From b3b0ba4c6d9ffc2e400cbb9c86b6d1ec86125833 Mon Sep 17 00:00:00 2001
From: stephentoub <stoub@microsoft.com>
Date: Sun, 7 Jun 2015 07:56:56 -0400
Subject: [PATCH] Add Dictionary<TKey,TValue>.TryAdd(TKey, TValue) method

---
 .../System/Collections/Generic/Dictionary.cs  | 25 +++++++++++++------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/src/mscorlib/src/System/Collections/Generic/Dictionary.cs b/src/mscorlib/src/System/Collections/Generic/Dictionary.cs
index 08621485f404..2626476b8d9b 100644
--- a/src/mscorlib/src/System/Collections/Generic/Dictionary.cs
+++ b/src/mscorlib/src/System/Collections/Generic/Dictionary.cs
@@ -186,7 +186,14 @@ public TValue this[TKey key] {
         }
 
         public void Add(TKey key, TValue value) {
-            Insert(key, value, true);
+            if (!TryAdd(key, value)) {
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_AddingDuplicate);
+            }
+        }
+
+        public bool TryAdd(TKey key, TValue value)
+        {
+            return Insert(key, value, true);
         }
 
         void ICollection<KeyValuePair<TKey, TValue>>.Add(KeyValuePair<TKey, TValue> keyValuePair) {
@@ -313,7 +320,8 @@ private void Initialize(int capacity) {
             freeList = -1;
         }
 
-        private void Insert(TKey key, TValue value, bool add) {
+        // returns false if the key already existed in the collection, or true if it didn't
+        private bool Insert(TKey key, TValue value, bool add) {
         
             if( key == null ) {
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
@@ -329,12 +337,12 @@ private void Insert(TKey key, TValue value, bool add) {
 
             for (int i = buckets[targetBucket]; i >= 0; i = entries[i].next) {
                 if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key)) {
-                    if (add) { 
-                        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_AddingDuplicate);
+                    if (!add)
+                    {
+                        entries[i].value = value;
+                        version++;
                     }
-                    entries[i].value = value;
-                    version++;
-                    return;
+                    return false;
                 } 
 
 #if FEATURE_RANDOMIZED_STRING_HASHING
@@ -386,6 +394,7 @@ private void Insert(TKey key, TValue value, bool add) {
 #endif // FEATURE_CORECLR
 
 #endif
+            return true;
 
         }
 
@@ -422,7 +431,7 @@ public virtual void OnDeserialization(Object sender) {
                     if ( array[i].Key == null) {
                         ThrowHelper.ThrowSerializationException(ExceptionResource.Serialization_NullKey);
                     }
-                    Insert(array[i].Key, array[i].Value, true);
+                    Add(array[i].Key, array[i].Value);
                 }
             }
             else {