Add CreateProtector convenience extension method

src/Microsoft.AspNet.Security.DataProtection/DataProtectionExtensions.cs

@@ -2,6 +2,8 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Diagnostics;
+using Microsoft.AspNet.Cryptography;
 
 namespace Microsoft.AspNet.Security.DataProtection
 {
@@ -21,6 +23,37 @@ namespace Microsoft.AspNet.Security.DataProtection
                 ?? new TimeLimitedDataProtector(protector.CreateProtector(TimeLimitedDataProtector.PurposeString));
         }
 
+        /// <summary>
+        /// Creates an IDataProtector given an array of purposes.
+        /// </summary>
+        /// <param name="provider">The provider from which to generate the purpose chain.</param>
+        /// <param name="purposes">
+        /// This is a convenience method used for chaining several purposes together
+        /// in a single call to CreateProtector. See the documentation of
+        /// IDataProtectionProvider.CreateProtector for more information.
+        /// </param>
+        /// <returns>An IDataProtector tied to the provided purpose chain.</returns>
+        public static IDataProtector CreateProtector([NotNull] this IDataProtectionProvider provider, params string[] purposes)
+        {
+            if (purposes == null || purposes.Length == 0)
+            {
+                throw new ArgumentException(Resources.DataProtectionExtensions_NullPurposesArray, nameof(purposes));
+            }
+
+            IDataProtectionProvider retVal = provider;
+            foreach (string purpose in purposes)
+            {
+                if (String.IsNullOrEmpty(purpose))
+                {
+                    throw new ArgumentException(Resources.DataProtectionExtensions_NullPurposesArray, nameof(purposes));
+                }
+                retVal = retVal.CreateProtector(purpose) ?? CryptoUtil.Fail<IDataProtector>("CreateProtector returned null.");
+            }
+
+            Debug.Assert(retVal is IDataProtector); // CreateProtector is supposed to return an instance of this interface
+            return (IDataProtector)retVal;
+        }
+
         /// <summary>
         /// Cryptographically protects a piece of plaintext data.
         /// </summary>

src/Microsoft.AspNet.Security.DataProtection/Properties/Resources.Designer.cs

@@ -186,6 +186,22 @@ namespace Microsoft.AspNet.Security.DataProtection
             return string.Format(CultureInfo.CurrentCulture, GetString("TimeLimitedDataProtector_PayloadExpired"), p0);
         }
 
+        /// <summary>
+        /// The purposes array cannot be null or empty and cannot contain null or empty elements.
+        /// </summary>
+        internal static string DataProtectionExtensions_NullPurposesArray
+        {
+            get { return GetString("DataProtectionExtensions_NullPurposesArray"); }
+        }
+
+        /// <summary>
+        /// The purposes array cannot be null or empty and cannot contain null or empty elements.
+        /// </summary>
+        internal static string FormatDataProtectionExtensions_NullPurposesArray()
+        {
+            return GetString("DataProtectionExtensions_NullPurposesArray");
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNet.Security.DataProtection/Resources.resx

@@ -150,4 +150,7 @@
   <data name="TimeLimitedDataProtector_PayloadExpired" xml:space="preserve">
     <value>The payload expired at {0}.</value>
   </data>
+  <data name="DataProtectionExtensions_NullPurposesArray" xml:space="preserve">
+    <value>The purposes array cannot be null or empty and cannot contain null or empty elements.</value>
+  </data>
 </root>

test/Microsoft.AspNet.Security.DataProtection.Test/DataProtectionExtensionsTests.cs

@@ -39,6 +39,43 @@ namespace Microsoft.AspNet.Security.DataProtection.Test
             Assert.Same(innerProtector, timeLimitedProtector.InnerProtector);
         }
 
+        [Theory]
+        [InlineData(new object[] { null })]
+        [InlineData(new object[] { new string[0] })]
+        [InlineData(new object[] { new string[] { null } })]
+        [InlineData(new object[] { new string[] { "the next value is bad", "" } })]
+        public void CreateProtector_Chained_FailureCases(string[] purposes)
+        {
+            // Arrange
+            var mockProtector = new Mock<IDataProtector>();
+            mockProtector.Setup(o => o.CreateProtector(It.IsAny<string>())).Returns(mockProtector.Object);
+            var provider = mockProtector.Object;
+
+            // Act & assert
+            var ex = Assert.Throws<ArgumentException>(() => provider.CreateProtector(purposes));
+            ex.AssertMessage("purposes", Resources.DataProtectionExtensions_NullPurposesArray);
+        }
+
+        [Fact]
+        public void CreateProtector_Chained_SuccessCase()
+        {
+            // Arrange
+            var finalExpectedProtector = new Mock<IDataProtector>().Object;
+
+            var thirdMock = new Mock<IDataProtector>();
+            thirdMock.Setup(o => o.CreateProtector("third")).Returns(finalExpectedProtector);
+            var secondMock = new Mock<IDataProtector>();
+            secondMock.Setup(o => o.CreateProtector("second")).Returns(thirdMock.Object);
+            var firstMock = new Mock<IDataProtector>();
+            firstMock.Setup(o => o.CreateProtector("first")).Returns(secondMock.Object);
+
+            // Act
+            var retVal = firstMock.Object.CreateProtector("first", "second", "third");
+
+            // Assert
+            Assert.Same(finalExpectedProtector, retVal);
+        }
+
         [Fact]
         public void Protect_InvalidUtf_Failure()
         {

test/Microsoft.AspNet.Security.DataProtection.Test/ExceptionHelpers.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Xunit;
+
+namespace Microsoft.AspNet.Security.DataProtection.Test
+{
+    internal static class ExceptionHelpers
+    {
+        public static void AssertMessage(this ArgumentException exception, string parameterName, string message)
+        {
+            Assert.Equal(parameterName, exception.ParamName);
+
+            // We'll let ArgumentException handle the message formatting for us and treat it as our control value
+            var controlException = new ArgumentException(message, parameterName);
+            Assert.Equal(controlException.Message, exception.Message);
+        }
+    }
+}