Simulate variadic templates in C#
Is there a well-known way for simulating the variadic template feature in C#?
For instance, I'd like to write a method that takes a lambda with an arbitrary set of parameters. Here is in pseudo code what I'd like to have:
void MyMethod<T1,T2,...,TRet开发者_如何转开发urn>(Fun<T1,T2, ..., TReturn> f)
{
}
C# generics are not the same as C++ templates. C++ templates are expanded compiletime and can be used recursively with variadic template arguments. The C++ template expansion is actually Turing Complete, so there is no theoretically limit to what can be done in templates.
C# generics are compiled directly, with an empty "placeholder" for the type that will be used at runtime.
To accept a lambda taking any number of arguments you would either have to generate a lot of overloads (through a code generator) or accept a LambdaExpression.
There is no varadic support for generic type arguments (on either methods or types). You will have to add lots of overloads.
varadic support is only available for arrays, via params, i.e.
void Foo(string key, params int[] values) {...}
Improtantly - how would you even refer to those various T* to write a generic method? Perhaps your best option is to take a Type[] or similar (depending on the context).
I know this is an old question, but if all you want to do is something simple like print those types out, you can do this very easily without Tuple or anything extra using 'dynamic':
private static void PrintTypes(params dynamic[] args)
{
foreach (var arg in args)
{
Console.WriteLine(arg.GetType());
}
}
static void Main(string[] args)
{
PrintTypes(1,1.0,"hello");
Console.ReadKey();
}
Will print "System.Int32" , "System.Double", "System.String"
If you want to perform some action on these things, as far as I know you have two choices. One is to trust the programmer that these types can do a compatible action, for example if you wanted to make a method to Sum any number of parameters. You could write a method like the following saying how you want to receive the result and the only prerequisite I guess would be that the + operation works between these types:
private static void AddToFirst<T>(ref T first, params dynamic[] args)
{
foreach (var arg in args)
{
first += arg;
}
}
static void Main(string[] args)
{
int x = 0;
AddToFirst(ref x,1,1.5,2.0,3.5,2);
Console.WriteLine(x);
double y = 0;
AddToFirst(ref y, 1, 1.5, 2.0, 3.5, 2);
Console.WriteLine(y);
Console.ReadKey();
}
With this, the output for the first line would be "9" because adding to an int, and the second line would be "10" because the .5s didn't get rounded, adding as a double. The problem with this code is if you pass some incompatible type in the list, it will have an error because the types can't get added together, and you won't see that error at compile time, only at runtime.
So, depending on your use case there might be another option which is why I said there were two choices at first. Assuming you know the choices for the possible types, you could make an interface or abstract class and make all of those types implement the interface. For example, the following. Sorry this is a bit crazy. And it can probably be simplfied.
public interface Applyable<T>
{
void Apply(T input);
T GetValue();
}
public abstract class Convertable<T>
{
public dynamic value { get; set; }
public Convertable(dynamic value)
{
this.value = value;
}
public abstract T GetConvertedValue();
}
public class IntableInt : Convertable<int>, Applyable<int>
{
public IntableInt(int value) : base(value) {}
public override int GetConvertedValue()
{
return value;
}
public void Apply(int input)
{
value += input;
}
public int GetValue()
{
return value;
}
}
public class IntableDouble : Convertable<int>
{
public IntableDouble(double value) : base(value) {}
public override int GetConvertedValue()
{
return (int) value;
}
}
public class IntableString : Convertable<int>
{
public IntableString(string value) : base(value) {}
public override int GetConvertedValue()
{
// If it can't be parsed return zero
int result;
return int.TryParse(value, out result) ? result : 0;
}
}
private static void ApplyToFirst<TResult>(ref Applyable<TResult> first, params Convertable<TResult>[] args)
{
foreach (var arg in args)
{
first.Apply(arg.GetConvertedValue());
}
}
static void Main(string[] args)
{
Applyable<int> result = new IntableInt(0);
IntableInt myInt = new IntableInt(1);
IntableDouble myDouble1 = new IntableDouble(1.5);
IntableDouble myDouble2 = new IntableDouble(2.0);
IntableDouble myDouble3 = new IntableDouble(3.5);
IntableString myString = new IntableString("2");
ApplyToFirst(ref result, myInt, myDouble1, myDouble2, myDouble3, myString);
Console.WriteLine(result.GetValue());
Console.ReadKey();
}
Will output "9" the same as the original Int code, except the only values you can actually pass in as parameters are things that you actually have defined and you know will work and not cause any errors. Of course, you would have to make new classes i.e. DoubleableInt , DoubleableString, etc.. in order to re-create the 2nd result of 10. But this is just an example, so you wouldn't even be trying to add things at all depending on what code you are writing and you would just start out with the implementation that served you the best.
