refactor(striped backups): Making the comparers singleton

Author: AdityaNPL

src/BackupChain.cs

@@ -24,7 +24,7 @@ private BackupChain(IList<BackupMetadata> recentBackups)
       if(recentBackups == null || recentBackups.Count == 0)
         throw new BackupChainException("There are no recent backups to form a chain");
 
-      var backups = recentBackups.Distinct(new BackupMetadataEqualityComparer())
+      var backups = recentBackups.Distinct(BackupMetadataEqualityComparer.Instance)
         .Where(IsValidFilePath) // A third party application caused invalid path strings to be inserted into backupmediafamily
         .ToList();
 
@@ -94,7 +94,7 @@ private static IEnumerable<BackupMetadata> NextLogBackup(IEnumerable<BackupMetad
       return backups.Where(b => b.BackupType == BackupFileTools.BackupType.Log &&
                                 prevBackup.LastLsn >= b.FirstLsn &&
                                 prevBackup.LastLsn <= b.LastLsn &&
-                                !new StripedBackupEqualityComparer().Equals(prevBackup, b));
+                                !StripedBackupEqualityComparer.Instance.Equals(prevBackup, b));
     }
 
     private static bool IsValidFilePath(BackupMetadata meta)

src/BackupMetadata.cs

@@ -7,6 +7,20 @@ namespace AgDatabaseMove
 
   public class StripedBackupEqualityComparer : IEqualityComparer<BackupMetadata>
   {
+    private static StripedBackupEqualityComparer _instance = null;
+    private StripedBackupEqualityComparer() { }
+
+    public static StripedBackupEqualityComparer Instance {
+      get
+      {
+        if (_instance == null)
+        {
+          _instance = new StripedBackupEqualityComparer();
+        }
+        return _instance;
+      }
+    }
+
     public bool Equals(BackupMetadata x, BackupMetadata y)
     {
       return x.LastLsn == y.LastLsn &&
@@ -37,8 +51,23 @@ public int GetHashCode(BackupMetadata obj)
   /// </summary>
   public class BackupMetadataEqualityComparer : IEqualityComparer<BackupMetadata>
   {
-    private readonly StripedBackupEqualityComparer _stripedBackupEqualityComparer = new StripedBackupEqualityComparer();
-    
+    private readonly StripedBackupEqualityComparer _stripedBackupEqualityComparer = StripedBackupEqualityComparer.Instance;
+
+    private static BackupMetadataEqualityComparer _instance = null;
+    private BackupMetadataEqualityComparer() { }
+
+    public static BackupMetadataEqualityComparer Instance
+    {
+      get
+      {
+        if (_instance == null)
+        {
+          _instance = new BackupMetadataEqualityComparer();
+        }
+        return _instance;
+      }
+    }
+
     public bool Equals(BackupMetadata x, BackupMetadata y)
     {
       return _stripedBackupEqualityComparer.Equals(x, y)
@@ -93,7 +122,7 @@ private StripedBackupSet(IEnumerable<BackupMetadata> stripedBackups)
     public static IEnumerable<StripedBackupSet> GetStripedBackupSetChain(IEnumerable<BackupMetadata> backups)
     {
       var chain = backups
-        .GroupBy(b => b, new StripedBackupEqualityComparer())
+        .GroupBy(b => b, StripedBackupEqualityComparer.Instance)
         .Select(group => new StripedBackupSet(group));
       return chain;
     }

tests/AgDatabaseMove.Unit/StripedBackupSetTests.cs

@@ -9,8 +9,8 @@ namespace AgDatabaseMove.Unit
   public class StripedBackupSetTests
   {
 
-    private static readonly StripedBackupEqualityComparer stripedBackupComparer = new StripedBackupEqualityComparer();
-    private static readonly BackupMetadataEqualityComparer backupComparer = new BackupMetadataEqualityComparer();
+    private static readonly StripedBackupEqualityComparer stripedBackupComparer = StripedBackupEqualityComparer.Instance;
+    private static readonly BackupMetadataEqualityComparer backupComparer = BackupMetadataEqualityComparer.Instance;
 
     [Fact]
     public void CombinesStripedBackups()