确定反射类型是否可以转换为另一种反射类型

Question

在 .net (C#) 中，如果您通过反射发现了两种类型，是否可以确定一种是否可以转换为另一种？（隐式和/或显式）。

我想要做的是创建一个库，允许用户指定将一种类型的属性映射到另一种类型的属性。如果这两个属性具有匹配的类型，一切都很好，但我希望能够允许它们映射可以使用隐式/显式转换的属性。所以如果他们有

class from  
{
  public int IntProp{get;set;}
}

class to
{
  public long LongProp{get;set;}
  public DateTime DateTimeProp{get;set;}
}

他们可以说 from.IntProp 将被分配给 to.LongProp （因为存在隐式转换）。但是，如果他们说它映射到 DateTimeProp，我将能够确定没有可用的强制转换并抛出异常。

Answer 1

public static bool HasConversionOperator( Type from, Type to )
        {
            Func<Expression, UnaryExpression> bodyFunction = body => Expression.Convert( body, to );
            ParameterExpression inp = Expression.Parameter( from, "inp" );
            try
            {
                // If this succeeds then we can cast 'from' type to 'to' type using implicit coercion
                Expression.Lambda( bodyFunction( inp ), inp ).Compile();
                return true;
            }
            catch( InvalidOperationException )
            {
                return false;
            }
        }

这应该可以解决隐式和显式转换（包括数字类型、类等）

Answer 2

这是一个不太漂亮的实现，但我相信它涵盖了所有情况（隐式/显式运算符、可空装箱/拆箱、原始类型转换、标准转换）。请注意，说转换可能会成功与转换将会成功（这几乎不可能确定）之间是有区别的。有关更多详细信息、全面的单元测试和隐式版本，请在此处查看我的帖子。

public static bool IsCastableTo(this Type from, Type to)
{
    // from https://web.archive.org/web/20141017005721/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;
    }

    // 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);
    }

    if (from.IsValueType)
    {
        try
        {
            ReflectionHelpers.GetMethod(() => AttemptExplicitCast<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, @"^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
        return from.IsNonValueTypeExplicitlyCastableTo(to);
    }
}

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 = Microsoft.CSharp.RuntimeBinder.Binder.Convert(CSharpBinderFlags.ConvertExplicit, typeof(TTo), typeof(TypeHelpers));
    var callSite = CallSite<Func<CallSite, TFrom, TTo>>.Create(binder);
    callSite.Target(callSite, default(TFrom));
}

Answer 3

最好研究一下 TypeConverter。