Source: http://blogs.tedneward.com/patterns/Builder-CSharp/

class GuardedFluentBuilder
{
	private Product product;
	public FluentBuilder Begin(string name) {
		product = new Product(name);
		return this;
	}
	public FluentBuilder Engine {
		get 
		{
			product.Parts.Add ("Engine");
			return this;
		}
	}
	public FluentBuilder SteeringWheel
	{
		get 
		{
			product.Parts.Add("Steering Wheel");
			return this;
		}
	}
	public FluentBuilder Tire() 
	{
		product.Parts.Add("Tire");
		return this;
	}
    public Product Build() 
    {
        if (product.Parts.Length < 4) throw new Exception("Product doesn't have enough parts to it yet"); if (product.Parts.Contains("Engine") == false) throw new Exception("Product must have an Engine"); if (product.Parts.Find((str) => str == "Tire").Length < 2)
            throw new Exception("Product must have at least 2 Tires");

        return product;
    }
}

var builder = new FluentBuilder();
product = builder.Begin("MyProduct")
    .Engine
    .SteeringWheel
    .Tire()
    .Tire()
    .Build();