Reflecting a private field from a base class

You can manually go up in the inheritance chain to get the base fields:

Given these classes:

class SuperClass1
{
    private int myField;
}

class SuperClass2 : SuperClass1
{
}

class MyClass : SuperClass2
{

}

This should work:

var myObj = new MyClass();
var myField = typeof(MyClass).BaseType
                             .BaseType
                             .GetField("myField", BindingFlags.Instance | BindingFlags.NonPublic);

There's a more generic solution in this SO answer: Not getting fields from GetType().GetFields with BindingFlag.Default

In a similar vein to BrokenGlass's solution, you could do this to make it a bit more generic:

class Base { private int _baseField; }
class Derived : Base { }
class Mine : Derived { }

And then:

Type t = typeof(Mine);
FieldInfo fi = null;

while (t != null) 
{
    fi = t.GetField("_baseField", BindingFlags.Instance | BindingFlags.NonPublic);

    if (fi != null) break;

    t = t.BaseType; 
}

if (fi == null)
{
    throw new Exception("Field '_baseField' not found in type hierarchy.");
}

As a utility method:

public static void SetField(object target, string fieldName, object value)
{
    if (target == null)
    {
        throw new ArgumentNullException("target", "The assignment target cannot be null.");
    }

    if (string.IsNullOrEmpty(fieldName))
    {
        throw new ArgumentException("fieldName", "The field name cannot be null or empty.");
    }

    Type t = target.GetType();
    FieldInfo fi = null;

    while (t != null)
    {
        fi = t.GetField(fieldName, BindingFlags.Instance | BindingFlags.NonPublic);

        if (fi != null) break;

        t = t.BaseType; 
    }

    if (fi == null)
    {
        throw new Exception(string.Format("Field '{0}' not found in type hierarchy.", fieldName));
    }

    fi.SetValue(target, value);
}

And then:

Mine m = new Mine();

SetField(m, "_baseField", 10);