What is the difference between ref and out parameters in C#?

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ref and out parameters are quite similar in their meaning. ref and out keywords are used to pass parameters to a method by reference. By default parameters are being passed to methods by value. Passing reference to variable instead of value means referenced variable might (or must) be modified/assigned by the method accepting this reference. The main difference is out parameters must be set during the execution of the method and ref parameters are not required to be set before returning. But there is more.

First of all out parameter doesn't need to be initialized before passing it to the method, because it will be initialized or set to new value by the method accepting it. Code where out parameter hasn't been set in method's body simply won't compile. Contrary, ref parameter should be initialized before passing it to a method. As such, the called method is not required to set ref parameter before using it.

Also, there is difference in semantic meaning between these two keywords. They both allow called method to modify a parameter. Any changes made to the parameter will be reflected in the variable which parameter refers to. But, since ref parameter should be set before passing it to a method and also might be changed in this method, it can be considered both input and (optionally) output of it. out parameter can't be considered input, because it's not required to be initialized before passing it to a method. Such parameters are strictly output parameters. We can think of them as an additional return value of a method that is already using the return value for something.

Code examples:

public static void MethodWithOutParameter(out int id) 
{ 
    id = 1; 
}  

static void Main(string[] args) 
{ 
    int i; 
    MethodWithOutParameter(out i); 
    Console.WriteLine(i); 
} 


Output: 1

public static void MethodWithRefParameter(ref int id) 
{ 
    id = id + 1; 
}  

static void Main(string[] args) 
{ 
    int i = 1; 
    MethodWithRefParameter(ref i); 
    Console.WriteLine(i); 
} 


Output: 2

Short answer

  • Both out and ref keywords are used in C# to pass parameters to methods by reference, which allows method to modify referenced variable.
  • ref parameter should be initialized before passing it, for out parameters it's optional.
  • out parameter should be set in method's body, for ref parameters it's optional.
  • Use ref parameters as input/output of a method, use out parameters as an additional returning value of it.

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