Changing read only properties with reflection

Question

Is it possible? With reflection or any other way?

Answer 1

As other stated, if you need to do that, you're facing a design issue to begin with. Now, if you want to know if it's possible just for the sake of knowing, or if there's no other way on earth to do it, it's indeed possible, with the help of a very small helper library and an extension method.

Consider the following code:

class Person {      int age;     string name;      public int Age { get { return age; } }     public string Name { get { return name; } } }  // ...  using Mono.Reflection; using System.Reflection;  // ...  Person person = new Person (27, "jb"); PropertyInfo nameProperty = typeof (Person).GetProperty ("Name"); FieldInfo nameField = nameProperty.GetBackingField (); nameField.SetValue (person, "jbe"); 

Using this code, you can get the backing field of a property with just the property, and assign a new value to the backing field. You can read more details about the implementation.

Also note that it works only for simple properties, such as:

public int Age { get { return age; } }  public string Name {     get { return name; }     set { name = value; } }  public double Velocity { get; private set; } 

If you have complex properties with custom code (which includes expression-bodied member like int Answer=> 42;), the backing field resolver will fail as there is no backing field in such case.

Answer 2

An alternate to Simon Mattes answer would be

Assuming you have:

public class MyClass {      public int MyNumber {get;} } 

You could do this if its for test purpose:

var field = typeof(MyClass).GetField("<MyNumber>k__BackingField", BindingFlags.Instance | BindingFlags.NonPublic); field.SetValue(anIstanceOfMyClass, 3); 

Answer 3

As a very dirty workaround for auto generated read only properties:

class MyTestClass
{
    public string MyProperty { get; }

    public MyTestClass( string MyProperty )
    {
        this.MyProperty = MyProperty;
    }
}

You can modify the auto generated backing field in the following way:

MyTestClass MyClass = new MyTestClass( "Hello" );
FieldInfo MyWriteableField = MyClass.GetType().GetRuntimeFields().Where( a => Regex.IsMatch( a.Name, $"\\A<{nameof( MyClass.MyProperty )}>k__BackingField\\Z" ) ).FirstOrDefault();
MyWriteableField.SetValue( MyClass, "Another new value" );

PS: when you are using a .NET version < 4.6 you might have to change some of the code to work. Enjoy!