is a "null character". It s used to terminate strings in C and some portions of C++. Pex is doing a test to see how your code handles the null character, likely looking for the Poison Null Byte security exploit.

Most C# code has nothing to fear; if you pass your string to unmanaged code, however, you may have problems.

Edit:

Just to be explicit... Pex is passing a string containing a null character. This is not a null reference.

It s a string with a null character. Older string libraries — like that used in C or older C++ libraries — used the character to indicate the end of the string.

Newer environments like .Net use a different system, but there is a lot of history around ending a string with , such that it s a common point of error. Testing libraries like Pex will use it to make sure your program handles it correctly.

It s a string containing the character . C# doesn t treat this in any particularly special way - it s just unicode character U+0000. If you write:

int firstCodePoint = text[0];

then you ll find firstCodePoint is 0.

Escape Sequence  
    
Character Name 
Null    
Unicode Encoding     
0x0000

See this link.

A string of length 1, containing the character u0000 (aka NUL). This character is not treated specially.

In C, which uses to terminate string, you also allocate a string of length 1. In this case the standard string functions will report a length of 0, since the string contains as well as being terminated with it. You could safely modify str[0], or strncat a single character into it.

I just found a good example in which is very important and necessary.

Assume we want to remove the last unwanted , in the following code.

for (int i = 0; i < 5; i++)
{
    foreach (var a in "Money")
        Console.Write($"{a},");
}

If we only add Console.Write(" "); as follows,

for (int i = 0; i < 5; i++)
{
    foreach (var a in "Money")
        Console.Write($"{a},");
    Console.Write("
");
}

The output will be still the same.

But if we add as follows,

for (int i = 0; i < 5; i++)
{
    foreach (var a in "Money")
        Console.Write($"{a},");
    Console.Write("
");
} 

The unwanted trailing , vanishes.