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?

Answer 1

Use BindingFlags.NonPublic and BindingFlags.Instance flags

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

Answer 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)
    ...

Answer 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.

Answer 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);
    }
}

Answer 5

在考虑私有成员时需要注意的一件事是，如果您的应用程序以中等信任度运行（例如，当您在共享托管环境中运行时），它将找不到它们—— BindingFlags.NonPublic 选项将被忽略。

Answer 6

typeof(MyType).GetField("fieldName", BindingFlags.NonPublic | BindingFlags.Instance)

Answer 7

I use this method personally

if (typeof(Foo).GetFields(BindingFlags.NonPublic | BindingFlags.Instance).Any(c => c.GetCustomAttributes(typeof(SomeAttribute), false).Any()))
{ 
    // do stuff
}

Answer 8

Here is some extension methods for simple get and set private fields and properties (properties with setter):

usage example:

 public class Foo { private int Bar = 5; } var targetObject = new Foo(); var barValue = targetObject.GetMemberValue("Bar");//Result is 5 targetObject.SetMemberValue("Bar", 10);//Sets Bar to 10

Code:

    /// <summary>
    /// Extensions methos for using reflection to get / set member values
    /// </summary>
    public static class ReflectionExtensions
    {
        /// <summary>
        /// Gets the public or private member using reflection.
        /// </summary>
        /// <param name="obj">The source target.</param>
        /// <param name="memberName">Name of the field or property.</param>
        /// <returns>the value of member</returns>
        public static object GetMemberValue(this object obj, string memberName)
        {
            var memInf = GetMemberInfo(obj, memberName);

            if (memInf == null)
                throw new System.Exception("memberName");

            if (memInf is System.Reflection.PropertyInfo)
                return memInf.As<System.Reflection.PropertyInfo>().GetValue(obj, null);

            if (memInf is System.Reflection.FieldInfo)
                return memInf.As<System.Reflection.FieldInfo>().GetValue(obj);

            throw new System.Exception();
        }

        /// <summary>
        /// Gets the public or private member using reflection.
        /// </summary>
        /// <param name="obj">The target object.</param>
        /// <param name="memberName">Name of the field or property.</param>
        /// <returns>Old Value</returns>
        public static object SetMemberValue(this object obj, string memberName, object newValue)
        {
            var memInf = GetMemberInfo(obj, memberName);


            if (memInf == null)
                throw new System.Exception("memberName");

            var oldValue = obj.GetMemberValue(memberName);

            if (memInf is System.Reflection.PropertyInfo)
                memInf.As<System.Reflection.PropertyInfo>().SetValue(obj, newValue, null);
            else if (memInf is System.Reflection.FieldInfo)
                memInf.As<System.Reflection.FieldInfo>().SetValue(obj, newValue);
            else
                throw new System.Exception();

            return oldValue;
        }

        /// <summary>
        /// Gets the member info
        /// </summary>
        /// <param name="obj">source object</param>
        /// <param name="memberName">name of member</param>
        /// <returns>instanse of MemberInfo corresponsing to member</returns>
        private static System.Reflection.MemberInfo GetMemberInfo(object obj, string memberName)
        {
            var prps = new System.Collections.Generic.List<System.Reflection.PropertyInfo>();

            prps.Add(obj.GetType().GetProperty(memberName,
                                               System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.Instance |
                                               System.Reflection.BindingFlags.FlattenHierarchy));
            prps = System.Linq.Enumerable.ToList(System.Linq.Enumerable.Where( prps,i => !ReferenceEquals(i, null)));
            if (prps.Count != 0)
                return prps[0];

            var flds = new System.Collections.Generic.List<System.Reflection.FieldInfo>();

            flds.Add(obj.GetType().GetField(memberName,
                                            System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance |
                                            System.Reflection.BindingFlags.FlattenHierarchy));

            //to add more types of properties

            flds = System.Linq.Enumerable.ToList(System.Linq.Enumerable.Where(flds, i => !ReferenceEquals(i, null)));

            if (flds.Count != 0)
                return flds[0];

            return null;
        }

        [System.Diagnostics.DebuggerHidden]
        private static T As<T>(this object obj)
        {
            return (T)obj;
        }
    }

Answer 9

Yes, however you will need to set your Binding flags to search for private fields (if your looking for the member outside of the class instance).

The binding flag you will need is: System.Reflection.BindingFlags.NonPublic

Answer 10

I came across this while searching for this on google so I realise I'm bumping an old post. However the GetCustomAttributes requires two params.

typeof(Foo).GetFields(BindingFlags.NonPublic | BindingFlags.Instance)
.Where(x => x.GetCustomAttributes(typeof(SomeAttribute), false).Length > 0);

The second parameter specifies whether or not you wish to search the inheritance hierarchy

Answer 11

If your .Net framework is greater than 4.5. You can use GetRuntimeFields method.

This method returns all fields that are defined on the specified type, including inherited, non-public, instance, and static fields.

https://docs.microsoft.com/en-us/dotnet/api/system.reflection.runtimereflectionextensions.getruntimefields?view=net-6.0

var foo = new Foo();
var fooFields = foo.GetType().GetRuntimeFields()