Determine if reflected property can be assigned null

Question

I wish to automagically discover some information on a provided class to do something akin to form entry. Specifically I am using reflection to return a PropertyInfo value f

Answer 1

PropertyInfo propertyInfo = ...
bool canAssignNull = 
    !propertyInfo.PropertyType.IsValueType || 
    propertyInfo.PropertyType.IsGenericType &&
        propertyInfo.PropertyType.GetGenericTypeDefinition() == typeof(Nullable<>)

Answer 2

From http://msdn.microsoft.com/en-us/library/ms366789.aspx

if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))

Type would be your PropertyInfo.PropertyType

Answer 3

Marc and Jonas both have parts to determine if a generic type can be assigned null.

// A silly example. default(T) will return null if it is nullable. So no reason to check here. Except for the sake of having an example.
public U AssignValueOrDefault<U>(object item)
{
    if (item == null)
    {
        Type type = typeof(U); // Type from Generic Parameter

        // Basic Types like int, bool, struct, ... can't be null
        //   Except int?, bool?, Nullable<int>, ...
        bool notNullable = type.IsValueType ||
                           (type.IsGenericType && type.GetGenericTypeDefinition() != typeof(Nullable<>)));

        if (notNullable)
            return default(T);
    }

    return (U)item;
}

Note: Most of the time you can check if the variable is null. Then use default(T). It will return null by default of the object is a class.

Answer 4

You need to handle null references and Nullable<T>, so (in turn):

bool canBeNull = !type.IsValueType || (Nullable.GetUnderlyingType(type) != null);

Note that IsByRef is something different, that allows you to choose between int and ref int / out int.