How can I run a static initializer method in C# before the Main() method?

Simply do the initialization inside a static constructor for Foo.

From the documentation:

A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.

There are static constructors in C# that you can use.

public static class Foo
{
    // Class members...

    static Foo(){
        init();
        // other stuff
    }

    internal static init()
    {
        // Do some initialization...
    }
}

Move your code from an internal static method to a static constructor, like this:

public static class Foo
{
  // Class members...

  static Foo()
  {
    // Do some initialization...
  }
}

This way, you are quite sure that the static constructor will be run on first mention of your Foo class, whether it's a construction of an instance or access to a static member.