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32 bit implicit conversions fail generic overload resolution

I'm experimenting with custom integer types and came across an interesting issue involving generics, implicit conversions, and 32 bit integers.
Below is a stripped down example of how to reproduce the problem. If I have two implicit methods that convert int to MyInt and vice versa, I get a compilation error which looks like C# can't resolve which generic type to use. And it only happens with int or uint. All other integer types work fine: sbyte,byte,short,ushort,long,ulong.
If I remove one of the implicit conversion methods, it also works fine. Something to do with circular implicit conversions?
using Xunit;
public class MyInt
{
public int Value;
//If I remove either one of the implicit methods below, it all works fine.
public static implicit operator int(MyInt myInt)
{
return myInt.Value;
}
public static implicit operator MyInt(int i)
{
return new MyInt() { Value = i };
}
public override bool Equals(object obj)
{
if (obj is MyInt myInt)
{
return this.Value == myInt.Value;
}
else
{
int other_int = (int)obj;
return Value == other_int;
}
}
}
Below is the test code showing the compilation errors I get when both implicit methods are defined.
public class Test
{
[Fact]
public void EqualityTest()
{
MyInt myInt = new MyInt();
myInt.Value = 4 ;
Assert.Equal(4, myInt.Value); //Always OK which makes sense
//Compile errors when both implicit methods defined:
// Error CS1503 Argument 1: cannot convert from 'int' to 'string',
// Error CS1503 Argument 2: cannot convert from 'ImplicitConversion.MyInt' to 'string'
Assert.Equal(4, myInt);
}
}
I believe C# is complaining about not being able to convert both types to string as that is the type of the last Xunit.Assert.Equal() overload and all the others failed to match:
//Xunit.Assert.Equal methods:
public static void Equal<T>(T expected, T actual);
public static void Equal(double expected, double actual, int precision);
public static void Equal<T>(T expected, T actual, IEqualityComparer<T> comparer);
public static void Equal(decimal expected, decimal actual, int precision);
public static void Equal(DateTime expected, DateTime actual, TimeSpan precision);
public static void Equal<T>(IEnumerable<T> expected, IEnumerable<T> actual, IEqualityComparer<T> comparer);
public static void Equal<T>(IEnumerable<T> expected, IEnumerable<T> actual);
public static void Equal(string expected, string actual, bool ignoreCase = false, bool ignoreLineEndingDifferences = false, bool ignoreWhiteSpaceDifferences = false);
public static void Equal(string expected, string actual);
I don't think I've made a mistake with the implicit conversions as I can make other similar examples create the same problem when used with 32 bit ints.
I'm testing in a .NET Core 3.0 project.
Any help would be appreciated. Thanks!
Clarification:
What I would like to know is why this only fails with 32 bit integers. Implicit conversions are working (confirmed with debugging) when the types are anything else like the example below using a long.
using Xunit;
public class MyLong
{
public long Value;
public static implicit operator long(MyLong myInt)
{
return myInt.Value;
}
public static implicit operator MyLong(long i)
{
return new MyLong() { Value = i };
}
public override bool Equals(object obj)
{
if (obj is MyLong myInt)
{
return this.Value == myInt.Value;
}
else
{
long other_int = (long)obj;
return Value == other_int;
}
}
}
public class Test2
{
[Fact]
public void EqualityTest()
{
MyLong myLong = new MyLong();
myLong.Value = 4 ;
Assert.Equal(4, myLong); //NOTE! `4` is implicitly converted to a MyLong
//object for comparison. Confirmed with debugging.
}
}

