source.cs

var temp = new StringBuilder().Append("inter").Append("ned");

string internedString1 = temp.ToString();
string internedString2 = temp.ToString();
Console.WriteLine($"Interned? {nameof(internedString1)}: {String.IsInterned(internedString1) != null}");
Console.WriteLine($"Interned?: {nameof(internedString2)} {String.IsInterned(internedString2) != null}");

String.Intern(internedString1);

Console.WriteLine($"Interned? {nameof(internedString1)}: {String.IsInterned(internedString1) != null}");
Console.WriteLine($"Interned?: {nameof(internedString2)} {String.IsInterned(internedString2) != null}");

string fromMetadata1 = String.IsInterned(internedString1);
string fromMetadata2 = String.IsInterned(internedString2);

Console.WriteLine($"Point to same string?: {ReferenceEquals(fromMetadata1, fromMetadata2)}");

fixed (char* p = fromMetadata1)
{
    //gore
    p[0] = 'k';
}

Console.WriteLine($"Interned? {nameof(internedString1)}: {String.IsInterned(internedString1) != null}");
Console.WriteLine($"Interned?: {nameof(internedString2)} {String.IsInterned(internedString2) != null}");

Console.ReadKey();