How does appending to a null string work in C#?

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

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);
}

Answer 2

Here is what your code gets compiled to

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

is compiled to this IL code:

.entrypoint
  // Code size       16 (0x10)
  .maxstack  2
  .locals init ([0] string sample)
  IL_0000:  nop
  IL_0001:  ldnull
  IL_0002:  stloc.0
  IL_0003:  ldloc.0
  IL_0004:  ldstr      "test"
  IL_0009:  call       string [mscorlib]System.String::Concat(string,
                                                              string)
  IL_000e:  stloc.0
  IL_000f:  ret

And String.Concat takes care of NULL string.

Answer 3

The relevant citation should be ECMA-334 §14.7.4:

String concatenation:

string operator +(string x, string y);
string operator +(string x, object y);
string operator +(object x, string y);  

The binary + operator performs string concatenation when one or both operands are of type string. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string operand is converted to its string representation by invoking the virtual ToString method inherited from type object. If ToString returns null, an empty string is substituted.

Answer 4

it is because

In string concatenation operations, the C# compiler treats a null string the same as an empty string, but it does not convert the value of the original null string.

From How to: Concatenate Multiple Strings (C# Programming Guide)

The binary + operator performs string concatenation when one or both operands are of type string. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string argument is converted to its string representation by invoking the virtual ToString method inherited from type object. If ToString returns null, an empty string is substituted.

From Addition operator