If I have a PInvoke call like the following:
[DllImport("Advapi32.dll", CharSet=CharSet.Auto)]
static extern Boolean FileEncryptionStatus(String filename,
out UInt32 status);
What is the difference between
[DllImport("Advapi32.dll", CharSet=CharSet.Auto)] static extern Boolean FileEncryptionStatus(String filename, out UInt32 status);
and
[DllImport("Advapi32.dll", CharSet=CharSet.Auto)] static extern Boolean FileEncryptionStatus(String filename, ref UInt32 status);
Well, as long as I have tested it, they exactly the same. From the MSDN you can even read
“I could have selected the ref keyword here as well, and in fact both result in the same machine code at run time. The out keyword is simply a specialization of a by-ref parameter that indicates to the C# compiler that the data being passed is only being passed out of the called function. In contrast, with the ref keyword the compiler assumes that data may flow both in and out of the called function.”
“When marshaling pointers through P/Invoke, ref and out are only used with value types in managed code. You can tell a parameter is a value type when its CLR type is defined using the struct keyword. Out and ref are used to marshal pointers to these data types”
So what should you use? Well using the out keyword for PInvoke will just add some information or documentation to your method, but because these functions are implemented in C or C++ they might treat an out parameter as an IN parameter so I really prefere to use ref when I’m calling functions with PInvoke.