Tom DuPont .NET: CSharp

Showing posts with label CSharp. Show all posts

Thursday, June 30, 2016

Updating Default Parameters in .NET Assemblies

One of the things that I love about C# is how so many of it's features are just very conveniently designed compiler tricks. This means that, just like any other magic trick, once you know how the trick is performed you immediately realize that there really is nothing magical about it.

So, let's talk about default parameters. They are actually just constant values that get compiled into your code when you go to use a method that has them. Let's look at an example...

The Code

public enum Country

US,

CA

public static class SharedUtility

    public static bool CanDrink(

        int age,

        Country country = Country.CA)

        switch (country)

            case Country.US:

                return age >= 21;

            case Country.CA:

                return age >= 18;

            default:

                throw new ArgumentException(

                    "Invalid Country",

                    nameof(country));

public class SharedUtilityTests

    [Fact]

    public void CanDrink()

        var result = SharedUtility.CanDrink(20);

        // The line above will compile into the following:

        // var result = SharedUtility.CanDrink(20, Country.CA);

        // Thus, the Assert below will succeed!

        Assert.True(result, "The default was not US!");

So, now that you know how the trick is performed, how could this cause a problem for you? Specifically, what happens if you update a library that exposes methods with default parameters?

Nothing will change, until you recompile against the library!

If another library changes their default parameters, but you do not recompile your code against it, then your code will be using the old default values! Let's look back at the previous example and see why this could cause confusion...

Common Logging Extensions with Caller Information

Update: Added the BreakParameter.

I have made it an (arguably bad) habit of manually adding the class and method name as a prefix to all my log lines. It is not that I enjoy typing out the same strings over and over, it's that I do not always trust things like the NLog callsite. Using the stack frame to identify a calling method always requires a bit of cleverness on the part of the author, as you never can be totally sure when you are dealing with a wrapper class or an async call stack.

I was recently introduced to the Caller Information attributes in C# 5, and now I think I am in love.

Disclaimer: I have not used this very much yet, but I intend to start! I think that these attributes are absolutely brilliant in their simplicity: a compiler trick to insert debug information directly into your code. That is freak'n sweet, and it's performant! I am not sure how this flew under my radar, but now that I know about it....

Below is a little T4 template that I wrote up to generate overloads for Common Logging that will include caller information. To customize this for your needs, just update the the GetFormat method at the bottom of the template.

Unit Tests

   1:  using System;

   2:  using System.Collections.Generic;

   3:  using Common.Logging.Simple;

   4:  using Xunit;

5:

   6:  namespace Common.Logging.Tests

   7:  {

   8:      /// <summary>

   9:      /// Tests for Common Logging extensions that use Caller Information

  10:      /// https://msdn.microsoft.com/en-us/library/mt653988.aspx

  11:      /// </summary>

  12:      public class LogCallTests

  13:      {

  14:          [Fact]

  15:          public void LogFromMethod()

  16:          {

  17:              var log = new QueueSimpleLogger();

  18:              var ex = new Exception("Boom");

19:

  20:              log.Debug("Hello");

  21:              log.Debug("World", ex);

22:

  23:              log.DebugCall("Hello");

  24:              log.DebugCall("World", ex);

25:

  26:              log.WarnFormat("Hello - {0}", "Zero");

  27:              log.WarnFormat("World - {0}", ex, "Zero");

28:

  29:              log.WarnFormatCall("Hello - {0}", "Zero");

  30:              log.WarnFormatCall("World - {0}", ex, "Zero");

31:

  32:              Assert.Equal(8, log.Queue.Count);

33:

  34:              Assert.Equal("Hello", log.Queue.Dequeue());

  35:              Assert.Equal("World - Ex: Boom", log.Queue.Dequeue());

36:

  37:              Assert.Equal("LogCallTests.LogFromMethod(23) - Hello", log.Queue.Dequeue());

