Find a private field with Reflection?

Question

Given this class

class Foo
{
    // Want to find _bar with reflection
    [SomeAttribute]
    private string _bar;

    public string BigBar
    {
        get { return this._bar; }
    }
}

I want to find the private item _bar that I will mark with a attribute. Is that possible?

I have done this with properties where I have looked for an attribute, but never a private member field.

What are the binding flags that I need to set to get the private fields?

Solution 1

Use BindingFlags.NonPublic and BindingFlags.Instance flags

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

Solution 2

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)
    ...

Solution 3

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.

Solution 4

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);
    }
}

Solution 5

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.

View more solutions

Comments

[Andy McCluggage]

I could only get this to work by also supplying the "BindingFlags.Instance" binding flag.

[lubos hasko]

I have fixed your answer. It's too confusing otherwise. Abe Heidebrecht's answer was the most complete though.

[Ramo]

Works great - FYI VB.NET version Me.GetType().GetFields(Reflection.BindingFlags.NonPublic Or Reflection.BindingFlags.Instance)

[ksun]

Using the instance binding flag is only if you want to get instance methods. If you wanted to get a private static method you can use (BindingFlags.NonPublic | BindingFlags.Static)

[Brian Sweeney]

jammycakes could you please give an example of shared hosting environment? i'm thinking iis with multiple apps is what your're getting?

[jammycakes]

I'm talking about where IIS is locked down to partial trust at the machine.config level. You usually only find this on cheap-and-nasty shared web hosting plans these days (the likes of which I no longer use) -- if you have full control over your server then it won't really be relevant since full trust is the default.

[Michael Freidgeim]

Similar answer describes easy to use function GetInstanceField(typeof(YourClass), instance, "someString") as string How to get the value of private field in C#?

[amnesia]

Sorry for extreme necro-posting, but this threw me off. GetCustomAttributes(Type) will not return null if the attribute isn't found, it simply returns an empty array.

[Hao Nguyen]

To find the field name, it's easy to do in Visual Studio. Set breakpoint at the variable, view its fields (including the private, usually started with m_fieldname). Replace that m_fieldname in to the command above.

[tayoung]

Dude, this was PERFECT for accessing a protected variable without exposing it to NLua in my code! Awesome!

[Leaky]

BindingFlags.Instance trips me EVERY time I use reflection on non-public members. If you use GetFields() with Default=0 BindingFlags, you get instance members automatically. But when you set a binding flag explicitly, it deletes the default inclusion of instance members. It makes no sense. Flags enums should be additive.