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CA1021: Avoid out parameters

TypeName

AvoidOutParameters

CheckId

CA1021

Category

Microsoft.Design

Breaking Change

Breaking

A public or protected method in a public type has an out parameter.

Passing types by reference (using out or ref) requires experience with pointers, understanding how value types and reference types differ, and handling methods with multiple return values. Also, the difference between out and ref parameters is not widely understood.

When a reference type is passed "by reference," the method intends to use the parameter to return a different instance of the object. Passing a reference type by reference is also known as using a double pointer, pointer to a pointer, or double indirection. By using the default calling convention, which is pass "by value," a parameter that takes a reference type already receives a pointer to the object. The pointer, not the object to which it points, is passed by value. Pass by value means that the method cannot change the pointer to have it point to a new instance of the reference type. However, it can change the contents of the object to which it points. For most applications this is sufficient and yields the desired behavior.

If a method must return a different instance, use the return value of the method to accomplish this. See the System.String class for a variety of methods that operate on strings and return a new instance of a string. When this model is used, the caller must decide whether the original object is preserved.

Although return values are commonplace and heavily used, the correct application of out and ref parameters requires intermediate design and coding skills. Library architects who design for a general audience should not expect users to master working with out or ref parameters.

To fix a violation of this rule that is caused by a value type, have the method return the object as its return value. If the method must return multiple values, redesign it to return a single instance of an object that holds the values.

To fix a violation of this rule that is caused by a reference type, make sure that the desired behavior is to return a new instance of the reference. If it is, the method should use its return value to do this.

It is safe to suppress a warning from this rule. However, this design could cause usability issues.

The following library shows two implementations of a class that generates responses to the feedback of a user. The first implementation (BadRefAndOut) forces the library user to manage three return values. The second implementation (RedesignedRefAndOut) simplifies the user experience by returning an instance of a container class (ReplyData) that manages the data as a single unit.

using System;

namespace DesignLibrary
{
   public enum Actions
   {
      Unknown,
      Discard,
      ForwardToManagement,
      ForwardToDeveloper
   }

   public enum TypeOfFeedback
   {
      Complaint, 
      Praise,
      Suggestion,
      Incomprehensible
   }

   public class BadRefAndOut
   {
      // Violates rule: DoNotPassTypesByReference. 

      public static bool ReplyInformation (TypeOfFeedback input, 
         out string reply, ref Actions action)
      {
         bool returnReply = false;
         string replyText = "Your feedback has been forwarded " + 
                            "to the product manager.";

         reply = String.Empty;
         switch (input)
         {
            case TypeOfFeedback.Complaint:
            case TypeOfFeedback.Praise :
               action = Actions.ForwardToManagement;
               reply = "Thank you. " + replyText;
               returnReply = true;
               break;
            case TypeOfFeedback.Suggestion:
               action = Actions.ForwardToDeveloper;
               reply = replyText;
               returnReply = true;
               break;
            case TypeOfFeedback.Incomprehensible:
            default:
               action = Actions.Discard;
               returnReply = false;
               break;
         }
         return returnReply;
      }
   }

   // Redesigned version does not use out or ref parameters; 
   // instead, it returns this container type. 

   public class ReplyData
   {
      string reply;
      Actions action;
      bool returnReply;

      // Constructors. 
      public ReplyData()
      {
         this.reply = String.Empty;
         this.action = Actions.Discard;
         this.returnReply = false;
      }

      public ReplyData (Actions action, string reply, bool returnReply)
      {
         this.reply = reply;
         this.action = action;
         this.returnReply = returnReply;
      }

      // Properties. 
      public string Reply { get { return reply;}}
      public Actions Action { get { return action;}}

      public override string ToString()
      {
         return String.Format("Reply: {0} Action: {1} return? {2}", 
            reply, action.ToString(), returnReply.ToString());
      }
   }

   public class RedesignedRefAndOut
   {
      public static ReplyData ReplyInformation (TypeOfFeedback input)
      {
         ReplyData answer;
         string replyText = "Your feedback has been forwarded " + 
            "to the product manager.";

         switch (input)
         {
            case TypeOfFeedback.Complaint:
            case TypeOfFeedback.Praise :
               answer = new ReplyData(
                  Actions.ForwardToManagement,
                  "Thank you. " + replyText,
                  true);
               break;
            case TypeOfFeedback.Suggestion:
               answer =  new ReplyData(
                  Actions.ForwardToDeveloper,
                  replyText,
                  true);
               break;
            case TypeOfFeedback.Incomprehensible:
            default:
               answer = new ReplyData();
               break;
         }
         return answer;
      }
   }
}

