If you have a char array that contains the string you want but is padded out with null terminators e.g. '\0' you can use the String.TrimEnd method to get the exact string you need.
A 12 element char array contains the following chars: M y S t r i n g \0 \0 \0 \0
To convert to a string you can use the String constructor to pass in a Char type. Unfortunately a .NET string can happily contain null characters so your resulting string would be MyString\0\0\0\0
Using the TrimEnd('\0') will return the correctly terminated string.
Note: Outputting a string containing null characters to other classes like form controls may automatically terminate the strings correctly at the first null character but behind the scenes the string still contains the null characters.
MSDN String.TrimEnd Method
Strings In .NET and C# (Jon Skeet)