  38:              Assert.Equal("LogCallTests.LogFromMethod(24) - World - Ex: Boom", log.Queue.Dequeue());

39:

  40:              Assert.Equal("Hello - Zero", log.Queue.Dequeue());

  41:              Assert.Equal("World - Zero - Ex: Boom", log.Queue.Dequeue());

42:

  43:              Assert.Equal("LogCallTests.LogFromMethod(29) - Hello - Zero", log.Queue.Dequeue());

  44:              Assert.Equal("LogCallTests.LogFromMethod(30) - World - Zero - Ex: Boom", log.Queue.Dequeue());

  45:          }

46:

  47:          [Fact]

  48:          public void LogFromAction()

  49:          {

  50:              var log = new QueueSimpleLogger();

  51:              var ex = new Exception("Boom");

  52:              Action action = () =>

  53:              {

  54:                  log.Debug("Hello");

  55:                  log.Debug("World", ex);

56:

  57:                  log.DebugCall("Hello");

  58:                  log.DebugCall("World", ex);

59:

  60:                  log.WarnFormat("Hello - {0}", "Zero");

  61:                  log.WarnFormat("World - {0}", ex, "Zero");

62:

  63:                  log.WarnFormatCall("Hello - {0}", "Zero");

  64:                  log.WarnFormatCall("World - {0}", ex, "Zero");

  65:              };

66:

  67:              action();

68:

  69:              Assert.Equal(8, log.Queue.Count);

70:

  71:              Assert.Equal("Hello", log.Queue.Dequeue());

  72:              Assert.Equal("World - Ex: Boom", log.Queue.Dequeue());

73:

  74:              Assert.Equal("LogCallTests.LogFromAction(57) - Hello", log.Queue.Dequeue());

  75:              Assert.Equal("LogCallTests.LogFromAction(58) - World - Ex: Boom", log.Queue.Dequeue());

76:

  77:              Assert.Equal("Hello - Zero", log.Queue.Dequeue());

  78:              Assert.Equal("World - Zero - Ex: Boom", log.Queue.Dequeue());

79:

  80:              Assert.Equal("LogCallTests.LogFromAction(63) - Hello - Zero", log.Queue.Dequeue());

  81:              Assert.Equal("LogCallTests.LogFromAction(64) - World - Zero - Ex: Boom", log.Queue.Dequeue());

  82:          }

83:

  84:          private class QueueSimpleLogger : AbstractSimpleLogger

  85:          {

  86:              public readonly Queue<string> Queue = new Queue<string>();

87:

  88:              public QueueSimpleLogger()

  89:                  : base(string.Empty, LogLevel.All, true, true, true, string.Empty)

  90:              {

  91:              }

92:

  93:              protected override void WriteInternal(LogLevel level, object message, Exception exception)

  94:              {

  95:                  var s = message.ToString();

  96:                  if (exception != null) s += " - Ex: " + exception.Message;

  97:                  Queue.Enqueue(s);

  98:              }

  99:          }

 100:      }

 101:  }

C# Interfaces and Default Parameters

How does a default parameter work?

So, let's talk about default parameters. They are actually just constant values that get compiled into your code when you go to use a method that has them. Let's look at an example...

public class DefaultParamTests1

    [Fact]

    public void WhatYouWrite()

        var actual = Double();

        Assert.Equal(2, actual);

    private static int Double(int i = 1)

        return i * 2;

public class DefaultParamTests2

    [Fact]

    public void WhatItCompilesTo()

        var actual = Double(1);

        Assert.Equal(2, actual);

    private static int Double(int i)

        return i * 2;

What happens when interfaces and methods don't match?

So, now that you know how the trick is performed, what happens if you use a different default value for a parameter defined by an interface and a class?

The answer is simple: if your object is cast as the class, then it will use the class value. If your object is cast as the interface, it will use the interface value instead. Let's take a look at another example...

Tom DuPont .NET