fix: do not recreate Random each time

Box.V2/Box.V2.csproj

@@ -276,6 +276,7 @@
     <Compile Include="Utility\ContentTypeMapper.cs" />
     <Compile Include="Utility\Retry.cs" />
     <Compile Include="Utility\SharedLinkUtils.cs" />
+    <Compile Include="Utility\ThreadSafeRandom.cs" />
     <Compile Include="Wrappers\BoxError.cs" />
     <Compile Include="Wrappers\BoxErrorContextInfo.cs" />
     <Compile Include="Wrappers\BoxBinaryRequest.cs" />
@@ -343,4 +344,4 @@
     <None Include="packages.config" />
   </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
-</Project>
+</Project>

Box.V2/Utility/ExponentialBackoff.cs

@@ -10,9 +10,10 @@ public TimeSpan GetRetryTimeout(int numRetries)
             const double RETRY_RANDOMIZATION_FACTOR = 0.5;
             var minRandomization = 1 - RETRY_RANDOMIZATION_FACTOR;
             var maxRandomization = 1 + RETRY_RANDOMIZATION_FACTOR;
-            var random = new Random();
 
-            var randomization = random.NextDouble() * (maxRandomization - minRandomization) + minRandomization;
+            var randomization = ThreadSafeRandom.Instance.NextDouble()
+                * (maxRandomization - minRandomization) + minRandomization;
+
             var exponential = Math.Pow(2, numRetries - 1);
             var result = Math.Ceiling(exponential * baseInterval.TotalSeconds * randomization);
             return TimeSpan.FromSeconds(result);

Box.V2/Utility/ThreadSafeRandom.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace Box.V2.Utility
+{
+    /// <summary>
+    /// A thread safe implementation of <see cref="Random"/>, following best practices
+    /// for .NET Framework and .NET Standard.
+    /// </summary>
+    /// <seealso href="https://learn.microsoft.com/en-us/dotnet/fundamentals/runtime-libraries/system-random"/>
+    internal static class ThreadSafeRandom
+    {
+        //
+        // Adapted from https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Random.cs
+        //
+        // NOTE: when/if this library updates to .NET 6+, this code can be replaced with `Random.Shared`
+        //
+
+        [ThreadStatic]
+        private static Random _random;
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private static Random CreateRandom() => _random = new Random();
+
+        /// <summary>
+        /// An instance of <see cref="Random"/> specific to the calling thread.
+        /// Do not pass this instance to other threads or contexts.
+        /// </summary>
+        public static Random Instance => _random ?? CreateRandom();
+    }
+}