Note: the proposed API was modified significantly during API design review.

See notes below: #34978 (comment)

The type will be used as a modreq on the return type of the set accessor:
public modreq(typeof(IsExternalInit) void set_InitOnlyProperty(string value) { ... }

Out-of-date:

As part of the C# 9 records feature, we want to allow some properties to be settable during "initialization". Initialization means construction and also object creation. So we would allow new C() { InitOnlyProperty = "hello" }.

To declare such a property, we'll allow the following syntax:

public string InitOnlyProperty
{
   get { ... }
   init { ... } // this is an init-only setter
}

We will emit this property as:

// pseudo code
private readonly string backingField;
[InitOnly]
public void InitOnlyProperty_setAccessor(modreq(typeof(InitOnlyAttribute)) string value) { ... }

The attribute is used on the set accessor and in a modreq.
There are two potential expansion of the feature: init-only fields and init-only methods. We can discuss whether it is better to include those now, or add later.

So we are proposing to add the following API:

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
    public sealed class InitOnlyAttribute : Attribute
    {
    }
}

Tagging @jaredpar @agocke FYI

Updates:

• removed AttributeTargets.Method, added AttributeTargets.Field (jcouv)
• removed [InitOnly] attribute on backing field of auto-property (jcouv)
• added AttributeTargets.Method back, with explanation (jcouv)
• added [InitOnly] on set accessor (jcouv)