如何判断 A 型是否可隐式转换为 B 型

Question

给定类型 a 和类型 b，我如何在运行时确定是否存在从 a 到 b 的隐式转换？

如果这没有意义，请考虑以下方法：

public PropertyInfo GetCompatibleProperty<T>(object instance, string propertyName)
{
   var property = instance.GetType().GetProperty(propertyName);

   bool isCompatibleProperty = !property.PropertyType.IsAssignableFrom(typeof(T));
   if (!isCompatibleProperty) throw new Exception("OH NOES!!!");

   return property;   
}

这是我想要工作的调用代码：

// Since string.Length is an int property, and ints are convertible
// to double, this should work, but it doesn't. :-(
var property = GetCompatibleProperty<double>("someStringHere", "Length");

Answer 1

请注意，IsAssignableFrom 不能解决您的问题。你必须像这样使用反射。注意处理原始类型的明确需要；这些列表符合规范的第 6.1.2 节（隐式数字转换）。

static class TypeExtensions { 
    static Dictionary<Type, List<Type>> dict = new Dictionary<Type, List<Type>>() {
        { typeof(decimal), new List<Type> { typeof(sbyte), typeof(byte), typeof(short), typeof(ushort), typeof(int), typeof(uint), typeof(long), typeof(ulong), typeof(char) } },
        { typeof(double), new List<Type> { typeof(sbyte), typeof(byte), typeof(short), typeof(ushort), typeof(int), typeof(uint), typeof(long), typeof(ulong), typeof(char), typeof(float) } },
        { typeof(float), new List<Type> { typeof(sbyte), typeof(byte), typeof(short), typeof(ushort), typeof(int), typeof(uint), typeof(long), typeof(ulong), typeof(char), typeof(float) } },
        { typeof(ulong), new List<Type> { typeof(byte), typeof(ushort), typeof(uint), typeof(char) } },
        { typeof(long), new List<Type> { typeof(sbyte), typeof(byte), typeof(short), typeof(ushort), typeof(int), typeof(uint), typeof(char) } },
        { typeof(uint), new List<Type> { typeof(byte), typeof(ushort), typeof(char) } },
        { typeof(int), new List<Type> { typeof(sbyte), typeof(byte), typeof(short), typeof(ushort), typeof(char) } },
        { typeof(ushort), new List<Type> { typeof(byte), typeof(char) } },
        { typeof(short), new List<Type> { typeof(byte) } }
    };
    public static bool IsCastableTo(this Type from, Type to) { 
        if (to.IsAssignableFrom(from)) { 
            return true; 
        }
        if (dict.ContainsKey(to) && dict[to].Contains(from)) {
            return true;
        }
        bool castable = from.GetMethods(BindingFlags.Public | BindingFlags.Static) 
                        .Any( 
                            m => m.ReturnType == to &&  
                            (m.Name == "op_Implicit" ||  
                            m.Name == "op_Explicit")
                        ); 
        return castable; 
    } 
} 

用法：

bool b = typeof(A).IsCastableTo(typeof(B));

Answer 2

您需要考虑的隐式转换：

• 身份
• sbyte 到 short、int、long、float、double 或 decimal
• byte 到 short、ushort、int、uint、long、ulong、float、double 或 decimal
• 短于 int、long、float、double 或 decimal
• ushort 到 int、uint、long、ulong、float、double 或 decimal
• int 到 long、float、double 或 decimal
• uint 到 long、ulong、float、double 或 decimal
• 长浮点数、双精度数或十进制数
• ulong 为浮点数、双精度数或十进制数
• char 到 ushort、int、uint、long、ulong、float、double 或 decimal
• 浮点数加倍
• 可空类型转换
• 对象的引用类型
• 派生类到基类
• 实现接口的类
• 基本接口的接口
• 当数组具有相同的维数时，从源元素类型到目标元素类型存在隐式转换，并且源元素类型和目标元素类型是引用类型时，数组到数组
• System.Array 的数组类型
• IList 及其基接口的数组类型
• 将类型委托给 System.Delegate
• 拳击转换
• 枚举类型为 System.Enum
• 用户定义的转换 (op_implicit)

我假设您正在寻找后者。您需要编写类似于编译器的东西来涵盖所有这些。值得注意的是 System.Linq.Expressions.Expression 并没有尝试这一壮举。

Answer 3

这个问题的公认答案可以处理很多情况，但不是全部。例如，这里只是一些没有正确处理的有效转换/转换：

// explicit
var a = (byte)2;
var b = (decimal?)2M;

// implicit
double? c = (byte)2;
decimal? d = 4L;

