追加到空字符串在 C# 中如何工作?

问题描述

我很惊讶地看到一个字符串被初始化为 null 然后在生产环境中附加了一些东西的例子.只是闻起来不对.

I was surprised to see an example of a string being initialised to null and then having something appended to it in a production environment. It just smelt wrong.

我确信它会抛出一个空对象异常，但这个大大简化的例子也有效:

I was sure it would have thrown a null object exception but this greatly reduced example also works:

string sample = null;
sample += "test";
// sample equals "test"

*请注意，我发现的原始代码将字符串属性设置为 null 并将其附加到其他地方，因此涉及编译器在编译时优化 null 的答案无关紧要.

有人可以解释为什么这样做没有错误吗?

Can someone explain why this works without error?

根据 Leppie 的回答，我使用 Reflector 来查看 string.Concat 中的内容.现在很明显为什么会发生这种转换(根本没有魔法):

Based on Leppie's answer I used Reflector to see what is inside string.Concat. It is now really obvious why that conversion takes place (no magic at all):

public static string Concat(string str0, string str1)
{
    if (IsNullOrEmpty(str0))
    {
        if (IsNullOrEmpty(str1))
        {
            return Empty;
        }
        return str1;
    }
    if (IsNullOrEmpty(str1))
    {
        return str0;
    }
    int length = str0.Length;
    string dest = FastAllocateString(length + str1.Length);
    FillStringChecked(dest, 0, str0);
    FillStringChecked(dest, length, str1);
    return dest;
}

**注意:我正在研究的特定实现(在 Microsoft 的 .Net 库中)不会像 C# 标准和大多数答案所建议的那样转换为空字符串，而是使用了一些测试来缩短流程.最终结果和它一样，但你去:)

推荐答案

字符串的 + 运算符只是 string.Concat 的简写，它只是将 null 在连接前将参数转换为空字符串.

the + operator for strings are just shorthand for string.Concat which simply turns null arguments into empty strings before the concatenation.

更新:

string.Concat 的通用版本:

The generalized version of string.Concat:

public static string Concat(params string[] values)
{
    int num = 0;
    if (values == null)
    {
        throw new ArgumentNullException("values");
    }
    string[] array = new string[values.Length];
    for (int i = 0; i < values.Length; i++)
    {
        string text = values[i];
        array[i] = ((text == null) ? string.Empty : text);
        num += array[i].Length;
        if (num < 0)
        {
            throw new OutOfMemoryException();
        }
    }
    return string.ConcatArray(array, num);
}