Conversions.cs

// ************** Implementation **************

namespace CodeDucky
{
    using Microsoft.CSharp.RuntimeBinder;
    using System;
    using System.Collections;
    using System.Collections.Concurrent;
    using System.Collections.Generic;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Reflection;
    using System.Runtime.CompilerServices;
    using System.Text;
    using System.Text.RegularExpressions;
    using System.Threading.Tasks;
    using CSharpBinder = Microsoft.CSharp.RuntimeBinder.Binder;

    public static class TypeHelpers
    {
        #region ---- Explicit casts ----
        public static bool IsCastableTo(this Type from, Type to)
        {
            // from http://www.codeducky.org/10-utilities-c-developers-should-know-part-one/ 
            Throw.IfNull(from, "from");
            Throw.IfNull(to, "to");

            // explicit conversion always works if to : from OR if 
            // there's an implicit conversion
            if (from.IsAssignableFrom(to) || from.IsImplicitlyCastableTo(to))
            {
                return true;
            }

            var key = new KeyValuePair<Type, Type>(from, to);
            bool cachedValue;
            if (CastCache.TryGetCachedValue(key, out cachedValue))
            {
                return cachedValue;
            }

            // for nullable types, we can simply strip off the nullability and evaluate the underyling types
            var underlyingFrom = Nullable.GetUnderlyingType(from);
            var underlyingTo = Nullable.GetUnderlyingType(to);
            if (underlyingFrom != null || underlyingTo != null)
            {
                return (underlyingFrom ?? from).IsCastableTo(underlyingTo ?? to);
            }

            bool result;

            if (from.IsValueType)
            {
                try
                {
                    ReflectionHelpers.GetMethod(() => AttemptExplicitCast<object, object>())
                        .GetGenericMethodDefinition()
                        .MakeGenericMethod(from, to)
                        .Invoke(null, new object[0]);
                    result = true;
                }
                catch (TargetInvocationException ex)
                {
                    result = !(
                        ex.InnerException is RuntimeBinderException
                        // if the code runs in an environment where this message is localized, we could attempt a known failure first and base the regex on it's message
                        && Regex.IsMatch(ex.InnerException.Message, @"^Cannot convert type '.*' to '.*'$")
                    );
                }
            }
            else
            {
                // if the from type is null, the dynamic logic above won't be of any help because 
                // either both types are nullable and thus a runtime cast of null => null will 
                // succeed OR we get a runtime failure related to the inability to cast null to 
                // the desired type, which may or may not indicate an actual issue. thus, we do 
                // the work manually
                result = from.IsNonValueTypeExplicitlyCastableTo(to);
            }
            
            CastCache.UpdateCache(key, result);
            return result;
        }

        private static bool IsNonValueTypeExplicitlyCastableTo(this Type from, Type to)
        {
            if ((to.IsInterface && !from.IsSealed)
                || (from.IsInterface && !to.IsSealed))
            {
                // any non-sealed type can be cast to any interface since the runtime type MIGHT implement
                // that interface. The reverse is also true; we can cast to any non-sealed type from any interface
                // since the runtime type that implements the interface might be a derived type of to.
                return true;
            }

            // arrays are complex because of array covariance 
            // (see http://msmvps.com/blogs/jon_skeet/archive/2013/06/22/array-covariance-not-just-ugly-but-slow-too.aspx).
            // Thus, we have to allow for things like var x = (IEnumerable<string>)new object[0];
            // and var x = (object[])default(IEnumerable<string>);
            var arrayType = from.IsArray && !from.GetElementType().IsValueType ? from
                : to.IsArray && !to.GetElementType().IsValueType ? to
                : null;
            if (arrayType != null)
            {
                var genericInterfaceType = from.IsInterface && from.IsGenericType ? from
                    : to.IsInterface && to.IsGenericType ? to
                    : null;
                if (genericInterfaceType != null)
                {
                    return arrayType.GetInterfaces()
                        .Any(i => i.IsGenericType
                            && i.GetGenericTypeDefinition() == genericInterfaceType.GetGenericTypeDefinition()
                            && i.GetGenericArguments().Zip(to.GetGenericArguments(), (ia, ta) => ta.IsAssignableFrom(ia) || ia.IsAssignableFrom(ta)).All(b => b));
                }
            }

