Add cast from IDictionary to ConcurrentDictionary (#9810)

Author: gave92

src/Files.Shared/Extensions/LinqExtensions.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
@@ -53,7 +54,14 @@ public static class LinqExtensions
                 var defaultValue = defaultValueFunc();
                 if (defaultValue is Task<TValue?> value)
                 {
-                    dictionary.Add(key, value);
+                    if (dictionary is ConcurrentDictionary<TKey, Task<TValue?>> cDict)
+                    {
+                        cDict.TryAdd(key, value);
+                    }
+                    else
+                    {
+                        dictionary.Add(key, value);
+                    }
                 }
                 return defaultValue;
             }