Hopefully someone can improve on what I wrote here or use it to see how this can be done in C#.
Another alternative besides those mentioned above is to use Tuple<,> and reflection, for example:
class PrintVariadic<T>
{
public T Value { get; set; }
public void Print()
{
InnerPrint(Value);
}
static void InnerPrint<Tn>(Tn t)
{
var type = t.GetType();
if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Tuple<,>))
{
var i1 = type.GetProperty("Item1").GetValue(t, new object[]{});
var i2 = type.GetProperty("Item2").GetValue(t, new object[]{ });
InnerPrint(i1);
InnerPrint(i2);
return;
}
Console.WriteLine(t.GetType());
}
}
class Program
{
static void Main(string[] args)
{
var v = new PrintVariadic<Tuple<
int, Tuple<
string, Tuple<
double,
long>>>>();
v.Value = Tuple.Create(
1, Tuple.Create(
"s", Tuple.Create(
4.0,
4L)));
v.Print();
Console.ReadKey();
}
}
I don't necessarily know if there's a name for this pattern, but I arrived at the following formulation for a recursive generic interface that allows an unlimited amount of values to be passed in, with the returned type retaining type information for all passed values.
public interface ITraversalRoot<TRoot>
{
ITraversalSpecification<TRoot> Specify();
}
public interface ITraverser<TRoot, TCurrent>: ITraversalRoot<TRoot>
{
IDerivedTraverser<TRoot, TInclude, TCurrent, ITraverser<TRoot, TCurrent>> AndInclude<TInclude>(Expression<Func<TCurrent, TInclude>> path);
}
public interface IDerivedTraverser<TRoot, TDerived, TParent, out TParentTraverser> : ITraverser<TRoot, TParent>
{
IDerivedTraverser<TRoot, TInclude, TDerived, IDerivedTraverser<TRoot, TDerived, TParent, TParentTraverser>> FromWhichInclude<TInclude>(Expression<Func<TDerived, TInclude>> path);
TParentTraverser ThenBackToParent();
}
There's no casting or "cheating" of the type system involved here: you can keep stacking on more values and the inferred return type keeps storing more and more information. Here is what the usage looks like:
var spec = Traversal
.StartFrom<VirtualMachine>() // ITraverser<VirtualMachine, VirtualMachine>
.AndInclude(vm => vm.EnvironmentBrowser) // IDerivedTraverser<VirtualMachine, EnvironmentBrowser, VirtualMachine, ITraverser<VirtualMachine, VirtualMachine>>
.AndInclude(vm => vm.Datastore) // IDerivedTraverser<VirtualMachine, Datastore, VirtualMachine, ITraverser<VirtualMachine, VirtualMachine>>
.FromWhichInclude(ds => ds.Browser) // IDerivedTraverser<VirtualMachine, HostDatastoreBrowser, Datastore, IDerivedTraverser<VirtualMachine, Datastore, VirtualMachine, ITraverser<VirtualMachine, VirtualMachine>>>
.FromWhichInclude(br => br.Mountpoints) // IDerivedTraverser<VirtualMachine, Mountpoint, HostDatastoreBrowser, IDerivedTraverser<VirtualMachine, HostDatastoreBrowser, Datastore, IDerivedTraverser<VirtualMachine, Datastore, VirtualMachine, ITraverser<VirtualMachine, VirtualMachine>>>>
.Specify(); // ITraversalSpecification<VirtualMachine>
As you can see the type signature becomes basically unreadable near after a few chained calls, but this is fine so long as type inference works and suggests the right type to the user.
In my example I am dealing with Funcs arguments, but you could presumably adapt this code to deal with arguments of arbitrary type.
