Question

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.

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"

*Note the original code I found sets a string property to null and appends to it elsewhere so answers involving the compiler optimizing out the null at compile-time are irrelevant.

Can someone explain why this works without error?

Follow-up:

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;
}

**Note: the specific implementation I was investigating (in the .Net library by Microsoft) does not convert to empty strings as is suggested by the C# standards and most of the answers, but uses a few tests to shortcut the process. The end result is the same as if it did but there you go 🙂

Answer 1

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

Update:

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);
}