Question

Is there any way in which you can test whether an instance of a generic parameter in a generic method has been assigned to when it can be either a value or reference type? I would like to be able to do this in a generic method I have to persist types, where T is the instance and K is the type of the identifier field for that type (which all objects I am persisting have because they inherit from a base type). I do not want to limit K to being a value type. The code is something like this:

public static bool Save<T, K>(T instance) {     // variable to hold object identifer     K instanceId = default(K);      PropertyInfo[] properties = typeof(T).GetProperties();      // loop through properties of the T     // if property is decorated with a specific attribute then assign to instanceId     // end loop      // check that we have a value assigned to instanceId other than default(K)     // if not return false otherwise continue to persist item } 

As K can be a value type checking if it is equal to default(K) results in an error as it relies on it being comparable. Is there a way around this?

Note that I have got around the need for this in the current case by placing a condition on the generic type T that it must inherit from the base type BaseObject so my question is about the general issue around generics and testing assignment.

Answer 1

If you’re going to read from instanceId later, it has to be definitely assigned from a compiler point of view. I would assign it with default(K) and separately have a flag saying whether or not it’s been given a useful value:

public static bool Save<T, K>(T instance) {     bool idAssigned = false;     // variable to hold object identifer     K instanceId = default(K)      PropertyInfo[] properties = typeof(T).GetProperties();      foreach(PropertyInfo property in properties)     {         if (SomeCondition(property))         {             instanceId = GetId(property);             idAssigned = true;         }     }      if (!idAssigned)     {         return false;     }     Persist(whatever);     return true; } 

EDIT: Comparing the value of instanceId with any specific value is a non-starter, unless you know for a fact that the value will never be used for a ‘normal’ value explicitly assigned to instanceId.

Basically you’ve got two bits of information here, so keep them separately – the flag is the way to go.