BaseBehaviour.cs

using System;
using UnityEngine;

public abstract class BaseBehaviour : MonoBehaviour
{
	public T RequireComponent<T>(Action<T> a) where T : Component
	{
		T component = gameObject.GetComponent<T>();
		if (component == null)
		{
			component = gameObject.AddComponent<T>();
			a(component);
		}
		return component;
	}
}

ExampleBehaviour.cs

using UnityEngine;

public class ExampleBehaviour : BaseBehaviour
{
	private Rigidbody2D body;
	private CircleCollider2D circle;

	void Awake()
	{
		// Multiple lines of code during setup
		body = RequireComponent<Rigidbody2D>(b => 
		{
			b.mass = 10;
			b.drag = 1;
			b.freezeRotation = true;
		});

		// Single line of code during setup
		circle = RequireComponent<CircleCollider2D>(c => c.radius = 2f);
	}
}