C# Language Specification

Output parameters

A parameter declared with an out modifier is an output parameter. Similar to a reference parameter, an output parameter does not create a new storage location. Instead, an output parameter represents the same storage location as the variable given as the argument in the method invocation.

When a formal parameter is an output parameter, the corresponding argument in a method invocation must consist of the keyword out followed by a variable-reference (§5.3.3) of the same type as the formal parameter. A variable need not be definitely assigned before it can be passed as an output parameter, but following an invocation where a variable was passed as an output parameter, the variable is considered definitely assigned.

Within a method, just like a local variable, an output parameter is initially considered unassigned and must be definitely assigned before its value is used.

Every output parameter of a method must be definitely assigned before the method returns.

A method declared as a partial method (§10.2.7) or an iterator (§10.14) cannot have output parameters.

Output parameters are typically used in methods that produce multiple return values. For example:

using System;

class Test

{
static void SplitPath(string path, out string dir, out string name) {
int i = path.Length;
while (i > 0) {
char ch = path[i – 1];
if (ch == '\\' || ch == '/' || ch == ':') break;
i--;
}
dir = path.Substring(0, i);
name = path.Substring(i);
}

static void Main() {

string dir, name;
SplitPath("c:\\Windows\\System\\hello.txt", out dir, out name);
Console.WriteLine(dir);
Console.WriteLine(name);
}
}

The example produces the output:

c:\Windows\System\
hello.txt

Note that the dir and name variables can be unassigned before they are passed to SplitPath, and that they are considered definitely assigned following the call.

4.