Making all assert ctor's private and the classes sealed. We want user…

…s to extend from Assert.That instead and not by creating instances of the Assert class which does not have any instance methods. (#224)

src/TestFramework/MSTest.Core/Assertions/Assert.cs

@@ -14,12 +14,16 @@ namespace Microsoft.VisualStudio.TestTools.UnitTesting
     /// unit tests. If the condition being tested is not met, an exception
     /// is thrown.
     /// </summary>
-    public class Assert
+    public sealed class Assert
     {
         private static Assert that;
 
         #region Singleton constructor
 
+        private Assert()
+        {
+        }
+
         /// <summary>
         /// Gets the singleton instance of the Assert functionality.
         /// </summary>

src/TestFramework/MSTest.Core/Assertions/CollectionAssert.cs

@@ -15,12 +15,16 @@ namespace Microsoft.VisualStudio.TestTools.UnitTesting
     /// with collections within unit tests. If the condition being tested is not
     /// met, an exception is thrown.
     /// </summary>
-    public class CollectionAssert
+    public sealed class CollectionAssert
     {
         private static CollectionAssert that;
 
         #region Singleton constructor
 
+        private CollectionAssert()
+        {
+        }
+
         /// <summary>
         /// Gets the singleton instance of the CollectionAssert functionality.
         /// </summary>

src/TestFramework/MSTest.Core/Assertions/StringAssert.cs

@@ -10,12 +10,16 @@ namespace Microsoft.VisualStudio.TestTools.UnitTesting
     /// <summary>
     /// The string assert.
     /// </summary>
-    public class StringAssert
+    public sealed class StringAssert
     {
         private static StringAssert that;
 
         #region Singleton constructor
 
+        private StringAssert()
+        {
+        }
+
         /// <summary>
         /// Gets the singleton instance of the CollectionAssert functionality.
         /// </summary>