Sunday, November 14, 2010

C# - Pass By Value, Pass By Reference and OUT Parameter

Pass by Value


When we pass the parameter to a function by value, the parameter value of the caller is copied to the called function's parameter. Consider the below Example:

//The Function
private void PassByVal_X(int x)
{
      //Increment the value passed in
      x++;
}

//Calling Code
int testPassByRef;
testPassByVal = 10;
PassByVal_X(testPassByVal)

In the example, the caller passed the testPassByRef by value to the function PassByVal_X. The value of 10 from testPassByValue is copied to the parameter x.


Pass by Reference


When we pass the parameter to a function by reference, the reference to the actual value of the caller is copied to the called function's parameter. So, at the end, there are two references pointing to the same value, one from the caller, and one from the function.

Have a look at the below example:
private void PassByRef_X(ref int x)
{
      //increment the value passed in
      x++;
}

//Calling Code
int testPassByRef;
testPassByRef = 10;
testPP.PassByRef_X(ref testPassByRef);

1) In this Example, the variable x and the variable testPassByRef pointing to the same location where the value 10 is stored.
2) Note the usage of the ref keyword. We are indicating the compiler that value is passed by reference.
3) Since the value is passed by reference, the increment of value x in side the function is reflected back to the caller in the variable testPassByRef.
4) When we are passing by reference, the variable should be initialized. Otherwise, we do get compilation error.
5) For standard types like int, float, double etc, we need the explicit ref keyword.



Pass by Reference with OUT


When we pass the parameter by reference, the compiler expects that we should initialize it. If there is situation that value is declared outside the function, and the same variable will be assigned with some value inside the function, then, we should go for Pass by reference, but, not with ref. Because, ref expects that the value should be initialized. The Out keyword is used in this situation.

Both ref and out are meaning that we are passing by reference. The only difference is for out, the variable is not required to be initialized before passing it as parameter.

Look at the below example:

private void PassByRefOut_X(out int x, int y)
{
      //Assign some value by multiplying y with y
      // x is assigned first here, after the assignment
      x = y * y;
}

//calling code
int testPassByRefOut;
testPP.PassByRefOut_X(out testPassByRefOut, 10);

Note the usage out keyword. It implies that the parameter is still treated as Pass By Reference. But, now we no need to initialize it and it must get assigned inside the function.



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