Check to see if a given object (reference or value type) is equal to its default

Question

I'm trying to find a way to check and see if the value of a given object is equal to its default value. I've looked around and come up with this:

    public static bool IsNullOrDefault<T>(T argument)     {         if (argument is ValueType || argument != null)         {             return object.Equals(argument, default(T));         }         return true;     } 

The problem I'm having is that I want to call it like this:

            object o = 0;             bool b = Utility.Utility.IsNullOrDefault(o); 

Yes o is an object, but I want to make it figure out the base type and check the default value of that. The base type, in this case, is an integer and I want to know in this case if the value is equal to default(int), not default(object).

I'm starting to think this might not be possible.

Answer 1

In your example, your integer is boxed and therefore your T is going to be object, and the default of object is null, so that's not valuable to you. If the object is a value type, you could get an instance of it (which would be the default) to use as a comparison. Something like:

if (argument is ValueType) {    object obj = Activator.CreateInstance(argument.GetType());    return obj.Equals(argument); } 

You'd want to deal with other possibilities before resorting to this. Marc Gravell's answer brings up some good points to consider, but for a full version of your method, you might have

public static bool IsNullOrDefault<T>(T argument) {     // deal with normal scenarios     if (argument == null) return true;     if (object.Equals(argument, default(T))) return true;      // deal with non-null nullables     Type methodType = typeof(T);     if (Nullable.GetUnderlyingType(methodType) != null) return false;      // deal with boxed value types     Type argumentType = argument.GetType();     if (argumentType.IsValueType && argumentType != methodType)      {         object obj = Activator.CreateInstance(argument.GetType());         return obj.Equals(argument);     }      return false; } 

Answer 2

if o is null, in a non-generic (object) method, you will have no access to the original type - and you can't do much about that.

Hence, the only time it matters is non-nullable value-types, so:

Type type = value.GetType(); if(!type.IsValueType) return false; // can't be, as would be null if(Nullable.GetUnderlyingType(type) != null) return false; // ditto, Nullable<T> object defaultValue = Activator.CreateInstance(type); // must exist for structs return value.Equals(defaultValue);