struct-class.cs

public struct Struct { public int Prop { get; set; } }
public class  Class  { public int Prop { get; set; } }

public static Class  Change(Class  cls) { cls.Prop = 1; return cls; }
public static Struct Change(Struct str) { str.Prop = 1; return str; }

...

var str = new Struct() { Prop = 2 };
var cls = new Class() { Prop = 2 };

var str2 = str;
var cls2 = cls;

str2.Prop = 3;
cls2.Prop = 3;

Console.WriteLine(str.Prop == str2.Prop);
Console.WriteLine(cls.Prop == cls2.Prop);

Console.WriteLine(str.Prop == Change(str).Prop);
Console.WriteLine(cls.Prop == Change(cls).Prop);