public static class Foo
{
    static Foo() { MainClass.Value = "Something"; }
    public static void DoSomething() { Thread.Sleep(100); }
}

public static class MainClass
{
    public static string Value = "Nothing";

    public static void Main()
    {
        Console.WriteLine(Value);
        Foo.DoSomething();
        Console.WriteLine(Value);
    }
}

Nothing
Something