            // look for conversion operators. Even though we already checked for implicit conversions, we have to look
            // for operators of both types because, for example, if a class defines an implicit conversion to int then it can be explicitly
            // cast to uint
            const BindingFlags conversionFlags = BindingFlags.Public | BindingFlags.Static | BindingFlags.FlattenHierarchy;
            var conversionMethods = from.GetMethods(conversionFlags)
                .Concat(to.GetMethods(conversionFlags))
                .Where(m => (m.Name == "op_Explicit" || m.Name == "op_Implicit")
                    && m.Attributes.HasFlag(MethodAttributes.SpecialName)
                    && m.GetParameters().Length == 1 
                    && (
                        // the from argument of the conversion function can be an indirect match to from in
                        // either direction. For example, if we have A : B and Foo defines a conversion from B => Foo,
                        // then C# allows A to be cast to Foo
                        m.GetParameters()[0].ParameterType.IsAssignableFrom(from)
                        || from.IsAssignableFrom(m.GetParameters()[0].ParameterType)
                    )
                );

            if (to.IsPrimitive && typeof(IConvertible).IsAssignableFrom(to))
            {
                // as mentioned above, primitive convertible types (i. e. not IntPtr) get special 
                // treatment in the sense that if you can convert from Foo => int, you can convert
                // from Foo => double as well
                return conversionMethods.Any(m => m.ReturnType.IsCastableTo(to));
            }

            return conversionMethods.Any(m => m.ReturnType == to);
        }

        private static void AttemptExplicitCast<TFrom, TTo>()
        {
            // based on the IL generated from
            // var x = (TTo)(dynamic)default(TFrom);

            var binder = CSharpBinder.Convert(CSharpBinderFlags.ConvertExplicit, typeof(TTo), typeof(TypeHelpers));
            var callSite = CallSite<Func<CallSite, TFrom, TTo>>.Create(binder);
            callSite.Target(callSite, default(TFrom));
        }
        #endregion

        #region ---- Implicit casts ----
        public static bool IsImplicitlyCastableTo(this Type from, Type to)
        {
            // from http://www.codeducky.org/10-utilities-c-developers-should-know-part-one/ 
            Throw.IfNull(from, "from");
            Throw.IfNull(to, "to");

            // not strictly necessary, but speeds things up and avoids polluting the cache
            if (to.IsAssignableFrom(from))
            {
                return true;
            }

            var key = new KeyValuePair<Type, Type>(from, to);
            bool cachedValue;
            if (ImplicitCastCache.TryGetCachedValue(key, out cachedValue))
            {
                return cachedValue;
            }

            bool result;
            try
            {
                // overload of GetMethod() from http://www.codeducky.org/10-utilities-c-developers-should-know-part-two/ 
                // that takes Expression<Action>
                ReflectionHelpers.GetMethod(() => AttemptImplicitCast<object, object>())
                    .GetGenericMethodDefinition()
                    .MakeGenericMethod(from, to)
                    .Invoke(null, new object[0]);
                result = true;
            }
            catch (TargetInvocationException ex)
            {
                result = !(
                    ex.InnerException is RuntimeBinderException
                    // if the code runs in an environment where this message is localized, we could attempt a known failure first and base the regex on it's message
                    && Regex.IsMatch(ex.InnerException.Message, @"^The best overloaded method match for 'System.Collections.Generic.List<.*>.Add(.*)' has some invalid arguments$")
                );
            }

            ImplicitCastCache.UpdateCache(key, result);
            return result;
        }

        private static void AttemptImplicitCast<TFrom, TTo>()
        {
            // based on the IL produced by:
            // dynamic list = new List<TTo>();
            // list.Add(default(TFrom));
            // We can't use the above code because it will mimic a cast in a generic method
            // which doesn't have the same semantics as a cast in a non-generic method

            var list = new List<TTo>(capacity: 1);
            var binder = CSharpBinder.InvokeMember(
                flags: CSharpBinderFlags.ResultDiscarded, 
                name: "Add", 
                typeArguments: null, 
                context: typeof(TypeHelpers),
                argumentInfo: new[] 
                { 
                    CSharpArgumentInfo.Create(flags: CSharpArgumentInfoFlags.None, name: null), 
                    CSharpArgumentInfo.Create(
                        flags: CSharpArgumentInfoFlags.UseCompileTimeType, 
                        name: null
                    ),
                }
            );
            var callSite = CallSite<Action<CallSite, object, TFrom>>.Create(binder);
            callSite.Target.Invoke(callSite, list, default(TFrom));
        }
        #endregion

