Breaking the C# type safety without "unsafe", with explicit struct layout.

FieldOffsetAbuse.cs

void Main()
{
    var u = new Union
    {
        Foo = null,
        Bar = new Bar()
    };
    
    u.Foo.S = "ohai!";
    
    u.Bar.I.Dump();
    u.Foo.S.Dump();
}

[StructLayout(LayoutKind.Explicit)]
struct Union
{
    [FieldOffset(0)]
    public Foo Foo;
    [FieldOffset(0)]
    public Bar Bar;
}

class Foo
{
    public string S { get;set; }
}

class Bar
{ 
    public int I { get;set; }
}