问题
At the run-time I get boxed instance of some type. How to unbox it to underlying type?
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;
回答1:
If you don't know the type at compile time, then you can't unbox because you have nowhere to put it - all you can do is store it in an object, which is: boxed.
The same also applies to reference-types like string: you can't cast it to the right type if you don't know the type at compile time: you have nowhere to put it.
You can special-case a few types, for example:
if(obj is int) {
int i = (int)obj;
...
} ...
Another trick that is sometimes (not often) helpful is to switch into generics; then instead of talking in terms of object you are talking in terms of T. This has... limited use though. The easiest way to do that is via dynamic, for example:
dynamic obj = ...
Foo(obj);
...
Foo<T>(T val) { ... code with T ... }
you can also add special cases to that appreach:
Foo(string val) { ... code with string ...}
Foo(int val) { ... code with int ...}
However, frankly I suggest it may be better to look hard at what you are trying to do.
回答2:
Now lets suppose, that real boxing occur:
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
Remark, if you substitute:
object convertedBack = Convert.ChangeType(o, type);
Console.WriteLine (convertedBack); //it still prints 5
Console.WriteLine (o); //it even print 5 here
The reason is that underlying object is still int. I've just used this example to show you, that boxing is irrelevant here. You need to rely on some abstraction in your operations and if you want to cast to int dynamically, what reference type do you wan to use.
回答3:
In such cases I'm going to use the strategy pattern by using a Dictionary<Type, Action<object>>:
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);
}
}
回答4:
Here's an example of exactly why you would do this:
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);
}
The reason for this is because the value is a boxed object... at runtime you do not know the type, so you would have to unbox to the prop.PropertyType
回答5:
A pragmatic solution; try and use a TypeConverter directly, and if that fails, convert to string and back again:-
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;
}
回答6:
Use expressions:
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);
}
回答7:
public static string GetType(object data)
{
Type type = data.GetType();
return Convert.ChangeType(data, type).GetType().Name;
}
Hi,this method receives object data and returns string type name of object. Hope this is what you need.
回答8:
you can try using the dynamic runtime
[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";
}
来源：https://stackoverflow.com/questions/15565196/how-to-unbox-from-object-to-type-it-contains-not-knowing-that-type-at-compile-t