        #region ---- Caching ----
        private const int MaxCacheSize = 5000;
        private static readonly Dictionary<KeyValuePair<Type, Type>, bool> CastCache = new Dictionary<KeyValuePair<Type, Type>, bool>(),
            ImplicitCastCache = new Dictionary<KeyValuePair<Type, Type>, bool>();

        private static bool TryGetCachedValue<TKey, TValue>(this Dictionary<TKey, TValue> cache, TKey key, out TValue value)
        {
            lock (cache.As<ICollection>().SyncRoot)
            {
                return cache.TryGetValue(key, out value);
            }
        }

        private static void UpdateCache<TKey, TValue>(this Dictionary<TKey, TValue> cache, TKey key, TValue value)
        {
            lock (cache.As<ICollection>().SyncRoot)
            {
                if (cache.Count > MaxCacheSize)
                {
                    cache.Clear();
                }
                cache[key] = value;
            }
        }
        #endregion
    }
}

// ************** Tests **************

namespace CodeDucky
{
    using Microsoft.CSharp;
    using Microsoft.VisualStudio.TestTools.UnitTesting;
    using System;
    using System.CodeDom.Compiler;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;

    [TestClass]
    public class ConversionsTest
    {
        [TestMethod]
        public void ImplicitlyCastable()
        {
            this.RunTests((from, to) => from.IsImplicitlyCastableTo(to), @implicit: true);
        }

        [TestMethod]
        public void ExplicitlyCastable()
        {
            this.RunTests((from, to) => from.IsCastableTo(to), @implicit: false);
        }

        /// <summary>
        /// Validates the given implementation function for either implicit or explicit conversion
        /// </summary>
        private void RunTests(Func<Type, Type, bool> func, bool @implicit)
        {
            // gather types
            var primitives = typeof(object).Assembly.GetTypes().Where(t => t.IsPrimitive).ToArray();
            var simpleTypes = new[] { typeof(string), typeof(DateTime), typeof(decimal), typeof(object), typeof(DateTimeOffset), typeof(TimeSpan), typeof(StringSplitOptions), typeof(DateTimeKind) };
            var variantTypes = new[] { typeof(string[]), typeof(object[]), typeof(IEnumerable<string>), typeof(IEnumerable<object>), typeof(Func<string>), typeof(Func<object>), typeof(Action<string>), typeof(Action<object>) };
            var conversionOperators = new[] { typeof(Operators), typeof(Operators2), typeof(DerivedOperators), typeof(OperatorsStruct) };
            var typesToConsider = primitives.Concat(simpleTypes).Concat(variantTypes).Concat(conversionOperators).ToArray();
            var allTypesToConsider = typesToConsider.Concat(typesToConsider.Where(t => t.IsValueType).Select(t => typeof(Nullable<>).MakeGenericType(t)));

            // generate test cases
            var cases = this.GenerateTestCases(allTypesToConsider, @implicit);

            // collect errors
            var mistakes = new List<string>();
            foreach (var @case in cases)
            {
                var result = func(@case.Item1, @case.Item2);
                if (result != (@case.Item3 == null))
                {
                   // func(@case.Item1, @case.Item2); // break here for easy debugging
                    mistakes.Add(string.Format("{0} => {1}: got {2} for {3} cast", @case.Item1, @case.Item2, result, @implicit ? "implicit" : "explicit"));
                }
            }
            Assert.IsTrue(mistakes.Count == 0, string.Join(Environment.NewLine, new[] { mistakes.Count + " errors" }.Concat(mistakes)));
        }

        private List<Tuple<Type, Type, CompilerError>> GenerateTestCases(IEnumerable<Type> types, bool @implicit)
        {
            // gather all pairs
            var typeCrossProduct = types.SelectMany(t => types, (from, to) => new { from, to })
                .Select((t, index) => new { t.from, t.to, index })
                .ToArray();

