fix issue 4528, use thread safe ConcurrentDictionary instead of Dictionary

src/Microsoft.Extensions.ML/PredictionEnginePool.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using Microsoft.Extensions.Options;
 using Microsoft.ML;
@@ -21,7 +22,7 @@ public class PredictionEnginePool<TData, TPrediction>
         private readonly IOptionsFactory<PredictionEnginePoolOptions<TData, TPrediction>> _predictionEngineOptions;
         private readonly IServiceProvider _serviceProvider;
         private readonly PoolLoader<TData,TPrediction> _defaultEnginePool;
-        private readonly Dictionary<string, PoolLoader<TData, TPrediction>> _namedPools;
+        private readonly ConcurrentDictionary<string, PoolLoader<TData, TPrediction>> _namedPools;
 
         public PredictionEnginePool(IServiceProvider serviceProvider,
                                     IOptions<MLOptions> mlContextOptions,
@@ -38,7 +39,7 @@ public PredictionEnginePool(IServiceProvider serviceProvider,
                 _defaultEnginePool = new PoolLoader<TData, TPrediction>(_serviceProvider, defaultOptions);
             }
 
-            _namedPools = new Dictionary<string, PoolLoader<TData, TPrediction>>();
+            _namedPools = new ConcurrentDictionary<string, PoolLoader<TData, TPrediction>>();
         }
 
         /// <summary>
@@ -100,7 +101,7 @@ public PredictionEngine<TData, TPrediction> GetPredictionEngine(string modelName
             //Here we are in the world of named models where the model hasn't been built yet.
             var options = _predictionEngineOptions.Create(modelName);
             var pool = new PoolLoader<TData, TPrediction>(_serviceProvider, options);
-            _namedPools.Add(modelName, pool);
+            _namedPools.TryAdd(modelName, pool);
             return pool.PredictionEnginePool.Get();
         }