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Why does C# have both 'ref' and 'out'?

Ref and out parameter passing modes are used to allow a method to alter variables passed in by the caller. The difference between ref and out is subtle but important. Each parameter passing mode is designed to apply to a slightly different programming scenario. The important difference between out and ref parameters is the definite assignment rules used by each.

The caller of a method which takes an out parameter is not required to assign to the variable passed as the out parameter prior to the call; however, the callee is required to assign to the out parameter before returning.

Here's a simple example:

class OutExample
{
      // Splits a string containing a first and last name separated
      // by a space into two distinct strings, one containing the first name and one containing the last name

      static void SplitName(string fullName, out string firstName, out string lastName)
      {
            // NOTE: firstName and lastName have not been assigned to yet.  Their values cannot be used.
            int spaceIndex = fullName.IndexOf(' ');
            firstName = fullName.Substring(0, spaceIndex).Trim();
            lastName = fullName.Substring(spaceIndex).Trim();
      }

      static void Main()
      {
            string fullName = "Yuan Sha";
            string firstName;
            string lastName;

            // NOTE: firstName and lastName have not been assigned yet.  Their values may not be used.
            SplitName(fullName, out firstName, out lastName);
            // NOTE: firstName and lastName have been assigned, because the out parameter passing mode guarantees it.

            System.Console.WriteLine("First Name '{0}'. Last Name '{1}'", firstName, lastName);
      }
}

One way to think of out parameters is that they are like additional return values of a method. They are very convenient when a method returns more than one value, in this example firstName and lastName. Out parameters can be abused however. As a matter of good programming style if you find yourself writing a method with many out parameters then you should think about refactoring your code. One possible solution is to package all the return values into a single struct.

In contrast ref parameters are considered initially assigned by the callee. As such, the callee is not required to assign to the ref parameter before use. Ref parameters are passed both into and out of a method.

Here's an example:

class RefExample
{
      static object FindNext(object value, object[] data, ref int index)
      {
            // NOTE: index can be used here because it is a ref parameter
            while (index < data.Length)
            {
                  if (data[index].Equals(value))
                  {
                        return data[index];
                  }
                   index += 1;
            }
             return null;
      }

      static void Main()
      {
            object[] data = new object[] {1,2,3,4,2,3,4,5,3};

            int index = 0;
            // NOTE: must assign to index before passing it as a ref parameter
            while (FindNext(3, data, ref index) != null)
            {
                  // NOTE: that FindNext may have modified the value of index
                  System.Console.WriteLine("Found at index {0}", index);
                  index += 1;
            }

            System.Console.WriteLine("Done Find");
      }
}

The two parameter passing modes addressed by out and ref are subtly different, however they are both very common. The subtle difference between these modes leads to some very common programming errors. These include:

  • not assigning a value to an out parameter in all control flow paths
  • not assigning a value to variable which is used as a ref parameter

Because the C# language assigns different definite assignment rules to these different parameter passing modes, these common coding errors are caught by the compiler as being incorrect C# code.

The crux of the decision to include both ref and out parameter passing modes was that allowing the compiler to detect these common coding errors was worth the additional complexity of having both ref and out parameter passing modes in the language.

Peter Hallam, C# Compiler Dev
Discuss this in C# Language Forum.