T Cast(double value) { ... }
// etc
}
For the implementation, you could just have a Func<int, T> field, a Func<double, T> field etc, and then have a big static constructor:
static Reinterpret()
{
if (typeof(T) == typeof(int))
{
// Assign all the fields using lambda expressions for ints.
// The actual assignment might be tricky, however - you may
// need to resort to some ghastly casting, e.g.
castDouble = (Func<double, T>)(Delegate)(Func<double, int>)
x => *((double*)&value;
}
....
}
Then for any type you didn't want to support, the fields would be null. Each Cast method would look like:
if (castIntMethod != null)
{
return castInt(value);
}
throw new InvalidOperationException("...");
To be honest, this isn't something I'd really want to do. I'd generally just use BitConverter. But it's an option.
Generics are not templates. They do not act like templates. They cannot be made to act like templates.
A "phantom" generic parameter is not going to help you simulate templates (and reinterpret_cast is not an actual template, anyway), because you soon are going to run into the fact that generics do not support specialization.
In particular, you asked "Is there any way to use explicit types in generics to ... keep methods distinct?" and commented that "the generic constraints ... keeps [sic] the methods distinct". But they actually do not. These methods are distinct only because the argument types are different. Generics are computed from overloads, they do not influence overloading.
I would like to determine if a parameter has this modifier using Reflection.I have looked at properties of ParameterInfo class, but couldn't find anything useful.I know the extension methods are just syntactic sugars but I believe there should be a way to determine whether a method is an extension method.
The only thing that distinguishes extension methods from other static methods (that are defined in a static, public class) is this modifier.
For example this is not an extension method:
public static int Square(int x) { return x * x; }
But this is:
public static int Square(this int x) { return x * x; }
So how can I distinguish between two methods using Reflection or something else if possible?
It's not exactly the same, but you can check whether the method has the ExtensionAttribute applied to it.
var method = type.GetMethod("Square");
if (method.IsDefined(typeof(ExtensionAttribute), false))
{
// Yup, it's an extension method
}
Now I say it's not exactly the same, because you could have written:
[Extension]
public static int Square(int x) { return x * x; }
... and the compiler would still pick it up as an extension method. So this does detect whether it's an extension method (assuming it's in a static top-level non-generic type) but it doesn't detect for certain whether the source code had the this modifier on the first parameter.
public static object GetObject(int x)
{
return new object { };
}
public static object GetObject(string x)
{
return new object { };
}
public static void ProcessObject<T>(T x) where T : int, string <= got error here:
{
object o = GetObject(x);
}
Got error "A type used as a constraint must be an interface, a non-sealed class or a type parameter."
How can I rewrite the code to get it work without write ProcessObject(int x) and ProcessObject(string x) twice?
So what you have now (according to accepted answer and your comments) is:
public static void ProcessObject<T>(T x)
{
object o;
if (typeof(T) == typeof(int))
o = GetObject((int)(object)x);
else if (typeof(T) == typeof(string))
o = GetObject((string)(object)x);
else
throw new Exception();
// do stuff with o
}
I'd recommend making public int and string overloads, but to prevent code duplication, internally call another method:
public static void ProcessObject(int x)
{
ProcessObject(GetObject(x));
}
public static void ProcessObject(string x)
{
ProcessObject(GetObject(x));
}
private static void ProcessObject(object o)
{
// do stuff with o
}
This makes your public methods' input values clear: int and string are the only acceptable types, while still not duplicating your actual logic (// do stuff with o).
You might dislike that the two public ProcessObject methods are duplicates of each other, (on the surface anyway; under the hood, they're calling two different GetObject overloads) but I think it's the best option.
You cannot do what you are trying to do: first, it is not possible to list several classes in a generic constraint; second, the type that you can put in a constraint must be such that you could inherit it (or implement it if it is an interface). Both int and string fail this check. In cases like this, you would be better off with two separate overloads.
Just remove the where part
public static void ProcessObject<T>(T x)
{
object o = GetObject(x);
}
And also don't use object in your other methods, instead use T
it's impossible in C# take a look on Constraints on Type Parameters. Try to use dynamic
Generally speaking, if your object reacts differently based on the generic type argument, you probably shouldn't be using generics in this case. Generics are great for situations where you want to do always the same thing, no matter what the actual type used.
Therefore, generic constraints will only allow you to list one base class for a type argument. Any actual types passed to the respective type arguments are meant to be a part of the given inheritance hierarchy starting with the base class you specified, so users of your class can specify any type that matches the base class or any of its subclasses.
At the same time, you, the author of the generic class, can safely assume that the specified type has (at least) the interface of the base class indicated by the constraint. Hence, you may access any members of the base class.
If you want to allow either string or int, imagine what members that could be. Both are derived directly from System.Object, hence the restriction would make no sense as it is no restriction; every type is derived from System.Object.
