diff --git a/src/mscorlib/shared/System/Collections/Generic/Dictionary.cs b/src/mscorlib/shared/System/Collections/Generic/Dictionary.cs
index 8792119c0460..09434e6f54b2 100644
--- a/src/mscorlib/shared/System/Collections/Generic/Dictionary.cs
+++ b/src/mscorlib/shared/System/Collections/Generic/Dictionary.cs
@@ -72,10 +72,14 @@ public Dictionary(int capacity, IEqualityComparer<TKey> comparer)
         {
             if (capacity < 0) ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity);
             if (capacity > 0) Initialize(capacity);
-            this.comparer = comparer ?? EqualityComparer<TKey>.Default;
+            if (comparer != EqualityComparer<TKey>.Default)
+            {
+                this.comparer = comparer;
+            }
 
-            if (this.comparer == EqualityComparer<string>.Default)
+            if (typeof(TKey) == typeof(string) && comparer == null)
             {
+                // To start, move off default comparer for string which is randomised
                 this.comparer = (IEqualityComparer<TKey>)NonRandomizedStringEqualityComparer.Default;
             }
         }
@@ -139,13 +143,7 @@ protected Dictionary(SerializationInfo info, StreamingContext context)
             HashHelpers.SerializationInfoTable.Add(this, info);
         }
 
-        public IEqualityComparer<TKey> Comparer
-        {
-            get
-            {
-                return comparer;
-            }
-        }
+        public IEqualityComparer<TKey> Comparer => comparer ?? EqualityComparer<TKey>.Default;
 
         public int Count
         {
@@ -283,10 +281,9 @@ public bool ContainsValue(TValue value)
             }
             else
             {
-                EqualityComparer<TValue> c = EqualityComparer<TValue>.Default;
                 for (int i = 0; i < count; i++)
                 {
-                    if (entries[i].hashCode >= 0 && c.Equals(entries[i].value, value)) return true;
+                    if (entries[i].hashCode >= 0 && (default(TValue) == null ? value.Equals(entries[i].value) : EqualityComparer<TValue>.Default.Equals(entries[i].value, value))) return true;
                 }
             }
             return false;
@@ -338,7 +335,7 @@ public virtual void GetObjectData(SerializationInfo info, StreamingContext conte
             }
 
             info.AddValue(VersionName, version);
-            info.AddValue(ComparerName, comparer, typeof(IEqualityComparer<TKey>));
+            info.AddValue(ComparerName, comparer ?? EqualityComparer<TKey>.Default, typeof(IEqualityComparer<TKey>));
             info.AddValue(HashSizeName, buckets == null ? 0 : buckets.Length); // This is the length of the bucket array
 
             if (buckets != null)
@@ -358,10 +355,10 @@ private int FindEntry(TKey key)
 
             if (buckets != null)
             {
-                int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF;
+                int hashCode = (comparer != null ? comparer.GetHashCode(key) : key.GetHashCode()) & 0x7FFFFFFF;
                 for (int i = buckets[hashCode % buckets.Length]; i >= 0; i = entries[i].next)
                 {
-                    if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key)) return i;
+                    if (entries[i].hashCode == hashCode && (comparer != null ? comparer.Equals(entries[i].key, key) : (default(TKey) == null ? key.Equals(entries[i].key) : EqualityComparer<TKey>.Default.Equals(entries[i].key, key)))) return i;
                 }
             }
             return -1;
@@ -384,13 +381,13 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
             }
 
             if (buckets == null) Initialize(0);
-            int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF;
+            int hashCode = (comparer != null ? comparer.GetHashCode(key) : key.GetHashCode()) & 0x7FFFFFFF;
             int targetBucket = hashCode % buckets.Length;
             int collisionCount = 0;
 
             for (int i = buckets[targetBucket]; i >= 0; i = entries[i].next)
             {
-                if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key))
+                if (entries[i].hashCode == hashCode && (comparer != null ? comparer.Equals(entries[i].key, key) : (default(TKey) == null ? key.Equals(entries[i].key) : EqualityComparer<TKey>.Default.Equals(entries[i].key, key))))
                 {
                     if (behavior == InsertionBehavior.OverwriteExisting)
                     {
@@ -437,9 +434,9 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
             // If we hit the collision threshold we'll need to switch to the comparer which is using randomized string hashing
             // i.e. EqualityComparer<string>.Default.
 
-            if (collisionCount > HashHelpers.HashCollisionThreshold && comparer is NonRandomizedStringEqualityComparer)
+            if (default(TKey) == null && collisionCount > HashHelpers.HashCollisionThreshold && comparer is NonRandomizedStringEqualityComparer)
             {
-                comparer = (IEqualityComparer<TKey>)EqualityComparer<string>.Default;
+                comparer = null;
                 Resize(entries.Length, true);
             }
 
@@ -514,7 +511,7 @@ private void Resize(int newSize, bool forceNewHashCodes)
                 {
                     if (newEntries[i].hashCode != -1)
                     {
-                        newEntries[i].hashCode = (comparer.GetHashCode(newEntries[i].key) & 0x7FFFFFFF);
+                        newEntries[i].hashCode = (comparer != null ? comparer.GetHashCode(newEntries[i].key) : newEntries[i].key.GetHashCode()) & 0x7FFFFFFF;
                     }
                 }
             }
@@ -545,7 +542,7 @@ public bool Remove(TKey key)
 
             if (buckets != null)
             {
-                int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF;
+                int hashCode = (comparer != null ? comparer.GetHashCode(key) : key.GetHashCode()) & 0x7FFFFFFF;
                 int bucket = hashCode % buckets.Length;
                 int last = -1;
                 int i = buckets[bucket];
@@ -553,7 +550,7 @@ public bool Remove(TKey key)
                 {
                     ref Entry entry = ref entries[i];
 
-                    if (entry.hashCode == hashCode && comparer.Equals(entry.key, key))
+                    if (entry.hashCode == hashCode && (comparer != null ? comparer.Equals(entry.key, key) : (default(TKey) == null ? key.Equals(entry.key) : EqualityComparer<TKey>.Default.Equals(entry.key, key))))
                     {
                         if (last < 0)
                         {
@@ -599,7 +596,7 @@ public bool Remove(TKey key, out TValue value)
 
             if (buckets != null)
             {
-                int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF;
+                int hashCode = (comparer != null ? comparer.GetHashCode(key) : key.GetHashCode()) & 0x7FFFFFFF;
                 int bucket = hashCode % buckets.Length;
                 int last = -1;
                 int i = buckets[bucket];
@@ -607,7 +604,7 @@ public bool Remove(TKey key, out TValue value)
                 {
                     ref Entry entry = ref entries[i];
 
-                    if (entry.hashCode == hashCode && comparer.Equals(entry.key, key))
+                    if (entry.hashCode == hashCode && (comparer != null ? comparer.Equals(entry.key, key) : (default(TKey) == null ? key.Equals(entry.key) : EqualityComparer<TKey>.Default.Equals(entry.key, key))))
                     {
                         if (last < 0)
                         {