How does ConfigureWebHostDefaults work

Question

In the following code snippet, I am having difficulty understanding where webBuilder came from. Assuming ConfigureWebHostDefaults is a method inside the parentheses, it should be a parameter being passed to the method. Within the parentheses, there is a function whose purpose I can understand. I also see that webBuilder is an IWebHostBuilder, but shouldn't we declare that variable somewhere before using it?

Here is the code snippet:

public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
              webBuilder.UseStartup<Startup>();
            });

Answer 1

This is basic c# 101.

A common pattern is passing a callback (delegate) to method. The method defines the the supported delegate signature (parameters and return type). this pattern is so common, the Action<> and Func<> generics are defined. See:

https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.hosting.generichostbuilderextensions.configurewebhostdefaults?view=aspnetcore-7.0

In your sample code a lambda is used as the delegate.