Fix Random.Shared giving identical sequences on threads started together

src/Polyfill/RandomPolyfill.cs

@@ -15,8 +15,22 @@ sealed class ThreadSafeRandom : Random
         [ThreadStatic]
         static Random? random;
 
+        // On .NET Framework `new Random()` is seeded from the tick count, so instances created
+        // together (e.g. by threads started at the same time) would produce identical sequences.
+        // Instead each thread's instance takes its seed from a single process-wide source.
+        static readonly Random seedSource = new();
+
         [MethodImpl(MethodImplOptions.NoInlining)]
-        static Random Create() => random = new();
+        static Random Create()
+        {
+            int seed;
+            lock (seedSource)
+            {
+                seed = seedSource.Next();
+            }
+
+            return random = new(seed);
+        }
 
         static Random LocalRandom => random ?? Create();
 

src/Split/net461/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/net462/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/net47/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/net471/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/net472/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/net48/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/net481/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/net5.0/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/netcoreapp2.0/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/netcoreapp2.1/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/netcoreapp2.2/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/netcoreapp3.0/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/netcoreapp3.1/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/netstandard2.0/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/netstandard2.1/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Split/uap10.0/RandomPolyfill.cs

@@ -11,8 +11,17 @@ sealed class ThreadSafeRandom : Random
 	{
 		[ThreadStatic]
 		static Random? random;
+		static readonly Random seedSource = new();
 		[MethodImpl(MethodImplOptions.NoInlining)]
-		static Random Create() => random = new();
+		static Random Create()
+		{
+			int seed;
+			lock (seedSource)
+			{
+				seed = seedSource.Next();
+			}
+			return random = new(seed);
+		}
 		static Random LocalRandom => random ?? Create();
 		public override int Next() => LocalRandom.Next();
 		public override int Next(int maxValue) => LocalRandom.Next(maxValue);

src/Tests/RandomPolyfillTests.cs

@@ -7,4 +7,32 @@ public async Task NextInt()
 
         await Assert.That(number).IsGreaterThanOrEqualTo(0).And.IsLessThanOrEqualTo(10);
     }
+
+    [Test]
+    public async Task SharedDiffersAcrossThreadsStartedTogether()
+    {
+        const int threadCount = 16;
+        var sequences = new string[threadCount];
+        using var barrier = new Barrier(threadCount);
+        var threads = new List<Thread>();
+        for (var index = 0; index < threadCount; index++)
+        {
+            var local = index;
+            var thread = new Thread(() =>
+            {
+                barrier.SignalAndWait();
+                var shared = Random.Shared;
+                sequences[local] = $"{shared.Next()},{shared.Next()},{shared.Next()},{shared.Next()}";
+            });
+            threads.Add(thread);
+            thread.Start();
+        }
+
+        foreach (var thread in threads)
+        {
+            thread.Join();
+        }
+
+        await Assert.That(sequences.Distinct().Count()).IsEqualTo(threadCount);
+    }
 }