VisitorImpl

Program.cs

using System;
using Visitor;

public class Program {
    public static void Main () {
        var baseEntry = new SimpleLogEntry () {
            Created = DateTime.Now,
            Message = "test"
        };

        var simpleEntry = new SimpleLogEntry (baseEntry) {
            AdditionalInfo = "additional"
        };

        var exceptionEntry = new ExceptionLogEntry (baseEntry) {
            ExceptionCode = 111
        };
        //it is possible to declare multiple visitors for particular hierarchy of objects
        // for example: PersistentLogEntryVisitors that allows to process LogEntries and save them to storage
        var visitor = new LogEntryVisitor ();
        visitor.ProcessLogEntry (simpleEntry);
        Assert.Contains ("simple", visitor.State);
        visitor.ProcessLogEntry (exceptionEntry);
        Assert.Contains ("exception", visitor.State);
        TestRunner.Print ();
    }
}

VisitorImpl.cs

using System;

namespace Visitor {
    public interface ILogEntryVisitor {
        void Visit (ExceptionLogEntry logEntry);
        void Visit (SimpleLogEntry logEntry);
    }
}
namespace Visitor {
    public class LogEntryVisitor : ILogEntryVisitor {
        public string State { get; set; }
        public void ProcessLogEntry (LogEntry logEntry) {
            logEntry.Accept (this);
        }
        void ILogEntryVisitor.Visit (ExceptionLogEntry exceptionLogEntry) {
            State = $"LogEntryVisitor[{nameof(exceptionLogEntry)}]";
        }
        void ILogEntryVisitor.Visit (SimpleLogEntry simpleLogEntry) {
            State = $"LogEntryVisitor[{nameof(simpleLogEntry)}]";
        }
    }
}

namespace Visitor {
    public class ExceptionLogEntry : LogEntry {
        public ExceptionLogEntry (LogEntry log) : base (log) { }

        public int ExceptionCode { get; set; }

        public override void Accept (ILogEntryVisitor visitor) {
            visitor.Visit (this);
        }
    }
}

namespace Visitor {
    public abstract class LogEntry {
        public DateTime Created { get; set; }
        public String Message { get; set; }

        public LogEntry (LogEntry log) {
            log.CopyTo (this);
        }

        public LogEntry () { }

        public abstract void Accept (ILogEntryVisitor visitor);

        public void CopyTo (LogEntry log) {
            log.Created = this.Created;
            log.Message = this.Message;
        }
    }
}

namespace Visitor {
    public class SimpleLogEntry : LogEntry {
        public SimpleLogEntry () { }

        public SimpleLogEntry (LogEntry log) : base (log) { }
        public string AdditionalInfo { get; set; }

        public override void Accept (ILogEntryVisitor visitor) {
            visitor.Visit (this);
        }
    }
}

XUnit.Utils.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;

public static class Assert {
    public static void Equal<T> (T a, T b, [CallerMemberName] string callerName = "", [CallerLineNumber] int callerLine = 0) {
        if (!a.Equals (b)) {
            // throw new Exception ($"{a} doesn't equal to {b}");
            TestRunner.AddResult (
                new TestRunResult () {
                    CallerName = callerName,
                        CallerLine = callerLine.ToString (),
                        Success = false,
                        Assertion = "Equal<T>",
                        ErrorMessage = $"{a} doesn't equal to {b}"
                });
        } else {
            TestRunner.AddResult (
                new TestRunResult () {
                    CallerName = callerName,
                        CallerLine = callerLine.ToString (),
                        Success = true,
                        Assertion = "Single<T>"
                });
        }
    }

