C#-与Kotlin中的with关键字类似?

你可以用 With 这样的方法:

// you need a separate overload to handle the case of not returning anything
public static void With<T>(T t, Action<T> block) {
    block(t);
}

public static R With<T, R>(T t, Func<T, R> block) => block(t);

using static 声明方法的类,并像这样使用它(从 here

var list = new List<string> {"one", "two", "three"};
With(list, x => {
    Console.WriteLine($"'with' is called with argument {x}");
    Console.WriteLine($"It contains {x.Count} elements");
    });
var firstAndLast = With(list, x => 
    $"The first element is {x.First()}. " +
    $"The last element is {x.Last()}.");

在您的示例中,它的用法如下:

static string PrintAlphabet() => With(new StringBuilder(), x => {
        for (var c = 'A' ; c <= 'Z' ; c = (char)(c + 1)) {
            x.Append(c);
        }
        return x.ToString();
    });