Something to do with circular implicit conversions?
Yes (though, you've already demonstrated that much by showing that it works fine when one of the conversions is eliminated).
The reason this is happening with int, and not with other types, is that the type of your literal is int. This means that during overload resolution, the compiler can go either way: convert int to MyInt, or convert MyInt to int. Neither option is clearly "better" than the other, so neither of those conversions survive consideration.
Then, having ruled out the closest possible generic version of the method, of the remaining overloads available the only one left is the Equal(string, string) overload (the only other one left with just two parameters is the Equal<T>(IEnumerable<T>, IEnumerable<T>), which is "worse" than the Equal(string, string) overload according to the overload resolution rules). Having found exactly one method that is clearly "better" than any others, the compiler then tries to use that method with your parameters, which of course don't fit, causing the errors to be emitted.
On the other hand…
When you try to call Equal(4, myLong), you've got two incompatible types. A literal having type int, and the MyLong value. In this case, the compiler tries each parameter one by one and finds that when it uses the MyLong type as the type parameter, it is possible to promote the int literal to a long and then implicitly convert that to MyLong. But it can't go the other way. It's not possible to select int as the generic type parameter, because MyLong can't be implicitly converted to int. So in that case, there is a "better" overload to choose, and so it's chosen.
By explicitly specifying the literal's type, you can try different combinations and see this pattern at work. First, I prefer a simpler wrapper class to test with:
public class Wrapper<T>
{
public T Value;
public static implicit operator T(Wrapper<T> wrapper) => wrapper.Value;
public static implicit operator Wrapper<T>(T value) => new Wrapper<T> { Value = value };
}
Then try these:
Wrapper<int> w1 = new Wrapper<int> { Value = 4 };
Wrapper<long> w2 = new Wrapper<long> { Value = 4 };
Assert.Equal(4, w1); // error
Assert.Equal((short)4, w1); // no error
Assert.Equal(4, w2); // no error
Assert.Equal(4L, w2); // error
The only thing that makes int special is that that's the default type for the numeric literal. Otherwise, a type that wraps int works exactly the same as a type that wraps anything else. As long as a conversion is available only in one direction between the two parameters, everything's fine. But when the conversion is available in both directions, the compiler has no choice but to throw up its hands and give up.

C# custom variables

I was experimenting with making a custom variables but got stuck.
I'm still new to C# so it's only expected for me to not know what's happening, I guess..
struct MyCustomStringVariable
{
public static implicit operator MyCustomStringVariable(string input)
{
return input;
}
}
class Program
{
static MyCustomStringVariable myCustomString = "This is a string!";
static void Main(string[] args)
{
Console.WriteLine(myCustomString);
Console.ReadLine();
}
}
The following exception is thrown
System.StackOverflowException: 'Exception of type 'System.StackOverflowException' was thrown.'

This is because the code is stuck in an infinit loop. Your implicit operator will call itself because it returns the original input string which does not throw an exception because of the defined operator.
public static implicit operator MyCustomStringVariable(string input)
{
return input; // returning string type will call this method again
}
should be
public static implicit operator MyCustomStringVariable(string input)
{
// and use input somewhere on the returned type
return new MyCustomStringVariable();
}
That said there is probably no reason for you to define a type named MyCustomStringVariable but that is hard to tell because you never share the code for this or how you intend to use it.
My final goal is to visualize the process of making a string variable in my head so that I can better understand the concept behind it.
I am not sure how your custom struct or its implicit operator fit in with this goal. Why not just use the type string?
static string myCustomString = "This is a string!";

It's because the implicit operator is called recursively. You'll need to implement your structure as so, encapsulating your string variable somehow.
struct MyCustomStringVariable
{
private string value;
public MyCustomStringVariable(string input)
{
value = input;
}
public static implicit operator MyCustomStringVariable(string input)
{
return new MyCustomStringVariable(input);
}
public string GetValue()
{
return value;
}
}
Then, calling it like
Console.WriteLine(myCustomString.GetValue());
You can refer to the documentation here.

Implicit conversion fails when changing struct to sealed class

Struct/class in question:
public struct HttpMethod
{
public static readonly HttpMethod Get = new HttpMethod("GET");
public static readonly HttpMethod Post = new HttpMethod("POST");
public static readonly HttpMethod Put = new HttpMethod("PUT");
public static readonly HttpMethod Patch = new HttpMethod("PATCH");
public static readonly HttpMethod Delete = new HttpMethod("DELETE");
private string _name;
public HttpMethod(string name)
{
// validation of name
_name = name.ToUpper();
}
public static implicit operator string(HttpMethod method)
{
return method._name;
}
public static implicit operator HttpMethod(string method)
{
return new HttpMethod(method);
}
public static bool IsValidHttpMethod(string method)
{
// ...
}
public override bool Equals(object obj)
{
// ...
}
public override int GetHashCode()
{
return _name.GetHashCode();
}
public override string ToString()
{
return _name;
}
}
The following code triggers the issue:
public class HttpRoute
{
public string Prefix { get; }
public HttpMethod[] Methods { get; }
public HttpRoute(string pattern, params HttpMethod[] methods)
{
if (pattern == null) throw new ArgumentNullException(nameof(pattern));
Prefix = pattern;
Methods = methods ?? new HttpMethod[0];
}
public bool CanAccept(HttpListenerRequest request)
{
return Methods.Contains(request.HttpMethod) && request.Url.AbsolutePath.StartsWith(Prefix);
}
}
The compiler error is created by changing the HttpMethod struct into a sealed class. The error is reported for return Methods.Contains(request.HttpMethod), note: request.HttpMethod in this case is a string. Which produces the following:
Error CS1929 'HttpMethod[]' does not contain a definition for 'Contains' and the best extension method overload 'Queryable.Contains<string>(IQueryable<string>, string)' requires a receiver of type 'IQueryable<string>'
My question is why? I can redesign the code to make it work, but I'm wanting to know why changing from struct to sealed class creates this weird error.
Edit: Adding a simplified set of example code (available here: https://dotnetfiddle.net/IZ9OXg). Take note that commenting out the implicit operator to string on the second class allows the code to compile:
public static void Main()
{
HttpMethod1[] Methods1 = new HttpMethod1[10];
HttpMethod2[] Methods2 = new HttpMethod2[10];
var res1 = Methods1.Contains("blah"); //works
var res2 = Methods2.Contains("blah"); //doesn't work
}
public struct HttpMethod1
{
public static implicit operator HttpMethod1(string method)
{
return new HttpMethod1();
}
public static implicit operator string (HttpMethod1 method)
{
return "";
}
}
public class HttpMethod2
{
public static implicit operator HttpMethod2(string method)
{
return new HttpMethod2();
}
//Comment out this method and it works fine
public static implicit operator string (HttpMethod2 method)
{
return "";
}
}