下面，我发布了这个函数的另一个版本，它专门回答了隐式转换和转换的问题。更多细节，我用来验证它的测试套件，以及显式转换版本，请查看my post on the subject。

public static bool IsImplicitlyCastableTo(this Type from, Type to)
{
    if (from == null) { throw new ArgumentNullException(nameof(from)); }
    if (to == null) { throw new ArgumentNullException(nameof(to)); }

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

    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]);
        return true;
    }
    catch (TargetInvocationException ex)
    {
        return = !(
            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$")
        );
    }
}

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 = Microsoft.CSharp.RuntimeBinder.Binder.InvokeMember(
        flags: CSharpBinderFlags.ResultDiscarded, 
        name: "Add", 
        typeArguments: null, 
        context: typeof(TypeHelpers), // the current type
        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));
}

Answer 4

这是一个通过以下所有测试的方法：

[Test] public void TestImplicitlyCastable()
{
    Assert.That( typeof(byte)    .IsImplicitlyCastableTo(typeof(short)));
    Assert.That( typeof(byte)    .IsImplicitlyCastableTo(typeof(byte?)));
    Assert.That( typeof(byte)    .IsImplicitlyCastableTo(typeof(long?)));
    Assert.That(!typeof(short)   .IsImplicitlyCastableTo(typeof(uint)));
    Assert.That( typeof(long)    .IsImplicitlyCastableTo(typeof(float)));
    Assert.That( typeof(long)    .IsImplicitlyCastableTo(typeof(decimal)));
    Assert.That(!typeof(double)  .IsImplicitlyCastableTo(typeof(decimal)));
    Assert.That(!typeof(decimal) .IsImplicitlyCastableTo(typeof(double)));
    Assert.That( typeof(List<int>).IsImplicitlyCastableTo(typeof(object)));
    Assert.That( typeof(float)   .IsImplicitlyCastableTo(typeof(IComparable<float>)));
    Assert.That( typeof(long?)   .IsImplicitlyCastableTo(typeof(IComparable<long>)));
    Assert.That(!typeof(object)  .IsImplicitlyCastableTo(typeof(string)));
    Assert.That( typeof(string[]).IsImplicitlyCastableTo(typeof(object[])));
    Assert.That( typeof(Foo)     .IsImplicitlyCastableTo(typeof(int)));
    Assert.That(!typeof(Foo)     .IsImplicitlyCastableTo(typeof(uint)));
    Assert.That( typeof(Foo)     .IsImplicitlyCastableTo(typeof(long)));
    Assert.That( typeof(Foo)     .IsImplicitlyCastableTo(typeof(long?)));
}
class Foo
{
    public static implicit operator int(Foo f) => 42;
}

它基于 dynamic 的一个技巧，灵感来自 ChaseMedallion 的回答：

using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Reflection;
using Microsoft.CSharp.RuntimeBinder;

public static class ReflectionHelpers
{
    [ThreadStatic]
    static readonly Dictionary<KeyValuePair<Type, Type>, bool> ImplicitCastCache;

    /// <summary>Returns true iff casting between values of the specified 
    /// types is possible based on the rules of C#.</summary>
    public static bool IsImplicitlyCastableTo(this Type from, Type to)
    {
        if (from == to)
            return true;

        var key = new KeyValuePair<Type, Type>(from, to);
        ImplicitCastCache ??= new Dictionary<KeyValuePair<Type, Type>, bool>();
        if (ImplicitCastCache.TryGetValue(key, out bool result))
            return result;

        if (to.IsAssignableFrom(from))
            return ImplicitCastCache[key] = true;

        var method = GetMethodInfo(() => IsImplicitlyCastableCore<int, int>())
            .GetGenericMethodDefinition().MakeGenericMethod(from, to);
        return ImplicitCastCache[key] = (bool)method.Invoke(null, Array.Empty<object>());
    }

    static bool IsImplicitlyCastableCore<TFrom,TTo>()
    {
        var testObject = new LinkedListNode<TTo>(default(TTo));
        try {
            ((dynamic)testObject).Value = default(TFrom);
            return true;
        } catch (Exception e) {
            // e.g. "Cannot implicitly convert type 'A' to 'B'. An explicit conversion exists (are you missing a cast?)"
            // The exception may be caused either because no conversion is available,
            // OR because it IS available but the conversion method threw something.
            // Assume RuntimeBinderException means the conversion does not exist.
            return !(e is RuntimeBinderException); 
        }
    }

    /// <summary><c>GetMethodInfo(() => M(args))</c> gets the MethodInfo object corresponding to M.</summary>
    public static MethodInfo GetMethodInfo(Expression<Action> shape) => ((MethodCallExpression)shape.Body).Method;
}