Dictionary.GetValueOrDefault

src/System.Collections/ref/System.Collections.cs

@@ -40,6 +40,15 @@ public static partial class StructuralComparisons
 }
 namespace System.Collections.Generic
 {
+#if netcoreapp11
+    public static class CollectionExtensions
+    {
+        public static TValue GetValueOrDefault<TKey, TValue>(System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key) { throw null; }
+        public static TValue GetValueOrDefault<TKey, TValue>(System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue) { throw null; }
+        public static TValue GetValueOrDefault<TKey, TValue>(System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> dictionary, TKey key) { throw null; }
+        public static TValue GetValueOrDefault<TKey, TValue>(System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue) { throw null; }
+    }
+#endif
     public abstract partial class Comparer<T> : System.Collections.Generic.IComparer<T>, System.Collections.IComparer
     {
         protected Comparer() { }
@@ -80,6 +89,10 @@ public void Clear() { }
         public bool ContainsValue(TValue value) { throw null; }
         public System.Collections.Generic.Dictionary<TKey, TValue>.Enumerator GetEnumerator() { throw null; }
         public virtual void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
+#if netcoreapp11
+        public TValue GetValueOrDefault(TKey key) { throw null; }
+        public TValue GetValueOrDefault(TKey key, TValue defaultValue) { throw null; }
+#endif
         public virtual void OnDeserialization(object sender) { }
         public bool Remove(TKey key) { throw null; }
         void System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Add(System.Collections.Generic.KeyValuePair<TKey, TValue> keyValuePair) { }

src/System.Collections/src/System.Collections.csproj

@@ -86,5 +86,8 @@
   <ItemGroup>
     <None Include="project.json" />
   </ItemGroup>
+  <ItemGroup>
+    <Compile Include="System\Collections\Generic\CollectionExtensions.cs" />
+  </ItemGroup>
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
-</Project>
+</Project>

src/System.Collections/src/System/Collections/Generic/CollectionExtensions.cs

@@ -0,0 +1,41 @@
+namespace System.Collections.Generic
+{
+    public static class CollectionExtensions
+    {
+        // Method similar to TryGetValue that returns the value instead of putting it in an out param.
+        public static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key) => dictionary.GetValueOrDefault(key, default(TValue));
+
+        // Method similar to TryGetValue that returns the value instead of putting it in an out param. If the entry
+        // doesn't exist, returns the defaultValue instead.
+        public static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue)
+        {
+            if (dictionary == null)
+            {
+                throw new ArgumentNullException(nameof(dictionary));
+            }
+
+            TValue value;
+            if (dictionary.TryGetValue(key, out value))
+                return value;
+            return defaultValue;
+        }
+
+        // Method similar to TryGetValue that returns the value instead of putting it in an out param.
+        public static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> dictionary, TKey key) => dictionary.GetValueOrDefault(key, default(TValue));
+
+        // Method similar to TryGetValue that returns the value instead of putting it in an out param. If the entry
+        // doesn't exist, returns the defaultValue instead.
+        public static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue)
+        {
+            if (dictionary == null)
+            {
+                throw new ArgumentNullException(nameof(dictionary));
+            }
+
+            TValue value;
+            if (dictionary.TryGetValue(key, out value))
+                return value;
+            return defaultValue;
+        }
+    }
+}

src/System.Collections/src/System/Collections/Generic/Dictionary.cs

@@ -537,18 +537,19 @@ public bool TryGetValue(TKey key, out TValue value)
             return false;
         }
 
-        // This is a convenience method for the internal callers that were converted from using Hashtable.
-        // Many were combining key doesn't exist and key exists but null value (for non-value types) checks.
-        // This allows them to continue getting that behavior with minimal code delta. This is basically
-        // TryGetValue without the out param
-        internal TValue GetValueOrDefault(TKey key)
+        // Method similar to TryGetValue that returns the value instead of putting it in an out param.
+        public TValue GetValueOrDefault(TKey key) => GetValueOrDefault(key, default(TValue));
+
+        // Method similar to TryGetValue that returns the value instead of putting it in an out param. If the entry
+        // doesn't exist, returns the defaultValue instead.
+        public TValue GetValueOrDefault(TKey key, TValue defaultValue)
         {
             int i = FindEntry(key);
             if (i >= 0)
             {
                 return entries[i].value;
             }
-            return default(TValue);
+            return defaultValue;
         }
 
         bool ICollection<KeyValuePair<TKey, TValue>>.IsReadOnly