.NET : How do you get the Type of a null object?

Question

I have a method with an out parameter that tries to do a type conversion. Basically:

public void GetParameterValue(out object destination)
{
    object param         


        

Answer 1

So is there any way to get the type of an object that is set to null? I would think there would have to be a way to know what type a storage location is without it being assigned anything.

Not necessarily. The best that you can say is that it is an object. A null reference does not point to any storage location, so there is no metadata from which it can make that determination.

The best that you could do is change it to be more generic, as in:

public void GetParameterValue<T>(out T destination)
{
    object paramVal = "Blah";
    destination = default(T);
    destination = Convert.ChangeType(paramVal, typeof(T));
}

The type of T can be inferred, so you shouldn't need to give a type parameter to the method explicitly.

Answer 2

I don't think it is possible to get the type when the value is null. Also, since you are calling inside GetParameterValue, the best you could do (when the value is null) is to get the type of the "destination" parameter which is "object". You might consider passing the Type as a parameter to GetParameterValue where you have more information, such as:

public void GetParameterValue(Type sourceType, out object destination) { //... }

Answer 3

The type of your destination variable is always System.Object. You could just return

Convert.ChangeType(paramVal, System.Object).

Answer 4

At a theoretical level isn't a null really the same as a void pointer in C, which is to say that it holds a memory address and that's it? If so then it is similar to the case of a division by zero in Mathematics where the result is undefined.

One could do the following for this line:

string val = GetParameterValue<string>("parameterName");

Just remove that first string and now there isn't the repetition:

var val = GetParameterValue<string>("parameterName");

Not necessarily what you are looking for, though there is the question of how does one interpret null?

Answer 5

If there is no instance, there is no instance type.

The best you can do is use the type of the reference, which means if you have an object reference (as in the method in the question), the reference type is object.

---

You probably shouldn't be trying to convert a null instance of one type into a null instance of another type...

Answer 6

Currently you have no way of knowing what gets passed into the method. You can convert it into a generic method. Like this:

public void GetParameterValue<T>(out T destination) { ... }