C# Property with no setter - how can it get set from constructor?

Question

How come you can set a get-only auto-property from a constructor? The code below shows how you can set the property from the constructor but using reflection shows that there really isn't a setter behind the scenes. How does it get set from the constructor call if the setter method doesn't even exist in the IL?

void Main()
{
    var obj = new GetOnlyProperty("original value");
    Console.WriteLine(obj.Thing); //works, property gets set from ctor

    //get the set method with reflection, is it just hidden..?
    //nope, null reference exception
    typeof(GetOnlyProperty)
        .GetProperty("Thing", BindingFlags.Instance | BindingFlags.Public)
        .GetSetMethod()
        .Invoke(obj, new object[]{"can't set me to this, setter doen't exist!"});
}

public class GetOnlyProperty
{
    public string Thing { get; }

    public GetOnlyProperty(string thing)
    {
        Thing = thing;
    }
}

Answer 1

A read-only automatically-implemented property is converted by the compiler into a read-only field and a read-only property. Assignments to the property in the constructor are compiled as assignments to the underlying field.

So your code here:

public class GetOnlyProperty
{
    public string Thing { get; }

    public GetOnlyProperty(string thing)
    {
        Thing = thing;
    }
}

is compiled into IL as if you'd written:

public class GetOnlyProperty
{
    private readonly string _thing;
    public string Thing => _thing;

    public GetOnlyProperty(string thing)
    {
        _thing = thing;
    }
}

... except that _thing is really given an "unspeakable name" that wouldn't be a valid C# identifier.