Question

在运行时我得到某种类型的盒装实例。如何将其拆箱到基础类型？

Object obj;
String variable = "Some text";

obj = variable // boxing;

// explicit unboxing, because we know the type of variable at compile time.

var x = (String)obj     

// Now let's pretend that we don't know the type of underlying object at compile time. 

Type desiredType = obj.GetType(); // But we can figure out.

//And now the question. 
//How to express something like this:

var y = (desiredType)obj; //Need to get unboxed instance of initial variable here;

Answer 1

如果您在编译时不知道类型，那么您无法取消装箱，因为您无处可放 - 您所能做的就是存储它在object中，即：盒装。

同样适用于像string这样的引用类型：如果在编译时不知道类型，则无法将其转换为正确的类型：无处可放< / em>的

你可以特殊情况下的几种类型，例如：

if(obj is int) { int i = (int)obj; ... } ...

另一个有时（不常见）有用的技巧是切换到 generics ;然后，不是用object来谈论T，而是用dynamic obj = ... Foo(obj); ... Foo<T>(T val) { ... code with T ... }来谈论。这有......限制使用。最简单的方法是通过动态，例如：

Foo(string val) { ... code with string ...} Foo(int val) { ... code with int ...}

您还可以为该appreach添加特殊情况：

{{1}}

但是，坦率地说，我建议最好仔细看看你想要做什么。

Answer 2

现在让我们假设真正的拳击发生：

int v = 5;

object o = v; //boxed 

Type type = o.GetType(); //will return typeof(int)

int convertedBack = (int)Convert.ChangeType(o, type);

Console.WriteLine (convertedBack); //prints 5

备注，如果您替换：

object convertedBack = Convert.ChangeType(o, type);

Console.WriteLine (convertedBack); //it still prints 5
Console.WriteLine (o); //it even print 5 here

原因是底层对象仍然是int。我刚用这个例子告诉你，拳击在这里无关紧要。你需要在操作中依赖一些抽象，如果你想动态地转换为int，你想使用哪种引用类型。

Answer 3

在这种情况下，我将使用Dictionary<Type, Action<object>>：

来使用策略模式

internal class Program
{
    private static void Main(string[] args)
    {
        var something = new Something();

        something.ComputeValue(13);
        something.ComputeValue(DateTime.Now);
        something.ComputeValue(DayOfWeek.Monday);

        Console.ReadKey();
    }
}

internal class Something
{
    private static Dictionary<Type, Action<object>> _Strategies;

    static Something()
    {
        // Prepare all available strategies.
        _Strategies = new Dictionary<Type, Action<object>>();
        _Strategies.Add(typeof(int), ComputeInteger);
        _Strategies.Add(typeof(DateTime), ComputeDateTime);
    }

    public void ComputeValue(object value)
    {
        Action<object> action;

        // Check if we have a matching strategy.
        if (!_Strategies.TryGetValue(value.GetType(), out action))
        {
            // If not, log error, throw exception, whatever.
            action = LogUnknownType;
        }

        // Perform the matching strategy on the given value.
        action(value);
    }

    private static void ComputeDateTime(object source)
    {
        // We get an object, but we are sure that it will always be an DateTime.
        var value = (DateTime)source;
        Console.WriteLine("We've got an date time: " + value);
    }

    private static void ComputeInteger(object source)
    {
        // We get an object, but we are sure that it will always be an int.
        var value = (int)source;
        Console.WriteLine("We've got an integer: " + value);
    }

    private static void LogUnknownType(object source)
    {
        // What should we do with the drunken sailor?
        var unknownType = source.GetType();
        Console.WriteLine("Don't know how to handle " + unknownType.FullName);
    }
}

Answer 4

这是一个确切原因的例子：

class MyClass
{
   public int Id {get;set;}
   public string Name {get;set;}
   public decimal Val {get;set;}
}
int i = 0;
var myClassImp = new MyClass();
foreach (var val in new [object]{"10", "My name", "100.21"} // Could be read from some data source, such as an excel spreadsheet
{
   var prop = typeof(MyClass).GetProperties().ElementAt(i++);
   // !!!!!! THROWS EXCEPTION  !!!!!!!
   prop.SetValue(myClassImp, System.Convert.ChangeType(val, prop.PropertyType), null);
}

原因是因为该值是一个盒装对象...在运行时你不知道类型，所以你必须取消装箱到prop.PropertyType

Answer 5

务实的解决方案;尝试直接使用TypeConverter，如果失败，转换为字符串并再次返回： -

private static T GetValueOfType<T>(this ManagementBaseObject MBO, String FieldName) {
    T lResult;

    try {
        Object lObj = MBO[FieldName];

        var lSrcType = lObj.GetType();
        var lDestType = typeof(T);

        if (lDestType.IsValueType && lDestType.IsAssignableFrom(lSrcType)) {
            lResult = (T)lObj;
            return lResult;
        }

        var lDestTC = TypeDescriptor.GetConverter(typeof(T));
        if (lDestTC.CanConvertFrom(lSrcType)) {
            lResult = (T)lDestTC.ConvertFrom(lObj);
        } else {
            var lSrcTC = TypeDescriptor.GetConverter(lSrcType);
            String lTmp = lSrcTC.ConvertToInvariantString(lObj);
            lResult = (T)lDestTC.ConvertFromInvariantString(lTmp);
        }
    } catch {
        lResult = default(T);
    }
    return lResult;
}

Answer 6

使用表达式：

var y = DynamicCast（obj，desiredType）;

static object DynamicCast(object source, Type type)
{
    var parameter = Expression.Parameter(typeof(object), "input");

    var cast = Expression.TypeAs(Expression.Convert(parameter, type), typeof(object));

    var lambda = Expression.Lambda<Func<object, object>>(cast, parameter);

    var func = lambda.Compile();

    return func(source);
}

Answer 7

public static string GetType(object data)
{
    Type type = data.GetType();
    return Convert.ChangeType(data, type).GetType().Name;
}

嗨，此方法接收对象数据并返回对象的字符串类型名称。希望这就是你所需要的。

Answer 8

您可以尝试使用动态运行时

    [Test]
    public void Test_UnboxingAtRuntime()
    {
        object boxed = "Hello";

        //this line is commented out as it does not compile
        // OverloadedMethod(boxed);

        var result = CallCorrectMethod(boxed);
        Assert.That(result, Is.EqualTo("string"));

        boxed = 1;
        result = CallCorrectMethod(boxed);
        Assert.That(result, Is.EqualTo("int"));
    }

    public string CallCorrectMethod(dynamic t)
    {
        return OverloadedMethod(t);
    }

    public string OverloadedMethod(string s)
    {
        return "string";
    }

    public string OverloadedMethod(int s)
    {
        return "int";
    }

如何从对象中取消对它包含的类型，在编译时不知道该类型？

8 个答案: