Get default value of class member ( C# )

Question

Let's assume I have a class ClassWithMember

class ClassWithMember
{
    int myIntMember = 10;
}

How do I get the default value 10 of the myIntMember member by System.Type?

I'm currently struggling around with reflections by all I retreive is the default value of int (0) not the classes default member (10)..

Answer 1

Try creating an instance an retreive the value with reflection.

Answer 2

You can try something like this:

var field = typeof(ClassWithMember).GetField("myIntMember",
    BindingFlags.Instance | BindingFlags.NonPublic);
var value = (int)field.GetValue(new ClassWithMember());

The trick here is to instantiate an instance.

Answer 3

Hi Ruben,

If you're in control of the code for ClassWithMember, you could take a completely different approach to this by using the [DefaultValue] attribute from System.ComponentModel. Basically, what you'd do is write something like this:

class ClassWithMember
{
    public ClassWithMember()
    {
        SetDefaultValues();
    }

    [DefaultValue(5)]
    public MyIntMember { get; set; }
}

And then have a function like this somewhere, perhaps in a base class:

public void SetDefaultValues()
{
    foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(this))
    {
        DefaultValueAttribute a = prop.Attributes[typeof(DefaultValueAttribute)] as DefaultValueAttribute;
        if (a == null) 
            continue;
        prop.SetValue(this, a.Value);
    }
}

So now, you have a situation where you can easily retrieve the default values using Reflection.

Keep in mind that this is going to be quite a lot slower due to the Reflection requirement, so if this code gets instantiated a lot, you'll probably want to find a different approach. Also, it won't work with non-value types, due to a limitation with the .NET Framework's attribute support.