在C#中附加到空字符串如何工作? [英] How does appending to a null string work in C#?

问题描述

我很惊讶地看到一个示例，该示例将字符串初始化为null，然后在生产环境中附加了一些内容.只是闻错了.

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.

我确定它会抛出一个空对象异常，但是这个大大简化的示例也可以工作:

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并将其附加到其他位置，因此涉及编译器在编译时优化null的答案是无关紧要的.

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?

基于Leppie的回答，我使用了Reflector来查看string.Concat中的内容.现在真的很清楚为什么会发生这种转换(一点也不神奇):

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 .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，而是使用一些测试来简化过程.最终结果与完成的结果相同，但是您可以:)

推荐答案

用于字符串的 + 运算符只是 string.Concat 的简写，它只是将 null在连接之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);
}

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