The following application illustrates the experience of the user. The call to the redesigned library (UseTheSimplifiedClass method) is more straightforward, and the information returned by the method is easily managed. The output from the two methods is identical.

using System;

namespace DesignLibrary
{
   public class UseComplexMethod
   {
      static void UseTheComplicatedClass()
      {
         // Using the version with the ref and out parameters.  
         // You do not have to initialize an out parameter. 

         string[] reply = new string[5];

         // You must initialize a ref parameter.
         Actions[] action = {Actions.Unknown,Actions.Unknown,
                             Actions.Unknown,Actions.Unknown,
                             Actions.Unknown,Actions.Unknown}; 
         bool[] disposition= new bool[5];
         int i = 0;

         foreach(TypeOfFeedback t in Enum.GetValues(typeof(TypeOfFeedback)))
         {
            // The call to the library.
            disposition[i] = BadRefAndOut.ReplyInformation(
               t, out reply[i], ref action[i]);
            Console.WriteLine("Reply: {0} Action: {1}  return? {2} ", 
               reply[i], action[i], disposition[i]);
            i++;
         }
      }

      static void UseTheSimplifiedClass()
      {
         ReplyData[] answer = new ReplyData[5];
         int i = 0;
         foreach(TypeOfFeedback t in Enum.GetValues(typeof(TypeOfFeedback)))
         {
            // The call to the library.
            answer[i] = RedesignedRefAndOut.ReplyInformation(t);
            Console.WriteLine(answer[i++]);
         }
      }

      public   static void Main()
      {
         UseTheComplicatedClass();

         // Print a blank line in output.
         Console.WriteLine("");

         UseTheSimplifiedClass();
      }
   }
}

The following example library illustrates how ref parameters for reference types are used and shows a better way to implement this functionality.

using System;

namespace DesignLibrary
{
   public class ReferenceTypesAndParameters
   {

      // The following syntax will not work. You cannot make a 
      // reference type that is passed by value point to a new 
      // instance. This needs the ref keyword. 

      public static void BadPassTheObject(string argument)
      {
         argument = argument + " ABCDE";
      }

      // The following syntax will work, but is considered bad design. 
      // It reassigns the argument to point to a new instance of string. 
      // Violates rule DoNotPassTypesByReference. 

      public static void PassTheReference(ref string argument)
      {
         argument = argument + " ABCDE";
      }

      // The following syntax will work and is a better design. 
      // It returns the altered argument as a new instance of string. 

      public static string BetterThanPassTheReference(string argument)
      {
         return argument + " ABCDE";
      }
   }
}

The following application calls each method in the library to demonstrate the behavior.

using System;

namespace DesignLibrary
{
   public class Test
   {
      public static void Main()
      {
         string s1 = "12345";
         string s2 = "12345";
         string s3 = "12345";

         Console.WriteLine("Changing pointer - passed by value:");
         Console.WriteLine(s1);
         ReferenceTypesAndParameters.BadPassTheObject (s1);
         Console.WriteLine(s1);

         Console.WriteLine("Changing pointer - passed by reference:");
         Console.WriteLine(s2);
         ReferenceTypesAndParameters.PassTheReference (ref s2);
         Console.WriteLine(s2);

         Console.WriteLine("Passing by return value:");
         s3 = ReferenceTypesAndParameters.BetterThanPassTheReference (s3);
         Console.WriteLine(s3);
      }
   }
}

This example produces the following output.

Changing pointer - passed by value:
12345
12345
Changing pointer - passed by reference:
12345
12345 ABCDE
Passing by return value:
12345 ABCDE

Methods that implement the Try<Something> pattern, such as Int32.TryParse, do not raise this violation. The following example shows a structure (value type) that implements the Int32.TryParse method.

using System;

namespace Samples
{
    public struct Point
    {
        private readonly int _X;
        private readonly int _Y;

        public Point(int axisX, int axisY)
        {
            _X = axisX;
            _Y = axisY;
        }

        public int X
        {
            get { return _X; }
        }

        public int Y
        {
            get { return _Y; }
        }

        public override int GetHashCode()
        {
            return _X ^ _Y;
        }

        public override bool Equals(object obj)
        {
            if (!(obj is Point))
                return false;

            return Equals((Point)obj);
        }

        public bool Equals(Point other)
        {
            if (_X != other._X)
                return false;

            return _Y == other._Y;
        }

        public static bool operator ==(Point point1, Point point2)
        {
            return point1.Equals(point2);
        }

        public static bool operator !=(Point point1, Point point2)
        {
            return !point1.Equals(point2);
        }

        // Does not violate this rule 
        public static bool TryParse(string value, out Point result)
        {
            // TryParse Implementation
            result = new Point(0,0);
            return false;
        }
    }
}
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