通过反射设置只读属性值

Question

I need to modify a property value which has only "get" option. I have found some trick with using GetField but I did not succeed.

See below the code:

namespace ConsoleApplication1
{
  abstract class Test
  {
    private int testValue;
    protected int TestValue {
        get { return testValue; }
    }
  }
  class Myclass : Test
  {
    public Myclass(): base(){}
  }  

  class Program
  {
    static void Main(string[] args)
    {
      Myclass test = new Myclass();

      var field = typeof(Myclass).GetField("<TestValue>k__BackingField", BindingFlags.NonPublic | BindingFlags.Instance);
      field.SetValue(test, 3);

    }
  }
}

Actually, I would like to set "TestValue" with a the value 3 but it is not working. I do probably something wrong.

Answer 1

The "trick" you found relies on auto-implemented properties ( string Foo { get; } ) that compile into a property with a backing field (hence the name of the member you're looking up). Do note that this is extremely fragile, as the generated member name isn't documented. But hey, relying on reflection to access a type's members is fragile to begin with.

You don't have an auto-implemented property though, you just need to access the private field named testValue which the property exposes.

But GetField() doesn't deal with inheritance for nonpublic members, so see GetFields of derived type . This method will help with that:

public static void Main()
{
    Myclass test = new Myclass();

    var field = GetInheritedPrivateField(test.GetType(), "testValue");

    field.SetValue(test, 3);
}

private static FieldInfo GetInheritedPrivateField(Type type, string fieldName)
{
    do
    {
        var field = type.GetField(fieldName, BindingFlags.NonPublic | BindingFlags.Instance);

        if (field != null)
        {
            return field;
        }

        type = type.BaseType;
    } 
    while (type != null);

    return null;
}

Answer 2

You can retrieve the field info for the Test class and use it to set the value on the derived class instance like so:

var field = typeof(Test).GetField("testValue", BindingFlags.NonPublic | BindingFlags.Instance);
field.SetValue(test, 3);

Answer 3

You cannot set a value on property without a "setter".

If you don't want the property to be overridden by some other object outside the scope, you might use "private set" as setter, then you may set the value within the scope.

Another approach would be to write a method to set the value of the getter of your property. ie

 class Myclass : Test
  {
    public Myclass(): base(){}

    public void SetValue(int value) { this.testValue=value; }
  }