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Eliminate heap allocations in Random/RandomNumberGenerator polyfills #85

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Merged
Tyrrrz merged 5 commits into from
Feb 27, 2026
Merged

Eliminate heap allocations in Random/RandomNumberGenerator polyfills #85

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0a92dcf
Initial plan
Copilot Feb 27, 2026
62d1c91
Eliminate unnecessary heap allocations using ArrayPool and cached RNG…
Copilot Feb 27, 2026
f988e73
Address review feedback: property syntax, clearArray for chars, forma…
Copilot Feb 27, 2026
b84a204
Fix over-generation in rented buffers, remove redundant Substring con…
Copilot Feb 27, 2026
55e43c1
Merge branch 'master' into copilot/optimize-nextbytes-allocation
Tyrrrz Feb 27, 2026
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37 changes: 26 additions & 11 deletions PolyShim/Net100/Random.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@
// ReSharper disable PartialTypeWithSinglePart

using System;
using System.Text;
using System.Buffers;
using System.Diagnostics.CodeAnalysis;

[ExcludeFromCodeCoverage]
Expand All @@ -17,25 +17,40 @@ internal static class MemberPolyfills_Net100_Random
// https://learn.microsoft.com/dotnet/api/system.random.gethexstring#system-random-gethexstring(system-int32-system-boolean)
public string GetHexString(int stringLength, bool lowercase = false)
{
var bytes = new byte[(stringLength + 1) / 2];
random.NextBytes(bytes);
var byteCount = (stringLength + 1) / 2;
var bytes = ArrayPool<byte>.Shared.Rent(byteCount);

var hex = lowercase ? Convert.ToHexStringLower(bytes) : Convert.ToHexString(bytes);
return hex.Substring(0, stringLength);
try
{
random.NextBytes(bytes.AsSpan(0, byteCount));

var hex = lowercase
? Convert.ToHexStringLower(bytes, 0, byteCount)
: Convert.ToHexString(bytes, 0, byteCount);

return hex.Substring(0, stringLength);
}
finally
{
ArrayPool<byte>.Shared.Return(bytes);
}
}

// https://learn.microsoft.com/dotnet/api/system.random.getstring
public string GetString(ReadOnlySpan<char> choices, int length)
{
var buffer = new StringBuilder(length);
var chars = ArrayPool<char>.Shared.Rent(length);
try
{
for (var i = 0; i < length; i++)
chars[i] = choices[random.Next(choices.Length)];

for (var i = 0; i < length; i++)
return new string(chars, 0, length);
}
finally
{
var index = random.Next(choices.Length);
buffer.Append(choices[index]);
ArrayPool<char>.Shared.Return(chars);
}

return buffer.ToString();
}
}
}
Expand Down
38 changes: 26 additions & 12 deletions PolyShim/Net60/Random.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
// ReSharper disable PartialTypeWithSinglePart

using System;
using System.Buffers;
using System.Diagnostics.CodeAnalysis;

file static class RandomEx
Expand All @@ -32,15 +33,22 @@ public long NextInt64(long minValue, long maxValue)

var range = (ulong)(maxValue - minValue);

ulong ulongRand;
do
var buffer = ArrayPool<byte>.Shared.Rent(8);
try
{
var buffer = new byte[8];
random.NextBytes(buffer);
ulongRand = BitConverter.ToUInt64(buffer, 0);
} while (ulongRand > ulong.MaxValue - (ulong.MaxValue % range + 1) % range);
ulong ulongRand;
do
{
random.NextBytes(buffer.AsSpan(0, 8));
ulongRand = BitConverter.ToUInt64(buffer, 0);
} while (ulongRand > ulong.MaxValue - (ulong.MaxValue % range + 1) % range);

return (long)(ulongRand % range) + minValue;
return (long)(ulongRand % range) + minValue;
}
finally
{
ArrayPool<byte>.Shared.Return(buffer);
}
}

// https://learn.microsoft.com/dotnet/api/system.random.nextint64#system-random-nextint64(system-int64)
Expand All @@ -52,11 +60,17 @@ public long NextInt64(long minValue, long maxValue)
// https://learn.microsoft.com/dotnet/api/system.random.nextsingle
public float NextSingle()
{
var buffer = new byte[4];
random.NextBytes(buffer);
var uintValue = BitConverter.ToUInt32(buffer, 0);

return (uintValue >> 8) * (1.0f / (1u << 24));
var buffer = ArrayPool<byte>.Shared.Rent(4);
try
{
random.NextBytes(buffer.AsSpan(0, 4));
var uintValue = BitConverter.ToUInt32(buffer, 0);
return (uintValue >> 8) * (1.0f / (1u << 24));
}
finally
{
ArrayPool<byte>.Shared.Return(buffer);
}
}

// https://learn.microsoft.com/dotnet/api/system.random.shared
Expand Down
17 changes: 10 additions & 7 deletions PolyShim/Net80/RandomNumberGenerator.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,6 @@

using System;
using System.Buffers;
using System.Text;
using System.Diagnostics.CodeAnalysis;
using System.Security.Cryptography;