Things I know:
Plainly the problem is in type inference.
In the first case, T is deduced to be HttpMethod1.
In the struct case, there is no conversion from HttpMethod1[] to IEnumerable<string> because covariance only works on reference types.
In the class case, there is no conversion from HttpMethod2[] to IEnumerable<string> because covariance only works on reference conversions, and this is a user-defined conversion.
Things I suspect but need to confirm:
Something about the slight difference between my last two points is confusing the type inference algorithm.
UPDATE:
It has nothing to do with covariant array conversions. The problem repros even without array conversions.
It does however have to do with covariant interface conversions.
It has nothing to do with strings. (Strings are often a bit weird because they have a hard-to-remember conversion to IEnumerable<char> that occasionally messes up type inference.)
Here's a program fragment that displays the problem; update your conversions to convert to C instead of string:
public interface IFoo<out T> {}
public class C {}
public class Program
{
public static bool Contains<T>(IFoo<T> items, T item)
{
System.Console.WriteLine(typeof(T));
return true;
}
public static void Main()
{
IFoo<HttpMethod1> m1 = null;
IFoo<HttpMethod2> m2 = null;
var res1 = Contains(m1, new C()); //works
var res2 = Contains(m2, new C()); //doesn't work
}
}
This looks like a possible bug in type inference, and if it is, it is my fault; many apologies if that is the case. Sadly I do not have time to look into it further today. You might want to open an issue on github and have someone who still does this for a living look into it. I would be fascinated to learn what the result was, and if it turns out to be a bug in either the design or the implementation of the inference algorithm.

Firstly, this is an observed behavioural difference between structs and classes. The fact that you have 'sealed' your class does not affect the outcome in this scenario.
Also we know the following statement will compile as expected for HttpMethod type declared as both a struct and class, thanks to the implicit operator.
string method = HttpMethods[0];
Dealing with Arrays introduces some lesser understood compiler nuances.
Covariance
When HttpMethod is a class (reference type), with an array such as HttpRoute.HttpMethods Array covariance (12.5 C# 5.0 Language Spec) comes into play that allows HttpMethod[x] to be treated as an object. Covariance will respect inbuilt implicit reference conversions (such as type inheritance or conversion to object) and it will respect explicit operators, but it will not respect or look for user defined implicit operators. (While a bit ambigous the actual spec doc lists specifically default implicit operators and explicit operators, it does not mention the user defined operators but seeing everything else is so highly specified you can infer that user defined operators are not supported.)
Basically Covariance takes precedence over many generic type evaluations. More on this in a moment.
Array covariance specifically does not extend to arrays of value-types. For example, no conversion exists that permits an int[] to be treated as an object[].
So when HttpMethod is a struct (value type), covariance is no longer an issue and the following generic extension from System.Linq namespace will apply:
public static bool Contains<TSource>(this IEnumerable<TSource> source, TSource value);
Because you have passed in a string comparator, the Contains statement will be evaluated as follows:
public static bool Contains<string>(this IEnumerable<string> source, string value);
When HttpMethod is a class (Reference Type), thanks to covariance, HttpMethod[] in it's current form comparable only with Object[] and thus IEnumerable, but not IEnumerable< T >, Why not? because the compiler needs to be able to determine the type to generate the generic implementation of IEnumerable< T > and to determine if it can perform an explicit cast from object to T.
Put another way, Compiler cannot determine if T can definetly be a String or not, so it doesn't find the match in the Linq extension methods that we were expecting.
So what can you do about it? (! Not this !)
The first common attempt might be to try using .Cast< string >() to cast the HttpMethod instances to strings for the comparison:
return HttpMethods.Cast<string>().Contains(request.Method) && request.Url.AbsolutePath.StartsWith(Prefix);
You will find that this does not work. Even though The parameter for Cast< T > is of type IEnumerable, not IEnumerable< T >. It is provided to allow you to use older collections that do not implement the generic version of IEnumerable with LINQ. Cast< T > is only designed to convert non-generic objects to their "true" type through the process of evaluating common origins for reference types or Un-Boxing for value types. If Boxing and Unboxing (C# Programming Guide) only applies to value types (structs) and since our HttpMethod type is a reference type (class) the only common origin between HttpMethod and String is Object. On HttpMethod there is no implicit, or even explicit operator that accepts Object and as it is not a value type there is no in built un-box operator that the compiler can use.
Note that this Cast<> will fail at runtime in this scenario when HttpMethod is a value type (class) the compiler will be happy to let it build.
Final Workaround
Instead of Cast< T > or relying on implicit conversions we will need to force the elements in the HttpMethods array to be explicitly cast to string (This will still use out implicit operator!) but Linq again makes this a trivial, but necessary task:
return HttpMethods.Select(c => (string)c).Contains(request.Method) && request.Url.AbsolutePath.StartsWith(Prefix);

