When no valid password could be generated, throw an Exception if desired

Fixes Issue prjseal#13 - Should throw instead of returning "Try again" when a valid password cannot be found

PasswordGenerator.Tests/BasicTests.cs

@@ -117,6 +117,23 @@ public void PasswordGenerator_LengthOnly_ShouldNotThrowAnError()
             string result = pwd.Next();
             Assert.AreEqual(21,result.Length);
         }
+
+        [Test]
+        public void PasswordGenerator_ThrowOnMaximumAttemptsExceededEnabled_ShouldThrow()
+        {
+            Assert.Throws<MaximumAttemptsExceededException>(() =>
+            {
+                // Multiple attempts are needed to make sure the test throws
+                for (int i = 0; i < 1000; i++)
+                {
+                    // Maximum complexity, minimal allowed attempts
+                    var pwd = new Password(includeLowercase: true, includeUppercase: true, includeNumeric: true, includeSpecial: true, passwordLength: 4, maximumAttempts: 1)
+                        .ThrowOnMaximumAttemptsExceeded();
+
+                    pwd.Next();
+                }
+            });
+        }
 
         [Test]
         public void PasswordGenerator_NoLengthTest_ShouldNotThrowAnError()

PasswordGenerator/MaximumAttemptsExceededException.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace PasswordGenerator
+{
+    /// <summary>
+    /// Represents errors that occurs during random password generation when the configured maximum attempts are exceeded.
+    /// </summary>
+    [Serializable]
+    public sealed class MaximumAttemptsExceededException : Exception
+    {
+        public MaximumAttemptsExceededException()
+        {
+        }
+
+        private MaximumAttemptsExceededException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
+    }
+}

PasswordGenerator/Password.cs

@@ -6,7 +6,7 @@
 
 namespace PasswordGenerator
 {
-/// <summary>
+    /// <summary>
     ///     Generates random passwords and validates that they meet the rules passed in
     /// </summary>
     public class Password : IPassword
@@ -18,6 +18,7 @@ public class Password : IPassword
         private const bool DefaultIncludeNumeric = true;
         private const bool DefaultIncludeSpecial = true;
         private static RNGCryptoServiceProvider _rng;
+        private bool _throwOnMaximumAttemptsExceeded;
 
         public Password()
         {
@@ -106,10 +107,25 @@ public IPassword LengthRequired(int passwordLength)
             Settings.PasswordLength = passwordLength;
             return this;
         }
+
+        /// <summary>
+        ///     Configures the password generator to throw an exception instead of returning "Try again" as a password when the maximum number of attempts exceeded.
+        /// </summary>
+        /// <remarks>See: https://github.com/prjseal/PasswordGenerator/issues/13</remarks>
+        /// <returns>The <see cref="IPassword"/> instance.</returns>
+        public IPassword ThrowOnMaximumAttemptsExceeded()
+        {
+            _throwOnMaximumAttemptsExceeded = true;
+            return this;
+        }
 
         /// <summary>
         ///     Gets the next random password which meets the requirements
         /// </summary>
+        /// <exception cref="MaximumAttemptsExceededException">
+        ///     Thrown when no valid password could be generated (within the configured attempts threshold).
+        ///     Only throws when throwing is configured.
+        /// </exception>
         /// <returns>A password as a string</returns>
         public string Next()
         {
@@ -128,7 +144,17 @@ public string Next()
                     passwordAttempts++;
                 } while (passwordAttempts < Settings.MaximumAttempts && !PasswordIsValid(Settings, password));
 
-                password = PasswordIsValid(Settings, password) ? password : "Try again";
+                if (PasswordIsValid(Settings, password))
+                {
+                    return password;
+                }
+
+                if (_throwOnMaximumAttemptsExceeded)
+                {
+                    throw new MaximumAttemptsExceededException();
+                }
+
+                password = "Try again";
             }
 
             return password;