Create sample for Intro To Nullable references (#491)

* first draft of code sample

Contributes to dotnet/docs#8208

* add snippet markers

Author: BillWagner

csharp/NullableIntroduction/NullableIntroduction.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 16
+VisualStudioVersion = 16.0.28315.86
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "NullableIntroduction", "NullableIntroduction\NullableIntroduction.csproj", "{B030A6FA-113E-4AC1-BDE4-7CF1DB29E22C}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{B030A6FA-113E-4AC1-BDE4-7CF1DB29E22C}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{B030A6FA-113E-4AC1-BDE4-7CF1DB29E22C}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{B030A6FA-113E-4AC1-BDE4-7CF1DB29E22C}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{B030A6FA-113E-4AC1-BDE4-7CF1DB29E22C}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {47456933-59EE-408A-AD9F-1887183AFCF8}
+	EndGlobalSection
+EndGlobal

csharp/NullableIntroduction/NullableIntroduction/NullableIntroduction.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <LangVersion>8.0</LangVersion>
+  </PropertyGroup>
+
+</Project>

csharp/NullableIntroduction/NullableIntroduction/Program.cs

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+using System;
+
+#nullable enable
+namespace NullableIntroduction
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            // <SnippetAddQuestions>
+            var surveyRun = new SurveyRun();
+            surveyRun.AddQuestion(QuestionType.YesNo, "Has your code ever thrown a NullReferenceException?");
+            surveyRun.AddQuestion(new SurveyQuestion(QuestionType.Number, "How many times (to the nearest 100) has that happened?"));
+            surveyRun.AddQuestion(QuestionType.Text, "What is your favorite color?");
+            // </SnippetAddQuestions>
+
+            // <SnippetRunSurvey>
+            surveyRun.PerformSurvey(50);
+            // </SnippetRunSurvey>
+
+            // <SnippetWriteAnswers>
+            foreach (var participant in surveyRun.AllParticipants)
+            {
+                Console.WriteLine($"Participant: {participant.Id}:");
+                if (participant.AnsweredSurvey)
+                {
+                    for (int i = 0; i < surveyRun.Questions.Count; i++)
+                    {
+                        var answer = participant.Answer(i);
+                        Console.WriteLine($"\t{surveyRun.GetQuestion(i)} : {answer}");
+                    }
+                }
+                else
+                    Console.WriteLine("\tNo responses");
+            }
+            // </SnippetWriteAnswers>
+        }
+    }
+}

csharp/NullableIntroduction/NullableIntroduction/SurveyQuestion.cs

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+#nullable enable
+namespace NullableIntroduction
+{
+    public enum QuestionType
+    {
+        YesNo,
+        Number,
+        Text
+    }
+
+    public class SurveyQuestion
+    {
+        public string QuestionText { get; }
+        public QuestionType TypeOfQuestion { get; }
+
+        public SurveyQuestion(QuestionType typeOfQuestion, string text) =>
+            (TypeOfQuestion, QuestionText) = (typeOfQuestion, text);
+    }
+}

csharp/NullableIntroduction/NullableIntroduction/SurveyResponse.cs

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+using System;
+using System.Collections.Generic;
+using System.Text;
+
+#nullable enable
+namespace NullableIntroduction
+{
+    public class SurveyResponse
+    {
+        // <SnippetRandom>
+        private static readonly Random randomGenerator = new Random();
+        public static SurveyResponse GetRandomId() => new SurveyResponse(randomGenerator.Next());
+        // </SnippetRandom>
+
+        public int Id { get; }
+
+        // <SnippetSurveyStatus>
+        public bool AnsweredSurvey => surveyResponses != null;
+        public string Answer(int index) => surveyResponses.GetValueOrDefault(index, "No answer")!;
+        // </SnippetSurveyStatus>
+
+        public SurveyResponse(int id) => Id = id;
+
+        // <SnippetAnswerSurvey>
+        private Dictionary<int, string>? surveyResponses;
+        public bool AnswerSurvey(IEnumerable<SurveyQuestion> questions)
+        {
+            if (ConsentToSurvey())
+            {
+                surveyResponses = new Dictionary<int, string>();
+                int index = 0;
+                foreach (var question in questions)
+                {
+                    var answer = GenerateAnswer(question);
+                    if (answer != null)
+                    {
+                        surveyResponses.Add(index, answer);
+                    }
+                    index++;
+                }
+            }
+            return surveyResponses != null;
+        }
+
+        private bool ConsentToSurvey() => randomGenerator.Next(0, 2) == 1;
+
+        private string? GenerateAnswer(SurveyQuestion question)
+        {
+            switch (question.TypeOfQuestion)
+            {
+                case QuestionType.YesNo:
+                    int n = randomGenerator.Next(-1, 2);
+                    return (n == -1) ? default : (n == 0) ? "No" : "Yes";
+                case QuestionType.Number:
+                    n = randomGenerator.Next(-30, 101);
+                    return (n < 0) ? default : n.ToString();
+                case QuestionType.Text:
+                default:
+                    switch (randomGenerator.Next(0, 5))
+                    {
+                        case 0:
+                            return default;
+                        case 1:
+                            return "Red";
+                        case 2:
+                            return "Green";
+                        case 3:
+                            return "Blue";
+                    }
+                    return "Red. No, Green. Wait.. Blue... AAARGGGGGHHH!";
+            }
+        }
+        // </SnippetAnswerSurvey>
+    }
+}

csharp/NullableIntroduction/NullableIntroduction/SurveyRun.cs

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+using System.Collections.Generic;
+using System.Linq;
+
+#nullable enable
+namespace NullableIntroduction
+{
+    public class SurveyRun
+    {
+        private List<SurveyQuestion> surveyQuestions = new List<SurveyQuestion>();
+
+        // <SnippetRunReport>
+        public IEnumerable<SurveyResponse> AllParticipants => (respondents ?? Enumerable.Empty<SurveyResponse>());
+        public ICollection<SurveyQuestion> Questions => surveyQuestions;
+        public SurveyQuestion GetQuestion(int index) => surveyQuestions[index];
+        // </SnippetRunReport>
+
+        public void AddQuestion(QuestionType type, string question) =>
+            AddQuestion(new SurveyQuestion(type, question));
+        public void AddQuestion(SurveyQuestion surveyQuestion) => surveyQuestions.Add(surveyQuestion);
+
+        // <SnippetPerformSurvey>
+        private List<SurveyResponse>? respondents;
+        public void PerformSurvey(int numberOfRespondents)
+        {
+            int repondentsConsenting = 0;
+            respondents = new List<SurveyResponse>();
+            while (repondentsConsenting < numberOfRespondents)
+            {
+                var respondent = SurveyResponse.GetRandomId();
+                if (respondent.AnswerSurvey(surveyQuestions))
+                    repondentsConsenting++;
+                respondents.Add(respondent);
+            }
+        }
+        // </SnippetPerformSurvey>
+    }
+}