How to cast implicitly on a reflected method call

I have a class Thing that is implicitly castable from a string. When I call a method with a Thing parameter directly the cast from string to Thing is done correctly.
However if I use reflection to call the same method it throws the exception
System.ArgumentException : Object of type 'System.String' cannot be
converted to type 'Things.Program+Thing'.
Maybe there is a good reason for this, but I can't figure it out. Does somebody have an idea how to get this working using reflection?
namespace Things
{
class Program
{
public class Thing
{
public string Some;
public static implicit operator Thing(string s)
{
return new Thing {Some = s};
}
}
public void showThing(Thing t)
{
Console.WriteLine("Some = " + t.Some);
}
public void Main()
{
showThing("foo");
MethodInfo showThingReflected = GetType().GetMethod("showThing");
showThingReflected.Invoke(this, new dynamic[] {"foo"});
}
}
}
Meta: Please, no discussions why implicit casting or reflection is bad.

The trick is to realize that the compiler creates a special static method called op_Implicit for your implicit conversion operator.
object arg = "foo";
// Program.showThing(Thing t)
var showThingReflected = GetType().GetMethod("showThing");
// typeof(Thing)
var paramType = showThingReflected.GetParameters()
.Single()
.ParameterType;
// Thing.implicit operator Thing(string s)
var converter = paramType.GetMethod("op_Implicit", new[] { arg.GetType() });
if (converter != null)
arg = converter.Invoke(null, new[] { arg }); // Converter exists: arg = (Thing)"foo";
// showThing(arg)
showThingReflected.Invoke(this, new[] { arg });

Found an answer which uses a TypeConverter (as Saeed mentions)
Seems to do the job.
TypeConverter For Implicit Conversion when using reflection

In this specific case you can make the conversion through the array type, that is
showThingReflected.Invoke(this, new Thing[] {"foo"});
but that's a kind of "cheating". In general, you cannot expect the Invoke to consider your user-defined implicit operator. This conversion must be inferred compile-time.

Can I create accessors on structs to automatically convert to/from other datatypes?

is it possible to do something like the following:
struct test
{
this
{
get { /*do something*/ }
set { /*do something*/ }
}
}
so that if somebody tried to do this,
test tt = new test();
string asd = tt; // intercept this and then return something else

Conceptually, what you want to do here is in fact possible within .NET and C#, but you're barking up the wrong tree with regards to syntax. It seems like an implicit conversion operator would be the solution here,
Example:
struct Foo
{
public static implicit operator string(Foo value)
{
// Return string that represents the given instance.
}
public static implicit operator Foo(string value)
{
// Return instance of type Foo for given string value.
}
}
This allows you to assign and return strings (or any other type) to/from objects of your custom type (Foo here).
var foo = new Foo();
foo = "foobar";
var string = foo; // "foobar"
The two implicit conversion operators don't have to be symmetric of course, though it's usually advisable.
Note: There are also explicit conversion operators, but I think you're more after implicit operators.

You can define implicit and explicit conversion operators to and from your custom type.
public static implicit operator string(test value)
{
return "something else";
}

Expanding on MikeP's answer you want something like:
public static implicit operator Test( string value )
{
//custom conversion routine
}
or
public static explicit operator Test( string value )
{
//custom conversion routine
}