如何判断类型 A 是否可以隐式转换为类型 B

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没有解决您的问题。你必须使用反射像这样。注意明确需要处理的基本类型;这些列表是每§6.1.2本说明书的（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 转为short、int、long、float、double 或decimal
• 字节转短整型、ushort、int、uint、long、ulong、float、double 或decimal
• 短整型、长整型、浮点型、双精度型或小数型
• ushort 到 int、uint、long、ulong、float、double 或decimal
• int 到 long、float、double 或decimal
• uint 转 long、ulong、float、double 或decimal
• long 为浮点型、双精度型或小数型
• ulong 为浮点型、双精度型或小数型
• char 到 ushort、int、uint、long、ulong、float、double 或decimal
• 浮动到双倍
• 可空类型转换
• 对象的引用类型
• 派生类到基类
• 类到实现的接口
• 接口到基本接口
• 数组到数组 当数组具有相同维数时，存在从源元素类型到目标元素类型的隐式转换，并且源元素类型和目标元素类型都是引用类型
• 数组类型为 System.Array
• IList<> 的数组类型及其基接口
• System.Delegate 的委托类型
• 拳击转换
• 枚举类型到 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 没有尝试这一壮举。

Answer 3

在接受这个问题的答案处理很多情况下，但不是全部。例如，这里仅仅是一些有效的石膏/未正确地处理的转换：

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

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

下面，我已为这一功能，其特异性应答隐式转换和转换的问题的替代版本。有关详细信息，测试套件我用来验证它，并明确投版本，请检查出的我的交关于这一主题。

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
    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));
}