Summarizing, if you really want to treat string and int differently, this is definitely a case for offering two overloads rather than one generic class.
In short: is it possible to define a generic method where the type parameter (T) is constrained to string or int[]? In pseudo-C#, what I want to do is:
public static int MyMethod<T> ( T arg1, T arg2 )
where T : (has an indexer that returns an int) {
// stuff with arg1[i], arg2[j], etc...
}
Note that in C#, due to the built-in string indexer (which returns a char) along with implicit conversion from char to int, the following expression means precisely the same thing whether source is a string or an int[]:
int someval = source[index];
Per the thread Constrain generic extension method to base types and string I realize I can't just make a list of unassociated types in the where T : x... constraint clause. Both int[] and string would comply with T: IEnumerable<int>, but IEnumerable<T> does not require implementers to have an indexer, which is exactly the common feature I am using from both types.
The purpose behind this is that I'm building some highly optimized string parsing and analyzing functions such as a fast implementation of the Damerau–Levenshtein distance algorithm. I've found that first converting my strings to arrays of int can sometimes yield significantly faster executions in repetitive character-by-character processing (as with the D-L algorithm). This is largely due to the fact that comparing int values is much faster that comparing char values.
The operative word is 'sometimes'. Sometimes it's faster to operate directly on the strings and avoid the cost of first converting and copying to arrays of int. So I now have methods that are truly identical except for the declarations.
Of course I can use dynamic, but the performance penalty from runtime checking completely destroys any gains made in the construction of the methods. (I did test it).
You cannot have a constraint that says “the type must have an indexer”.
However, you can have a constraint that says “the type must implement an interface that has an indexer”. Such an interface might be IList<char>, for example.
Unfortunately, string doesn’t implement IList<char>, so you would have to write a small wrapper class for it:
sealed class StringWrapper : IList<char>
{
public string String { get; private set; }
public StringWrapper(string str) { String = str; }
public static implicit operator StringWrapper(string str)
{
return new StringWrapper(str);
}
public char this[int index]
{
get { return String[index]; }
set { throw new NotSupportedException(); }
}
// Etc.... need to implement all the IList<char> methods
// (just throw NotSupportedException except in the ones that are trivial)
}
And then you can declare your method like this:
public static TElement MyMethod<TCollection, TElement>(TCollection arg)
where TCollection : IList<TElement>
{
return arg[0];
}
[...]
MyMethod<StringWrapper, char>("abc") // returns 'a'
MyMethod<int[], int>(new[] { 1, 2, 3 }) // returns 1
No, C# does not allow you to create a "composite" constraint like that.
The expression you show does mean the same thing, but you can't use that fact to your advantage. C# generics resemble C++ templates syntactically, but they work completely differently, so the fact that the indexer does the same thing ends up being irrelevant.
You could, of course, just create the two overloads with the specific types you need, but that does mean some annoying copy & paste. Any attempts at abstracting this to avoid repetition will cost you performance badly.
You could make your method signature
public static int MyMethod<T> ( T[] arg1, T[] arg2 )
and use String.ToCharArray() before passing string arguments (or maybe in an overload, you get the idea...)
I'm just learning about generics and have a question regarding method return values.
Say, I want a generic method in the sense that the required generic part of the method signature is only the return value. The method will always take one string as it's parameter but could return either a double or an int. Is this possible?
Effectively I want to take a string, parse the number contained within (which could be a double or an int) and then return that value.
Thanks.
Something like this?
void Main()
{
int iIntVal = ConvertTo<int>("10");
double dDoubleVal = ConvertTo<double>("10.42");
}
public T ConvertTo<T>(string val) where T: struct
{
return (T) System.Convert.ChangeType(val, Type.GetTypeCode(typeof(T)));
}
You cannot return either a double or an int from a generic method without it also returning any other type.
I might, for example, have a Foo class and your generic parse method, without any constraint, will allow this call to be made:
Foo result = Parse<Foo>("111");
The best that you can do with numbers is constrain on your function by only allowing struct (value-types) to be used.
T Parse<T>(string value) where T : struct;
But this will allow all number types, plus any other value-type.
You can constrain by interface type, but there isn't an INumeric interface on double or int so you're kind of stuck.
The only thing that you can do is throw an exception if the wrong type is passed in - which generally isn't very satisfying.
Your best approach, in this case, is to abandon generics and use separately named methods.
double ParseDouble(string value);
int ParseInteger(string value);
But, of course, this won't help you learn generics. Sorry.
Yes it's possible.
Example:
public T ParseValue<T>(String value) {
// ...
}
You could do something like ...
public TResult Parse<TResult>(string parameter)
{
/* do stuff */
}
And use it like ...
int result = Parse<int>("111");
And then it will depend on your implementation in the Parse method.
Hope it helps.