Implement new concept exercise for extension methods

concepts/extension-methods/.meta/config.json

@@ -0,0 +1,6 @@
+{
+  "blurb": "Extension methods allow you to add methods to existing types.",
+  "authors": [
+    "yzAlvin"
+  ]
+}

concepts/extension-methods/about.md

@@ -0,0 +1,22 @@
+# About
+
+[Extension methods][extension-methods] allow adding methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.
+
+Extension methods are brought into scope at the namespace level.
+
+Extension methods are static methods, but they're called as if they were instance methods on the extended type. It achieves this by using `this` before the type, indicating the instance we put the `.` on is passed as the first parameter. For client code, there's no apparent difference between calling an extension method and the methods defined in a type.
+
+```csharp
+public static int WordCount(this string str)
+{
+    return str.Split().Length;
+}
+
+"Hello World".WordCount();
+// => 2
+```
+
+A well-known example of extension methods are the [LINQ][linq] standard query operators that add query functionality to the existing IEnumerable types.
+
+[linq]: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/
+[extension-methods]: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods

concepts/extension-methods/introduction.md

@@ -0,0 +1,13 @@
+# Introduction
+
+Extension methods allow adding methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. They are defined as static methods but are called by using instance method syntax. Their first parameter is preceded by the `this` modifier, and specifies which type the method operates on, and are brought into scope at the namespace level.
+
+```csharp
+public static int WordCount(this string str)
+{
+    return str.Split().Length;
+}
+
+"Hello World".WordCount();
+// => 2
+```

concepts/extension-methods/links.json

@@ -0,0 +1,6 @@
+[
+  {
+    "url": "https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods",
+    "description": "extension-methods"
+  }
+]

config.json

@@ -420,6 +420,15 @@
           "overflow",
           "integral-numbers"
         ]
+      },
+      {
+        "slug": "log-analysis",
+        "name": "Log Analysis",
+        "uuid": "4a940e54-eda8-4500-bcb2-25d83af01fb1",
+        "concepts": ["extension-methods"],
+        "prerequisites": [
+          "strings"
+        ]
       }
     ],
     "practice": [
@@ -1931,6 +1940,11 @@
       "slug": "expression-bodied-members",
       "name": "Expression Bodied Members"
     },
+    {
+      "uuid": "c1e57d81-8965-4e3f-96d1-44ff43b768e4",
+      "slug": "extension-methods",
+      "name": "Extension Methods"
+    },
     {
       "uuid": "0d0ce715-a7f2-42f4-b97b-54ba99b4df73",
       "slug": "flag-enums",

exercises/concept/calculator-conundrum/CalculatorConundrum.cs

@@ -15,7 +15,6 @@ public static int Division(int operand1, int operand2)
     {
         return operand1 / operand2;
     }
-
     public static int Multiplication(int operand1, int operand2)
     {
         return operand1 * operand2;

exercises/concept/log-analysis/.docs/hints.md

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+# Hints
+
+## General
+
+- All extension methods must be marked `static`.
+- All extension methods must have `this` before the first argument
+
+## 1. Implement the extension method SubstringAfter
+
+- Different options to search for text in a string are explored in [this tutorial][tutorial-docs.microsoft.com-search-text-in-string].
+
+## 2. Implement the extension method SubstringBetween
+
+- Different options to search for text in a string are explored in [this tutorial][tutorial-docs.microsoft.com-search-text-in-string].
+
+## 3. Parse message in a log
+
+- You can re-use the `SubstringAfter()` extension method here.
+
+## 4. Parse log level in a log
+
+- You can re-use the `SubstringBetween()` extension method here.
+
+[tutorial-csharp.net-strings]: https://csharp.net-tutorials.com/data-types/strings/

exercises/concept/log-analysis/.docs/instructions.md

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+# Instructions
+
+In this exercise you'll be processing log-lines.
+
+Each log line is a string formatted as follows: `"[<LEVEL>]: <MESSAGE>"`.
+
+There are three different log levels:
+
+- `INFO`
+- `WARNING`
+- `ERROR`
+
+You have several tasks, each of which will take a log line and ask you to do something with it.
+
+## 1. Allow retrieving the string after a specific substring
+
+Looking at the logs of the last month, you see that the test message is always located after a specific substring. As you're anticipating having to extract the test message sometime in the near future, you decide to create a helper method to help you with that.
+
+Implement the (_static_) `LogAnalysis.SubstringAfter()` extension method, that takes in some string delimeter and returns the substring after the delimiter.
+
+```csharp
+var log = "[INFO]: File Deleted.";
+log.SubstringAfter(": "); // => returns "File Deleted."
+```
+
+## 2. Allow retrieving the string in between two substrings
+
+On further inspection, you see that the log level is always located between square brackets (`[` and `]`). As you're also anticipating having to extract the log level sometime in the near future, you decide to create a another helper method to help you with that.
+
+Implement the (_static_) `LogAnalysis.SubstringBetween()` extension method that takes in two string delimeters, and returns the substring that lies between the two delimeters.
+
+```csharp
+var log = "[INFO]: File Deleted.";
+log.SubstringBetween("[", "]"); // => returns "INFO"
+```
+
+## 3. Parse message in a log
+
+Implement the (_static_) `LogAnalysis.Message()` extension method to return the message contained in a log.
+
+```csharp
+var log = "[ERROR]: Missing ; on line 20.";
+log.Message(); // => returns "Missing ; on line 20."
+```
+
+## 4. Parse log level in a log
+
+Implement the (_static_) `LogAnalysis.LogLevel()` extension method to return the log level of a log.
+
+```csharp
+var log = "[ERROR]: Missing ; on line 20.";
+log.LogLevel(); // => returns "ERROR"
+```

exercises/concept/log-analysis/.docs/introduction.md

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+# Introduction
+
+Extension methods allow adding methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. They are defined as static methods but are called by using instance method syntax. Their first parameter is preceded by the 'this' modifier, and specifies which type the method operates on.
+
+```csharp
+public static int WordCount(this string str)
+{
+    return str.Split().Length;
+}
+"Hello World".WordCount();
+// => 2

exercises/concept/log-analysis/.meta/Exemplar.cs

@@ -0,0 +1,24 @@
+using System;
+
+public static class LogAnalysis 
+{
+    public static string SubstringAfter(this string str, string delimiter)
+    {
+        return str.Substring(str.IndexOf(delimiter) + delimiter.Length);
+    }
+
+    public static string SubstringBetween(this string str, string start, string stop)
+    {
+        return str.SubstringAfter(start).Substring(0, str.IndexOf(stop) - 1);
+    }
+
+    public static string Message(this string log)
+    {
+        return log.SubstringAfter("]: ");
+    }
+
+    public static string LogLevel(this string log)
+    {
+        return log.SubstringBetween("[", "]");
+    }
+}

exercises/concept/log-analysis/.meta/config.json

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+{
+  "blurb": "Learn about extension-methods by analysing logs.",
+  "icon": "log-levels",
+  "authors": [
+    "yzAlvin"
+  ],
+  "files": {
+    "solution": [
+      "LogAnalysis.cs"
+    ],
+    "test": [
+      "LogAnalysisTests.cs"
+    ],
+    "exemplar": [
+      ".meta/Exemplar.cs"
+    ]
+  }
+}

exercises/concept/log-analysis/.meta/design.md

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+# Design
+
+## Goal
+
+The goal of this exercise is to teach the student the Concept of Extension Methods in C#.
+
+## Learning objectives
+
+- Know what extension methods are.
+- Know how to define extension methods on reference and value types
+
+## Out of scope
+
+- Assemblies.
+
+## Concepts
+
+- `extension-methods`: know what extension methods are; know how to define extension methods on reference and value types.
+
+## Prerequisites
+
+- `classes`: know what instance methods are.
+- `namespaces`: know what namespaces are; know how to import namespaces.
+- `collections`: know what collections are.
+
+## Resources to refer to
+
+### Hints
+
+- [Extension methods][extension-methods]: how to define extension methods.
+- [Namespaces][namespaces]: how to define an import namespaces.
+- [Collections][collections]: what a collections is.
+
+### After
+
+- [Extension methods][extension-methods]: how to define extension methods.
+- [Namespaces][namespaces]: how to define an import namespaces.
+- [Collections][collections]: what a collections is.
+
+[extension-methods]: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods
+[namespaces]: https://docs.microsoft.com/en-us/dotnet/csharp/fundamentals/types/namespaces
+[collections]: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/collections

exercises/concept/log-analysis/LogAnalysis.cs

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+using System;
+
+public static class LogAnalysis 
+{
+    // TODO: define the 'SubstringAfter()' extension method on the `string` type
+
+    // TODO: define the 'SubstringBetween()' extension method on the `string` type
+
+    // TODO: define the 'Message()' extension method on the `string` type
+
+    // TODO: define the 'LogLevel()' extension method on the `string` type
+}

exercises/concept/log-analysis/LogAnalysis.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net5.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.3" />
+    <PackageReference Include="xunit" Version="2.4.1" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
+    <PackageReference Include="Exercism.Tests" Version="0.1.0-beta1" />
+  </ItemGroup>
+
+</Project>

exercises/concept/log-analysis/LogAnalysisTests.cs

@@ -0,0 +1,38 @@
+using Xunit;
+using Exercism.Tests;
+using System;
+
+public class LogAnalysisTests
+{
+    [Fact]
+    public void SubstringAfter_WithDelimeterOfLength1()
+    {
+        Assert.Equal(" am the 1st test", "I am the 1st test".SubstringAfter("I"));
+    }
+
+    [Fact(Skip = "Remove this Skip property to run this test")]
+    public void SubstringAfter_WithDelimeterOfLengthLongerThan1()
+    {
+        Assert.Equal(" test", "I am the 2nd test".SubstringAfter("2nd"));
+    }
+
+    [Fact(Skip = "Remove this Skip property to run this test")]
+    public void SubstringBetween()
+    {
+        Assert.Equal("INFO", "[INFO]: File Deleted.".SubstringBetween("[", "]"));
+    }
+
+    [Fact(Skip = "Remove this Skip property to run this test")]
+    public void Message()
+    {
+        var log = "[WARNING]: Library is deprecated.";
+        Assert.Equal("Library is deprecated.", log.Message());
+    }
+
+    [Fact(Skip = "Remove this Skip property to run this test")]
+    public void LogLevel()
+    {
+        var log = "[WARNING]: Library is deprecated.";
+        Assert.Equal("WARNING", log.LogLevel());;
+    }
+}