public class Foo
{
    public string? Bar {get; set;}
}

var foo = new Foo {
    Bar = "Lorem Ipsum"
};

var n = foo.Bar!.Length;

public record Foo(string Hello, string World);

class Program
{
    static void Main(string[] args)
    {
        var foo = new Foo("Hello", "World");
        Console.WriteLine($"{foo.Hello} {foo.World}");
    }
}

var foo = new Foo("Hello", "World");
var bar = foo with { World = "fkthat" };

public record CreateCustomerModel(
    string FirstName,
    string LastName,
    string Email,
    string Phone
);

[HttpPost("")]
[System.Diagnostics.CodeAnalysis.SuppressMessage("Performance",
    "CA1822:Mark members as static",
    Justification = "Action method")]
public void CreateCustomer(CreateCustomerModel model) => _ = model

[System.Diagnostics.CodeAnalysis.SuppressMessage("Usage",
    "CA1801:Review unused parameters",
    Justification = "Seems like C# 9 preview bug")]
public record CreateCustomerModel(
    string FirstName,
    string LastName,
    [EmailAddress] string Email,
    string Phone
);

// вот это вот работать будет
var joeDoe = JsonSerializer.Deserialize<Contact>(
    "{\"name\": \"Joe Doe\", \"phone\": \"+1\"}");

// а вот такое уже не скомпилируется
// joeDoe.Phone = "+7";

public record Contact(string Name, string Phone);