How to test whether a value is boxed in C# / .NET?

Question

I\'m looking for a way to write code that tests whether a value is boxed.

My preliminary investigations indicate that .NET goes out of its way to conceal the fact,

Answer 1

GetType() and IsValueType don't reveal the difference between a boxed value and an unboxed value.

GetType is a sealed (non-virtual) method on System.Object. Calling this method on a value-type will definitely box it. Not even Nullable<T> is able to get around this - calling GetType on a nullable will return the underlying type if it has a value (boxed as the underlying type) or throw a NullReferenceException if it doesn't (boxed to null, can't dereference a null-reference).

when looking at any variable or value, even if its type is "dynamic" or "object," whether it's boxed or not boxed.

In general, if you have an expression that "holds" a value-type, the value of that expression will be a reference to a box unless the expression's compile-time type is of the value-type itself (generics are slightly more complicated). Common reference-types that can hold references to boxed structures are object, dynamic and interface-types.

Answer 2

This approach is similar to Jared Par's answer. But I think !typeof(T).IsValueTypeis cleaner than enumerating all types which could contain a boxed value.

public static bool IsBoxed<T>(T value)
{
    return !typeof(T).IsValueType && (value != null) && value.GetType().IsValueType;
}

Unlike Jared's code this will handle the case where T is System.ValueType correctly.

Another subtle point is that value.GetType().IsValueType comes after !typeof(T).IsValueType since otherwise GetType() would create a temporary boxed copy of the value.

Answer 3

Here are some simple helper methods to check if a variable is a boxed integer:

public static bool IsBoxed(object item)
{
    return true;
}

public static bool IsBoxed<T>(T item) where T : struct
{
    return false;
}

Just call IsBoxed(...) on your variable:

IsBoxed(o1) // evaluates to true
IsBoxed(i1) // evaluates to false

This accomplishes nothing, of course. Why exactly do you need to know if a value is boxed or not?

Answer 4

I think actually the question is kind of misstated. Isn't the question actually, "How can I tell if an object is a box for another type?"

With reference to Allon's comment, if you have an object of type Object and the object is a primitive value type, it's a box. I'm not certain this is 100% correct, but (similar to Allon's implementation):

// Assume there is some object o.
bool isBoxed = o.GetType().IsPrimitive;

Answer 5

Similar to Allon's answer, but should return the correct answer for any type without generating a compile-time error:

int i = 123;
Console.WriteLine(IsBoxed(i));    // false

object o = 123;
Console.WriteLine(IsBoxed(o));    // true

IComparable c = 123;
Console.WriteLine(IsBoxed(c));    // true

ValueType v = 123;
Console.WriteLine(IsBoxed(v));    // true

int? n1 = 123;
Console.WriteLine(IsBoxed(n1));    // false
int? n2 = null;
Console.WriteLine(IsBoxed(n2));    // false

string s1 = "foo";
Console.WriteLine(IsBoxed(s1));    // false
string s2 = null;
Console.WriteLine(IsBoxed(s2));    // false

// ...

public static bool IsBoxed<T>(T item)
{
    return (item != null) && (default(T) == null) && item.GetType().IsValueType;
}

public static bool IsBoxed<T>(T? item) where T : struct
{
    return false;
}

(Although you could make the argument that the possible compile-time error caused by Allon's code is a feature, not a bug: if you hit a compile-time error then you're definitely not dealing with an unboxed value type!)

Answer 6

I'm not sure if this will be relevant to anyone, but since I encountered this post because boxing was actually impacting my very dynamic mapping.

Sigil proivdes a fantastic UnBoxAny method

Assuming you have the following:

public class Report { public decimal Total { get; set; } }

new Dictionary<string, object> { { "Total", 5m} }

So the decimal value is boxed.

var totalProperty = typeof(Report).GetProperty("Total");

var value = emit.DeclareLocal<object>();

//invoke TryGetValue on dictionary to populate local 'value'*
                                                    //stack: [bool returned-TryGetValue]
//either Pop() or use in If/Else to consume value **
                                                    //stack: 
//load the Report instance to the top of the stack 
//(or create a new Report)
                                                    //stack: [report]
emit.LoadLocal(value);                              //stack: [report] [object value]
emit.UnboxAny(totalProperty.PropertyType);          //stack: [report] [decimal value]

//setter has signature "void (this Report, decimal)" 
//so it consumes two values off the stack and pushes nothing

emit.CallVirtual(totalProperty.SetMethod);          //stack: 

* invoke TryGetValue

** use in If/Else