Builder Pattern

Builder separates the construction of a complex object from its representation, usually via a fluent, chainable API — most useful when an object has many optional parameters/configuration steps and a giant constructor (or worse, multiple overloaded constructors) would be unreadable.

You've almost certainly already used this: HostBuilder/WebApplicationBuilder in ASP.NET Core, DbContextOptionsBuilder, and StringBuilder are all Builder pattern instances — a good real-world anchor if asked "where have you seen this before."

public class HttpRequestMessageBuilder
{
    private readonly HttpRequestMessage _request = new();
    public HttpRequestMessageBuilder WithUrl(string url) { _request.RequestUri = new Uri(url); return this; }
    public HttpRequestMessageBuilder WithMethod(HttpMethod method) { _request.Method = method; return this; }
    public HttpRequestMessageBuilder WithHeader(string key, string value) { _request.Headers.Add(key, value); return this; }
    public HttpRequestMessage Build() => _request;
}