            // create the code to pass to the compiler
            var code = string.Join(
                Environment.NewLine,
                new[] { "namespace A { public class B { static T Get<T>() { return default(T); } public void C() {" }
                .Concat(typeCrossProduct.Select(t => string.Format("{0} var{1} = {2}Get<{3}>();", GetName(t.to), t.index, @implicit ? string.Empty : "(" + GetName(t.to) + ")", GetName(t.from))))
                    .Concat(new[] { "}}}" })
            );                

            // compile the code
            var provider = new CSharpCodeProvider();
            var compilerParams = new CompilerParameters();
            compilerParams.ReferencedAssemblies.Add(this.GetType().Assembly.Location); // reference the current assembly!
            compilerParams.GenerateExecutable = false;
            compilerParams.GenerateInMemory = true;
            var compilationResult = provider.CompileAssemblyFromSource(compilerParams, code);

            // determine the outcome of each conversion by matching compiler errors with conversions by line #
            var cases = typeCrossProduct.GroupJoin(
                    compilationResult.Errors.Cast<CompilerError>(),
                    t => t.index,
                    e => e.Line - 2,
                    (t, e) => Tuple.Create(t.from, t.to, e.FirstOrDefault())
                )
                .ToList();

            // add a special case
            // this can't be verified by the normal means, since it's a private class
            cases.Add(Tuple.Create(typeof(PrivateOperators), typeof(int), default(CompilerError)));

            return cases;
        }

        /// <summary>
        /// Gets a C# name for the given type
        /// </summary>
        private static string GetName(Type type)
        {
            if (!type.IsGenericType)
            {
                return type.ToString();
            }

            return string.Format("{0}.{1}<{2}>", type.Namespace, type.Name.Substring(0, type.Name.IndexOf('`')), string.Join(", ", type.GetGenericArguments().Select(GetName)));
        }

        private class PrivateOperators
        {
            public static implicit operator int(PrivateOperators o)
            {
                return 1;
            }
        }
    }

    public class Operators
    {
        public static implicit operator string(Operators o)
        {
            throw new NotImplementedException();
        }

        public static implicit operator int(Operators o)
        {
            return 1;
        }

        public static explicit operator decimal?(Operators o)
        {
            throw new NotImplementedException();
        }

        public static explicit operator StringSplitOptions(Operators o)
        {
            return StringSplitOptions.RemoveEmptyEntries;
        }
    }

    public class DerivedOperators : Operators 
    {
        public static explicit operator DateTime(DerivedOperators o)
        {
            return DateTime.Now;
        }
    }

    public struct OperatorsStruct
    {
        public static implicit operator string(OperatorsStruct o)
        {
            throw new NotImplementedException();
        }

        public static implicit operator int(OperatorsStruct o)
        {
            return 1;
        }

        public static explicit operator decimal?(OperatorsStruct o)
        {
            throw new NotImplementedException();
        }

        public static explicit operator StringSplitOptions(OperatorsStruct o)
        {
            return StringSplitOptions.RemoveEmptyEntries;
        }
    }

    public class Operators2
    {
        public static explicit operator bool(Operators2 o)
        {
            return false;
        }

        public static implicit operator Operators2(DerivedOperators o)
        {
            return null;
        }

        public static explicit operator Operators2(int i)
        {
            throw new NotImplementedException();
        }
    }
}

This is great! Would you consider putting it in a nuget package?

@BrannJoly I'm actually in the progress of working on one. See https://github.com/madelson/MedallionUtilities/tree/master/MedallionReflection

Have you tried this in .NET Core? I was getting an exception in this block:

ReflectionHelper.GetMethod(() => AttemptImplicitCast<object, object>())
	.GetGenericMethodDefinition()
	.MakeGenericMethod(from, to) // this line
	.Invoke(null, new object[0]);

"System.BadImageFormatException : An attempt was made to load a program with an incorrect format. (0x8007000B)". In the specific use case I had, from is a string (or more specifically, a ReadOnlySpan<char>) and to is an integer.

@howcheng Interesting. It's because ReadOnlySpan<char> is a ref struct type so you can't use it as a generic argument. Since dynamic doesn't work with ref structs, we'd need to fork the implementation upon detecting IsByRefType if we wanted it to work.

I imagine that in that case we're reduced to reflecting for op_implicit/op_explicit static methods on each type :-(

I've filed an issue to fix this in the library version here: madelson/MedallionUtilities#6