除了类参数的 null 之外，C# 可选参数？

Question

这个问题的最佳解决方案是什么？我正在尝试创建一个函数，该函数具有多个类类型的可选参数，其中 null 是一个有意义的值，不能用作默认值。如，

public void DoSomething(Class1 optional1, Class2 optional2, Class3 optional3)    {        if (! WasSpecified(optional1)) { optional1 = defaultForOptional1; }        if (! WasSpecified(optional2)) { optional2 = defaultForOptional2; }        if (! WasSpecified(optional3)) { optional3 = defaultForOptional3; }        // ... do the actual work ...    }

I can't use Class1 optional1 = null because null is meaningful. I can't use some placeholder class instance Class1 optional1 = defaultForOptional1 because of the compile-time constant requirement for these optional parameters I've come up with the following options:

1. Provide overloads with every possible combination, which means 8 overloads for this method.
2. Include a Boolean parameter for each optional parameter indicating whether or not to use the default, which I clutters up the signature.

Has anyone out there come up with some clever solution for this?

Thanks!

edit: I ended up writing a wrapper class for so I didn't have to keep repeating Boolean HasFoo.

    /// <summary>
    /// A wrapper for variables indicating whether or not the variable has
    /// been set.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public struct Setable<T>
    {
        // According to http://msdn.microsoft.com/en-us/library/aa288208%28v=vs.71%29.aspx,
        // "[s]tructs cannot contain explicit parameterless constructors" and "[s]truct
        // members are automatically initialized to their default values."  That's fine,
        // since Boolean defaults to false and usually T will be nullable.

        /// <summary>
        /// Whether or not the variable was set.
        /// </summary>
        public Boolean IsSet { get; private set; }

        /// <summary>
        /// The variable value.
        /// </summary>
        public T Value { get; private set; }

        /// <summary>
        /// Converts from Setable to T.
        /// </summary>
        /// <param name="p_setable"></param>
        /// <returns></returns>
        public static implicit operator T(Setable<T> p_setable)
        {
            return p_setable.Value;
        }

        /// <summary>
        /// Converts from T to Setable.
        /// </summary>
        /// <param name="p_tee"></param>
        /// <returns></returns>
        public static implicit operator Setable<T>(T p_tee)
        {
            return new Setable<T>
            {
                IsSet = true
              , Value = p_tee
            };
        }
    }

Answer 1

我至少会考虑为参数创建一个新类型:

public void DoSomething(DoSomethingOptions options)

... DoSomethingOptions 可能看起来像这样:

public class DoSomethingOptions
{
    private Class1 class1;
    public bool HasClass1 { get; private set; }

    public Class1 Class1 
    {
        get { return class1; }
        set
        {
            class1 = value;
            HasClass1 = true;
        }
    }

    ... for other properties ...
}

然后你可以调用它:

DoSomething(new DoSomethingOptions { Class1 = null, Class2 = new Class2() });

您不会以指数级的重载集结束，您仍然可以合理紧凑地调用它。

这类似于 Process 使用 ProcessStartInfo 的方法。