byref.cs

public delegate ref int RefIntFunc();

public delegate ref T RefFunc<T>();
public delegate ref TReturn RefFunc<T, TReturn>(T arg);
public delegate ref TReturn RefFunc<T1, T2, TReturn>(T1 arg1, T2 arg2);

public interface IWithRefReturns {
  ref int RefIntMethod();
  ref string RefStringMethod();
  ref int RefIntProperty { get; }
  ref object this[int key] { get; }
}

public interface ISpan<T> {
  ref T this[int index] { get; }
}

public abstract class WithRefReturns : IWithRefReturns {
  public abstract ref int RefIntMethod();
  public abstract ref string RefStringMethod();
  public abstract ref int RefIntProperty { get; }
  public abstract ref object this[int key] { get; }
}