Added GetOrAdd static value overload

Author: alex-jitbit

FastCache/FastCache.cs

@@ -156,6 +156,20 @@ public TValue GetOrAdd(TKey key, Func<TKey, TValue> valueFactory, TimeSpan ttl)
 			return _dict.GetOrAdd(key, (k, v) => new TtlValue(v.valueFactory(k), v.ttl), (ttl, valueFactory)).Value;
 		}
 
+		/// <summary>
+		/// Adds a key/value pair by using the specified function if the key does not already exist, or returns the existing value if the key exists.
+		/// </summary>
+		/// <param name="key">The key to add</param>
+		/// <param name="value">The value to add</param>
+		/// <param name="ttl">TTL of the item</param>
+		public TValue GetOrAdd(TKey key, TValue value, TimeSpan ttl)
+		{
+			if (TryGet(key, out var existingValue))
+				return existingValue;
+
+			return _dict.GetOrAdd(key, new TtlValue(value, ttl)).Value;
+		}
+
 		/// <summary>
 		/// Tries to remove item with the specified key
 		/// </summary>