What do ref, val and out mean on method parameters?

Question

I'm looking for a clear, concise and accurate answer.

Ideally as the actual answer, although links to good explanations welcome.

This also applies to VB.Net, but the keywords are different - ByRef and ByVal.

Answer 1

out means that the parameter will be initialised by the method:

int result; //not initialised

if( int.TryParse( "123", out result ) )
   //result is now 123
else
   //if TryParse failed result has still be 
   // initialised to its default value (0)

ref will force the underlying reference to be passed:

void ChangeMyClass1( MyClass input ) {
   input.MyProperty = "changed by 1";
   input = null;
   //can't see input anymore ... 
   // I've only nulled my local scope's reference
}

void ChangeMyClass2( ref MyClass input ) {
   input.MyProperty = "changed by 2";
   input = null;
   //the passed reference is now null too.
}

MyClass tester = new MyClass { MyProperty = "initial value" };

ChangeMyClass1( tester );
// now tester.MyProperty is "changed by 1"

ChangeMyClass2( ref tester );
// now tester is null

Answer 2

This article "Parameter passing in C#" is the most complete explanation I've read so far.

Answer 3

One of my own questions at stackoverflow handles this topic too.
It handles about "pass by reference" and "pass by value" in different types of languages, c# is included so maybe you can find some extra information there as well.

Basically it comes down to:

• ref: the parameter with the ref keyword will be passed by reference
• out: the parameter with the out keyword will be treated as an output parameter

but that's really the most basic answer you can give, as it is a little more complex than it is stated here

Answer 4

By default (in C#), passing an object to a function actually passes a copy of the reference to that object. Changing the parameter itself only changes the value in the parameter, and not the variable that was specified.

void Test1(string param)
{
    param = "new value";
}

string s1 = "initial value";
Test1(s1);
// s1 == "initial value"

Using out or ref passes a reference to the variable specified in the call to the function. Any changes to the value of an out or ref parameter will be passed back to the caller.

Both out and ref behave identically except for one slight difference: ref parameters are required to be initialised before calling, while out parameters can be uninitialised. By extension, ref parameters are guaranteed to be initialised at the start of the method, while out parameters are treated as uninitialised.

void Test2(ref string param)
{
    param = "new value";
}

void Test3(out string param)
{
    // Use of param here will not compile
    param = "another value";
}

string s2 = "initial value";
string s3;
Test2(ref s2);
// s2 == "new value"
// Test2(ref s3); // Passing ref s3 will not compile
Test3(out s2);
// s2 == "another value"
Test3(out s3);
// s3 == "another value"

Edit: As dp points out, the difference between out and ref is only enforced by the C# compiler, not by the CLR. As far as I know, VB has no equivalent for out and implements ref (as ByRef) only, matching the support of the CLR.

Answer 5

One additional note about ref vs. out: The distinction between the two is enforced by the C# compiler. The CLR does not distinguish between between out and ref. This means that you cannot have two methods whose signatures differ only by an out or ref

void foo(int value) {}

// Only one of the following would be allowed

// valid to overload with ref
void foo(ref int value) {}

// OR with out
void foo(out int value) {}