Determine Whether Two Date Ranges Overlap Tests

Program.cs

namespace ConsoleApplication
{
    using System;

    class Program
    {
        /// <param name="args"></param>
        static void Main(string[] args)
        {
            bool isoverlap;
            string testResult;
            
            DateTime? A = new DateTime(2017, 10, 1);
            DateTime? B = new DateTime(2017, 10, 10);

            DateTime? C = null;
            DateTime? D = null;

            //#1
            Console.WriteLine("SCENARIO#1:");
            Console.WriteLine("           A |---------------------| B");
            Console.WriteLine("      C |--------| D");
            C = A.Value.AddDays(-2);
            D = A.Value.AddDays(2);
            Console.ForegroundColor = ConsoleColor.Yellow;
            isoverlap = isoverlapping(A, B, C, D);
            testResult = (isoverlap == true) ? "PASSED" : "FAILED";
            Console.WriteLine("OVERLAP?: {0}", isoverlap.ToString());
            Console.ForegroundColor = (testResult == "PASSED") ? ConsoleColor.Green : ConsoleColor.Red;
            Console.WriteLine("TEST RESULT: {0}", testResult);
            Console.ResetColor();


            //#2
            Console.WriteLine("SCENARIO#2:");
            Console.WriteLine("            A |---------------------| B");
            Console.WriteLine("                              C |--------| D");
            C = A.Value.AddDays(2);
            D = B.Value.AddDays(2);
            Console.ForegroundColor = ConsoleColor.Yellow;
            isoverlap = isoverlapping(A, B, C, D);
            testResult = (isoverlap == true) ? "PASSED" : "FAILED";
            Console.WriteLine("OVERLAP?: {0}", isoverlap.ToString());
            Console.ForegroundColor = (testResult == "PASSED") ? ConsoleColor.Green : ConsoleColor.Red;
            Console.WriteLine("TEST RESULT: {0}", testResult);
            Console.ResetColor();


            //#3
            Console.WriteLine("SCENARIO#3:");
            Console.WriteLine("            A |---------------------| B");
            Console.WriteLine("            C |---------------------| D");
            C = A;
            D = B;
            Console.ForegroundColor = ConsoleColor.Yellow;
            isoverlap = isoverlapping(A, B, C, D);
            testResult = (isoverlap == true) ? "PASSED" : "FAILED";
            Console.WriteLine("OVERLAP?: {0}", isoverlap.ToString());
            Console.ForegroundColor = (testResult == "PASSED") ? ConsoleColor.Green : ConsoleColor.Red;
            Console.WriteLine("TEST RESULT: {0}", testResult);
            Console.ResetColor();


            //#4
            Console.WriteLine("SCENARIO#4:");
            Console.WriteLine("            A |---------------------| B");
            Console.WriteLine("                  C |--------| D");
            C = A.Value.AddDays(2);
            D = B.Value.AddDays(-2);
            Console.ForegroundColor = ConsoleColor.Yellow;
            isoverlap = isoverlapping(A, B, C, D);
            testResult = (isoverlap == true) ? "PASSED" : "FAILED";
            Console.WriteLine("OVERLAP?: {0}", isoverlap.ToString());
            Console.ForegroundColor = (testResult == "PASSED") ? ConsoleColor.Green : ConsoleColor.Red;
            Console.WriteLine("TEST RESULT: {0}", testResult);
            Console.ResetColor();


            //#5
            Console.WriteLine("SCENARIO#5:");
            Console.WriteLine("                          A |---------------------| B");
            Console.WriteLine("      C |--------| D");
            C = A.Value.AddDays(-2);
            D = A.Value.AddDays(-1);
            Console.ForegroundColor = ConsoleColor.Yellow;
            isoverlap = isoverlapping(A, B, C, D);
            testResult = (isoverlap == false) ? "PASSED" : "FAILED";
            Console.WriteLine("OVERLAP?: {0}", isoverlap.ToString());
            Console.ForegroundColor = (testResult == "PASSED") ? ConsoleColor.Green : ConsoleColor.Red;
            Console.WriteLine("TEST RESULT: {0}", testResult);
            Console.ResetColor();


            //#6
            Console.WriteLine("SCENARIO#6:");
            Console.WriteLine("            A |---------------------| B");
            Console.WriteLine("                                              C |--------| D");
            C = B.Value.AddDays(2);
            D = B.Value.AddDays(3);
            Console.ForegroundColor = ConsoleColor.Yellow;
            isoverlap = isoverlapping(A, B, C, D);
            testResult = (isoverlap == false) ? "PASSED" : "FAILED";
            Console.WriteLine("OVERLAP?: {0}", isoverlap.ToString());
            Console.ForegroundColor = (testResult == "PASSED") ? ConsoleColor.Green : ConsoleColor.Red;
            Console.WriteLine("TEST RESULT: {0}", testResult);
            Console.ResetColor();

            //#7
            A = new DateTime(2017, 10, 1);
            B = new DateTime(2017, 10, 1);
            C = A;
            D = B;
            Console.WriteLine("SCENARIO#7: (Same Day for both sets)");
            Console.WriteLine("            A || B");
            Console.WriteLine("            C || D");
            C = B;
            D = B;
            Console.ForegroundColor = ConsoleColor.Yellow;
            isoverlap = isoverlapping(A, B, C, D);
            testResult = (isoverlap == true) ? "PASSED" : "FAILED";
            Console.WriteLine("OVERLAP?: {0}", isoverlap.ToString());
            Console.ForegroundColor = (testResult == "PASSED") ? ConsoleColor.Green : ConsoleColor.Red;
            Console.WriteLine("TEST RESULT: {0}", testResult);
            Console.ResetColor();
             
            Console.ReadKey();
        }

        static bool isoverlapping(DateTime? A, DateTime? B, DateTime? C, DateTime? D)
        {
            //return ((A.HasValue ? (A < D) : true) && (B.HasValue ? (C < B) : true));

            //return !((B.HasValue ? (B <= C) : true) || (A.HasValue ? (A >= D) : true));

             return ((A == null || D == null || A <= D) 
                && (C == null || B == null || C <= B)
                && (A == null || B == null || A <= B)
                && (C == null || D == null || C <= D)); //or:    WORKS!!!
            //(A <= D) && (A <= B) && (C <= B) && (C <= D) or,
            //(A <= Min(B, D) && (C <= Min(B, D)) or:
            //(Max(A, C) <= Min(B, D));
        }
    }
}

what about d is null and c is smaller than a, getting wrong result.