Expand Down Expand Up @@ -56,6 +55,7 @@ public static string GetHexString(int stringLength, bool lowercase = false)
var hex = lowercase
? Convert.ToHexStringLower(bytes, 0, byteCount)
: Convert.ToHexString(bytes, 0, byteCount);

return hex.Substring(0, stringLength);
}
finally
Expand All @@ -67,15 +67,18 @@ public static string GetHexString(int stringLength, bool lowercase = false)
// https://learn.microsoft.com/dotnet/api/system.security.cryptography.randomnumbergenerator.getstring
public static string GetString(ReadOnlySpan<char> choices, int length)
{
var buffer = new StringBuilder(length);
var chars = ArrayPool<char>.Shared.Rent(length);
try
{
for (var i = 0; i < length; i++)
chars[i] = choices[RandomNumberGenerator.GetInt32(choices.Length)];

for (var i = 0; i < length; i++)
return new string(chars, 0, length);
}
finally
{
var index = RandomNumberGenerator.GetInt32(choices.Length);
buffer.Append(choices[index]);
ArrayPool<char>.Shared.Return(chars, clearArray: true);
}

return buffer.ToString();
}
}
}
Expand Down
2 changes: 1 addition & 1 deletion PolyShim/NetCore21/Random.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,7 @@ internal static class MemberPolyfills_NetCore21_Random
// https://learn.microsoft.com/dotnet/api/system.random.nextbytes#system-random-nextbytes(system-span((system-byte)))
public void NextBytes(Span<byte> buffer)
{
var bufferArray = buffer.ToArray();
var bufferArray = new byte[buffer.Length];
random.NextBytes(bufferArray);
bufferArray.CopyTo(buffer);
}
Expand Down
31 changes: 17 additions & 14 deletions PolyShim/NetCore21/RandomNumberGenerator.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,9 +6,15 @@
// ReSharper disable PartialTypeWithSinglePart

using System;
using System.Buffers;
using System.Diagnostics.CodeAnalysis;
using System.Security.Cryptography;

file static class RandomNumberGeneratorEx
{
public static RandomNumberGenerator Instance { get; } = RandomNumberGenerator.Create();
}

[ExcludeFromCodeCoverage]
internal static class MemberPolyfills_NetCore21_RandomNumberGenerator
{
Expand All @@ -20,9 +26,16 @@ public void GetBytes(Span<byte> data)
if (data.Length == 0)
return;

var buffer = new byte[data.Length];
rng.GetBytes(buffer);
buffer.CopyTo(data);
var buffer = ArrayPool<byte>.Shared.Rent(data.Length);
try
{
rng.GetBytes(buffer, 0, data.Length);
buffer.AsSpan(0, data.Length).CopyTo(data);
}
finally
{
ArrayPool<byte>.Shared.Return(buffer, clearArray: true);
}
}

// https://learn.microsoft.com/dotnet/api/system.security.cryptography.randomnumbergenerator.getnonzerobytes#system-security-cryptography-randomnumbergenerator-getnonzerobytes(system-span((system-byte)))
Expand All @@ -43,17 +56,7 @@ public static void Fill(Span<byte> data)
if (data.Length == 0)
return;

var rng = RandomNumberGenerator.Create();
try
{
rng.GetBytes(data);
}
finally
{
// Explicit cast needed for .NET Framework 3.5 where RandomNumberGenerator
// doesn't properly expose IDisposable for using statements.
((IDisposable)rng).Dispose();
}
RandomNumberGeneratorEx.Instance.GetBytes(data);
}
}
}
Expand Down
24 changes: 15 additions & 9 deletions PolyShim/NetCore30/RandomNumberGenerator.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,9 +6,15 @@
// ReSharper disable PartialTypeWithSinglePart

using System;
using System.Buffers;
using System.Diagnostics.CodeAnalysis;
using System.Security.Cryptography;

file static class RandomNumberGeneratorEx
{
public static RandomNumberGenerator Instance { get; } = RandomNumberGenerator.Create();
}

[ExcludeFromCodeCoverage]
internal static class MemberPolyfills_NetCore30_RandomNumberGenerator
{
Expand All @@ -18,27 +24,27 @@ internal static class MemberPolyfills_NetCore30_RandomNumberGenerator
public static int GetInt32(int fromInclusive, int toExclusive)
{
var range = (uint)(toExclusive - fromInclusive);
var rng = RandomNumberGenerator.Create();

// Reject values that would cause bias in the distribution.
// This ensures uniform distribution by rejecting values in the
// incomplete final bucket.
var rejectionThreshold = uint.MaxValue - (uint.MaxValue % range + 1) % range;

var buffer = ArrayPool<byte>.Shared.Rent(4);
try
{
// Reject values that would cause bias in the distribution.
// This ensures uniform distribution by rejecting values in the
// incomplete final bucket.
var rejectionThreshold = uint.MaxValue - (uint.MaxValue % range + 1) % range;

uint result;
do
{
var buffer = new byte[4];
rng.GetBytes(buffer);
RandomNumberGeneratorEx.Instance.GetBytes(buffer, 0, 4);
result = BitConverter.ToUInt32(buffer, 0);
} while (result > rejectionThreshold);

return (int)(result % range) + fromInclusive;
}
finally
{
((IDisposable)rng).Dispose();
ArrayPool<byte>.Shared.Return(buffer, clearArray: true);
}
}

Expand Down
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