Find a private field with Reflection?

Question

Use BindingFlags.NonPublic and BindingFlags.Instance flags

FieldInfo[] fields = myType.GetFields(
                         BindingFlags.NonPublic | 
                         BindingFlags.Instance);

You can do it just like with a property:

FieldInfo fi = typeof(Foo).GetField("_bar", BindingFlags.NonPublic | BindingFlags.Instance);
if (fi.GetCustomAttributes(typeof(SomeAttribute)) != null)
    ...

Get private variable's value using Reflection:

var _barVariable = typeof(Foo).GetField("_bar", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(objectForFooClass);

Set value for private variable using Reflection:

typeof(Foo).GetField("_bar", BindingFlags.NonPublic | BindingFlags.Instance).SetValue(objectForFoocClass, "newValue");

Where objectForFooClass is a non null instance for the class type Foo.

One thing that you need to be aware of when reflecting on private members is that if your application is running in medium trust (as, for instance, when you are running on a shared hosting environment), it won't find them -- the BindingFlags.NonPublic option will simply be ignored.

Nice Syntax With Extension Method

You can access any private field of an arbitrary type with code like this:

Foo foo = new Foo();
string c = foo.GetFieldValue<string>("_bar");

For that you need to define an extension method that will do the work for you:

public static class ReflectionExtensions {
    public static T GetFieldValue<T>(this object obj, string name) {
        // Set the flags so that private and public fields from instances will be found
        var bindingFlags = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;
        var field = obj.GetType().GetField(name, bindingFlags);
        return (T)field?.GetValue(obj);
    }
}