Question

In an application I have some code that has a FieldInfo for a Nullable<int> and I need to retrieve the nullable value (not the underlying value) like in the sample bellow:

class Test
{
    public int? value;
}

public class Program
{
    static void Main(string[] args)
    {
        var obj = new Test { value = 10 };
        var fld = typeof (Test).GetField("value");
        var v = fld.GetValue(obj);

        System.Diagnostics.Debug.WriteLine(v.GetType().FullName);
        System.Diagnostics.Debug.WriteLine(fld.FieldType.FullName);
     }
 }

My problem is that v is always assigned the underlying value (in this sample an int) instead the nullable (in this sample a Nullable<int>).

PS: The real application don’t has the type of the nullable at compile time, so a cast is not possible.

Thanks in advance for any help.

Answer 1

In this case v is of type object. If value is null then v will be null; if value is some integer then v will be that integer. If you want v to actually have the type Nullable<int> then you have to declare it as such: var v = (int?) fld.GetValue(obj);.

If you need to be able to reference v.Value and get the boxed value back, you’ll probably have to record the fact that fld was nullable (Nullable.GetUnderlyingType(fld.FieldType) != null). Note that generics will not help you here because you do not know the T at compile time.

Here’s a helper you can use:

struct NullableObject
{
    public object Value { get; private set; }

    public static object GetField(object Target, FieldInfo Field)
    {
        object value = Field.GetValue(Target);
        if (Nullable.GetUnderlyingType(Field.FieldType) != null)
            return new NullableObject { Value = value };
        return value;
    }
}

public static class NullableHelper
{
    public static object GetNullableValue(this FieldInfo field, object target)
    {
        return NullableObject.GetField(target, field);
    }
}

Then instead of calling var v = fld.GetValue(obj);, say var v = fld.GetNullableValue(obj);. If fld represents a Nullable type, you’ll get an object with a Value property; if not, you’ll just get the value.