For a simulation you can say:
void MyMethod<TSource, TResult>(Func<TSource, TResult> f) where TSource : Tparams {
where Tparams to be a variadic arguments implementation class. However, the framework does not provide an out-of-box stuff to do that, Action, Func, Tuple, etc., are all have limited length of their signatures. The only thing I can think of is to apply the CRTP .. in a way I've not find somebody blogged. Here's my implementation:
*: Thank @SLaks for mentioning Tuple<T1, ..., T7, TRest> also works in a recursive way. I noticed it's recursive on the constructor and the factory method instead of its class definition; and do a runtime type checking of the last argument of type TRest is required to be a ITupleInternal; and this works a bit differently.
Code
using System; namespace VariadicGenerics { public interface INode { INode Next { get; } } public interface INode<R>:INode { R Value { get; set; } } public abstract class Tparams { public static C<TValue> V<TValue>(TValue x) { return new T<TValue>(x); } } public class T<P>:C<P> { public T(P x) : base(x) { } } public abstract class C<R>:Tparams, INode<R> { public class T<P>:C<T<P>>, INode<P> { public T(C<R> node, P x) { if(node is R) { Next=(R)(node as object); } else { Next=(node as INode<R>).Value; } Value=x; } public T() { if(Extensions.TypeIs(typeof(R), typeof(C<>.T<>))) { Next=(R)Activator.CreateInstance(typeof(R)); } } public R Next { private set; get; } public P Value { get; set; } INode INode.Next { get { return this.Next as INode; } } } public new T<TValue> V<TValue>(TValue x) { return new T<TValue>(this, x); } public int GetLength() { return m_expandedArguments.Length; } public C(R x) { (this as INode<R>).Value=x; } C() { } static C() { m_expandedArguments=Extensions.GetExpandedGenericArguments(typeof(R)); } // demonstration of non-recursive traversal public INode this[int index] { get { var count = m_expandedArguments.Length; for(INode node = this; null!=node; node=node.Next) { if(--count==index) { return node; } } throw new ArgumentOutOfRangeException("index"); } } R INode<R>.Value { get; set; } INode INode.Next { get { return null; } } static readonly Type[] m_expandedArguments; } }
Note the type parameter for the inherited class C<> in the declaration of
public class T<P>:C<T<P>>, INode<P> {
is T<P>, and the class T<P> is nested so that you can do some crazy things such as:
Test
[Microsoft.VisualStudio.TestTools.UnitTesting.TestClass] public class TestClass { void MyMethod<TSource, TResult>(Func<TSource, TResult> f) where TSource : Tparams { T<byte>.T<char>.T<uint>.T<long>. T<byte>.T<char>.T<long>.T<uint>. T<byte>.T<long>.T<char>.T<uint>. T<long>.T<byte>.T<char>.T<uint>. T<long>.T<byte>.T<uint>.T<char>. T<byte>.T<long>.T<uint>.T<char>. T<byte>.T<uint>.T<long>.T<char>. T<byte>.T<uint>.T<char>.T<long>. T<uint>.T<byte>.T<char>.T<long>. T<uint>.T<byte>.T<long>.T<char>. T<uint>.T<long>.T<byte>.T<char>. T<long>.T<uint>.T<byte>.T<char>. T<long>.T<uint>.T<char>.T<byte>. T<uint>.T<long>.T<char>.T<byte>. T<uint>.T<char>.T<long>.T<byte>. T<uint>.T<char>.T<byte>.T<long>. T<char>.T<uint>.T<byte>.T<long>. T<char>.T<uint>.T<long>.T<byte>. T<char>.T<long>.T<uint>.T<byte>. T<long>.T<char>.T<uint>.T<byte>. T<long>.T<char>.T<byte>.T<uint>. T<char>.T<long>.T<byte>.T<uint>. T<char>.T<byte>.T<long>.T<uint>. T<char>.T<byte>.T<uint>.T<long> crazy = Tparams // trying to change any value to not match the // declaring type makes the compilation fail .V((byte)1).V('2').V(4u).V(8L) .V((byte)1).V('2').V(8L).V(4u) .V((byte)1).V(8L).V('2').V(4u) .V(8L).V((byte)1).V('2').V(4u) .V(8L).V((byte)1).V(4u).V('2') .V((byte)1).V(8L).V(4u).V('2') .V((byte)1).V(4u).V(8L).V('2') .V((byte)1).V(4u).V('2').V(8L) .V(4u).V((byte)1).V('2').V(8L) .V(4u).V((byte)1).V(8L).V('2') .V(4u).V(8L).V((byte)1).V('2') .V(8L).V(4u).V((byte)1).V('2') .V(8L).V(4u).V('9').V((byte)1) .V(4u).V(8L).V('2').V((byte)1) .V(4u).V('2').V(8L).V((byte)1) .V(4u).V('2').V((byte)1).V(8L) .V('2').V(4u).V((byte)1).V(8L) .V('2').V(4u).V(8L).V((byte)1) .V('2').V(8L).V(4u).V((byte)1) .V(8L).V('2').V(4u).V((byte)1) .V(8L).V('2').V((byte)1).V(4u) .V('2').V(8L).V((byte)1).V(4u) .V('2').V((byte)1).V(8L).V(4u) .V('7').V((byte)1).V(4u).V(8L); var args = crazy as TSource; if(null!=args) { f(args); } } [TestMethod] public void TestMethod() { Func< T<byte>.T<char>.T<uint>.T<long>. T<byte>.T<char>.T<long>.T<uint>. T<byte>.T<long>.T<char>.T<uint>. T<long>.T<byte>.T<char>.T<uint>. T<long>.T<byte>.T<uint>.T<char>. T<byte>.T<long>.T<uint>.T<char>. T<byte>.T<uint>.T<long>.T<char>. T<byte>.T<uint>.T<char>.T<long>. T<uint>.T<byte>.T<char>.T<long>. T<uint>.T<byte>.T<long>.T<char>. T<uint>.T<long>.T<byte>.T<char>. T<long>.T<uint>.T<byte>.T<char>. T<long>.T<uint>.T<char>.T<byte>. T<uint>.T<long>.T<char>.T<byte>. T<uint>.T<char>.T<long>.T<byte>. T<uint>.T<char>.T<byte>.T<long>. T<char>.T<uint>.T<byte>.T<long>. T<char>.T<uint>.T<long>.T<byte>. T<char>.T<long>.T<uint>.T<byte>. T<long>.T<char>.T<uint>.T<byte>. T<long>.T<char>.T<byte>.T<uint>. T<char>.T<long>.T<byte>.T<uint>. T<char>.T<byte>.T<long>.T<uint>. T<char>.T<byte>.T<uint>.T<long>, String> f = args => { Debug.WriteLine(String.Format("Length={0}", args.GetLength())); // print fourth value from the last Debug.WriteLine(String.Format("value={0}", args.Next.Next.Next.Value)); args.Next.Next.Next.Value='x'; Debug.WriteLine(String.Format("value={0}", args.Next.Next.Next.Value)); return "test"; }; MyMethod(f); } }
Another thing to note is we have two classes named T, the non-nested T:
public class T<P>:C<P> {
is just for the consistency of usage, and I made class C abstract to not directly being newed.
The Code part above needs to expand ther generic argument to calculate about their length, here are two extension methods it used:
Code(extensions)
using System.Diagnostics; using System; namespace VariadicGenerics { [DebuggerStepThrough] public static class Extensions { public static readonly Type VariadicType = typeof(C<>.T<>); public static bool TypeIs(this Type x, Type d) { if(null==d) { return false; } for(var c = x; null!=c; c=c.BaseType) { var a = c.GetInterfaces(); for(var i = a.Length; i-->=0;) { var t = i<0 ? c : a[i]; if(t==d||t.IsGenericType&&t.GetGenericTypeDefinition()==d) { return true; } } } return false; } public static Type[] GetExpandedGenericArguments(this Type t) { var expanded = new Type[] { }; for(var skip = 1; t.TypeIs(VariadicType) ? true : skip-->0;) { var args = skip>0 ? t.GetGenericArguments() : new[] { t }; if(args.Length>0) { var length = args.Length-skip; var temp = new Type[length+expanded.Length]; Array.Copy(args, skip, temp, 0, length); Array.Copy(expanded, 0, temp, length, expanded.Length); expanded=temp; t=args[0]; } } return expanded; } } }
For this implementation, I choosed not to break the compile-time type checking, so we do not have a constructor or a factory with the signature like params object[] to provide values; instead, use a fluent pattern of method V for mass object instantiation to keep type can be statically type checked as much as possible.
加载中,请稍侯......
精彩评论