如何在 C# 中将参数默认为 Guid.Empty? [英] How can I default a parameter to Guid.Empty in C#?

问题描述

我想说:

public void Problem(Guid optional = Guid.Empty)
{
}

但是编译器抱怨 Guid.Empty 不是编译时常量.

But the compiler complains that Guid.Empty is not a compile time constant.

因为我不想更改 API，所以我无法使用:

As I don’t wish to change the API I can’t use:

 Nullable<Guid>

推荐答案

解决方案

您可以使用 new Guid() 代替

public void Problem(Guid optional = new Guid())
{
  // when called without parameters this will be true
  var guidIsEmpty = optional == Guid.Empty;
}

你也可以使用default(Guid)

default(Guid) 也将与 new Guid() 完全一样工作.

You can also use default(Guid)

default(Guid) also will work exactly as new Guid().

因为 Guid 是值类型而不是引用类型，所以，例如 default(Guid) 不等于 null，而是等于调用默认构造函数.

Because Guid is a value type not reference type, so, default(Guid) is not equal to null for example, instead, it's equal to calling default constructor.

这意味着:

public void Problem(Guid optional = default(Guid))
{
  // when called without parameters this will be true
  var guidIsEmpty = optional == Guid.Empty;
}

它与原始示例完全相同.

It's exactly the same as the original example.

您收到错误的原因是因为 Empty 定义为:

The reason you are getting the error is because Empty is defined as:

public static readonly Guid Empty;

所以，它是一个变量，而不是一个常量(定义为 static readonly 而不是 const).编译器只能将编译器已知的值作为方法参数的默认值(而不是仅运行时已知的).

So, it is a variable, not a constant (defined as static readonly not as const). Compiler can only have compiler-known values as method parameters default values (not runtime-only-known).

根本原因是您不能拥有任何 struct 的 const，例如与 enum 不同.如果你尝试，它不会编译.

The root cause is that you cannot have a const of any struct, unlike enum for example. If you try it, it will not compile.

原因再次是 struct 不是原始类型.
有关 .NET 中所有原始类型的列表，请参阅 http://msdn.microsoft.com/en-gb/library/system.typecode.aspx
(注意enum通常会继承int，这是一个原语)

The reason once more is that struct is not a primitive type.
For a list of all primitive types in .NET see http://msdn.microsoft.com/en-gb/library/system.typecode.aspx
(note that enum usually inherits int, which is a primitive)

我并不是说它需要一个常数.它需要一些可以在编译时决定的东西.Empty 是一个字段，因此，它的值在编译时是未知的(仅在运行时开始时).

I'm not saying it needs a constant. It needs something that can be decided in compile time. Empty is a field, so, it's value is not known in compile time (only at very beginning of run time).

必须在编译时知道默认参数值，它可以是 const 值，或者使用 C# 特性定义的东西，在编译时知道值，如 default(Guid) 或 new Guid() (这是在编译时为 struct 决定的，因为您不能修改代码中的 struct 构造函数).

Default parameter value must be known at compile-time, which may be a const value, or something defined using a C# feature that makes value known at compile time, like default(Guid) or new Guid() (which is decided at compile time for structs as you cannot modify the struct constructor in code).

虽然您可以轻松提供 default 或 new，但您不能提供 const(因为它不是原始类型或 enum 如上所述).所以，再说一次，不是说可选参数本身需要一个常量，而是编译器已知的值.

While you can provide default or new easily, you cannot provide a const (because it's not a primitive type or an enum as explained above). So, again, not saying that optional parameter itself needs a constant, but compiler known value.

这篇关于如何在 C# 中将参数默认为 Guid.Empty?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！