    public static void Equal<T> (IEnumerable<T> a,
        IEnumerable<T> b, [CallerMemberName] string callerName = "", [CallerLineNumber] int callerLine = 0) {
        if (a.Count () != b.Count ()) {
            TestRunner.AddResult (
                new TestRunResult () {
                    CallerName = callerName,
                        CallerLine = callerLine.ToString (),
                        Success = false,
                        Assertion = "Equal<T>",
                        ErrorMessage = $"a.{a.Count()} != b.{b.Count()}"
                });
        } else if (!a.SequenceEqual (b)) {
            TestRunner.AddResult (
                new TestRunResult () {
                    CallerName = callerName,
                        CallerLine = callerLine.ToString (),
                        Success = false,
                        Assertion = "SequenceEqual<T>",
                        ErrorMessage = $"Different sequences"
                });
        } else {
            TestRunner.AddResult (
                new TestRunResult () {
                    CallerName = callerName,
                        CallerLine = callerLine.ToString (),
                        Success = true,
                        Assertion = "Equal<T>"
                });
        }
    }

    public static void Single<T> (IEnumerable<T> collection, [CallerMemberName] string callerName = "", [CallerLineNumber] int callerLine = 0) {
        if (collection.Count () != 1) {
            // throw new Exception ("Collection doesn't contain 1 element");
            TestRunner.AddResult (
                new TestRunResult () {
                    CallerName = callerName,
                        CallerLine = callerLine.ToString (),
                        Success = false,
                        Assertion = "Single<T>",
                        ErrorMessage = "Collection doesn't contain 1 element"
                });
        }
        TestRunner.AddResult (
            new TestRunResult () {
                CallerName = callerName,
                    CallerLine = callerLine.ToString (),
                    Success = true,
                    Assertion = "Single<T>"
            });
    }

    public static void Contains<T> (T token1, T token2, [CallerMemberName] string callerName = "", [CallerLineNumber] int callerLine = 0) {
        if (typeof (T) == typeof (System.String)) {
            if (!((token2 as string)).Contains (token1 as string)) {
                // throw new Exception ($"{token2} doesn't contain {token1}");
                TestRunner.AddResult (
                    new TestRunResult () {
                        CallerName = callerName,
                            CallerLine = callerLine.ToString (),
                            Success = false,
                            Assertion = "Contains<T>",
                            ErrorMessage = $"{token2} doesn't contain {token1}"
                    });
            } else {
                TestRunner.AddResult (
                    new TestRunResult () {
                        CallerName = callerName,
                            CallerLine = callerLine.ToString (),
                            Success = true,
                            Assertion = "Contains<T>"
                    });
            }
        } else {
            throw new Exception ("It is not possible to use .Contains() method");
        }
    }

    public static void All<T> (IEnumerable<T> collection, Func<T, bool> predicate, [CallerMemberName] string callerName = "", [CallerLineNumber] int callerLine = 0) {
        bool res = collection.All (predicate);
        var tr = new TestRunResult () {
                CallerName = callerName,
                    CallerLine = callerLine.ToString (),
                    Success = res,
                    Assertion = "All<T>"
            };
        TestRunner.AddResult (tr);
        if(!res) {
            tr.ErrorMessage = "Condition is not met for all members";
        }
    }

}

public static class TestRunner {
    public static List<TestRunResult> Results { get; } = new List<TestRunResult> ();

    public static void AddResult (TestRunResult res) {
        Results.Add (res);
    }

    public static void Print () {
        System.Console.WriteLine ("TestRunner.Print");
        System.Console.WriteLine ("============================================");
        foreach (var log in Results) {
            System.Console.WriteLine (log);
        }
        System.Console.WriteLine ("============================================");
    }
}

public class TestRunResult {
    public string CallerName { get; set; }
    public string CallerLine { get; set; }
    public string Assertion { get; set; }
    public bool Success { get; set; }
    public string ErrorMessage { get; set; }
    public override string ToString () {
        return $"Assertion: {Assertion, 15}; MethodName: {CallerName, 10}; Line: {CallerLine, 3}; " + (Success? $"Success: {Success}": $"ErrorMessage: {ErrorMessage}");
    }
}