ConstInterpolatedStrings.cs

using System;

const string s1 = $"abc";
const string s2 = $"{s1}edf";
Console.WriteLine(s2);

DoSomething_Old(42);
DoSomething_VeryOld(42);

[Obsolete($"Use {nameof(DoSomething_New)} instead")]
void DoSomething_Old(int x) { }
void DoSomething_VeryOld(int x)
{
    throw new InvalidOperationException($"{nameof(DoSomething_VeryOld)} is no longer supported");
}
void DoSomething_New(int x) { }