Fix SpiAdapter.Read buffer length mismatch in Mcp23xxx TransferFullDuplex by Copilot · Pull Request #2492 · dotnet/iot

Copilot · 2026-04-09T15:49:00Z

SpiAdapter.Read() passes a 2-byte writeBuffer and a (buffer.Length + 2)-byte readBuffer to TransferFullDuplex, which requires equal-length buffers. This throws ArgumentException on any real SPI device (e.g., UnixSpiDevice).

SpiAdapter.cs: Allocate writeBuffer at the same length as readBuffer. OpCode and register address occupy the first 2 bytes; remaining bytes are zero-filled don't-care padding per standard SPI full-duplex read semantics.
Mcp23xxxTest.cs: Fix SpiDeviceMock.TransferFullDuplex to only forward the 2-byte command prefix to Write(), preventing the don't-care padding from overwriting mock register state.

// Before: writeBuffer is 2 bytes, readBuffer is buffer.Length + 2 — mismatch
Span<byte> writeBuffer = stackalloc byte[] { opCode, registerAddress };
Span<byte> readBuffer = stackalloc byte[buffer.Length + 2];

// After: both buffers are buffer.Length + 2
Span<byte> writeBuffer = stackalloc byte[fullLength];
writeBuffer[0] = opCode;
writeBuffer[1] = registerAddress;
Span<byte> readBuffer = stackalloc byte[fullLength];

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

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- Triggering command: /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet restore --no-dependencies /home/REDACTED/work/iot/iot/tools/ArduinoCsCompiler/ArduinoCsCompiler.sln --packages /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
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- Triggering command: /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet restore --no-dependencies /home/REDACTED/work/iot/iot/src/System.Device.Gpio/System.Device.Gpio.sln --packages /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
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- Triggering command: /usr/bin/dotnet dotnet build src/devices/Mcp23xxx/Mcp23xxx.csproj (dns block)
- Triggering command: /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet restore --no-dependencies /home/REDACTED/work/iot/iot/samples/samples.sln --packages /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
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- Triggering command: /usr/bin/dotnet dotnet build src/devices/Mcp23xxx/Mcp23xxx.csproj (dns block)
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- Triggering command: /usr/bin/dotnet dotnet build src/devices/Mcp23xxx/Mcp23xxx.csproj (dns block)
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- Triggering command: /usr/bin/dotnet dotnet build src/devices/Mcp23xxx/Mcp23xxx.csproj (dns block)
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- Triggering command: /usr/bin/dotnet dotnet build src/devices/Mcp23xxx/Mcp23xxx.csproj (dns block)
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- Triggering command: /usr/bin/dotnet dotnet build src/devices/Mcp23xxx/Mcp23xxx.csproj (dns block)
- Triggering command: /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet restore --no-dependencies /home/REDACTED/work/iot/iot/samples/samples.sln --packages /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
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- Triggering command: /usr/bin/dotnet dotnet build src/devices/Mcp23xxx/Mcp23xxx.csproj (dns block)
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- Triggering command: /usr/bin/dotnet dotnet build src/devices/Mcp23xxx/Mcp23xxx.csproj (dns block)
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- Triggering command: /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet restore --no-dependencies /home/REDACTED/work/iot/iot/samples/samples.sln --packages /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
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- Triggering command: /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/.dotnet/dotnet restore --no-dependencies /home/REDACTED/work/iot/iot/src/System.Device.Gpio/System.Device.Gpio.sln --packages /tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/tmp/codeql-scratch-db3f4ba25dde505c/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
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TransferFullDuplex requires writeBuffer and readBuffer to have the same length. The Read method was creating a 2-byte writeBuffer and a (buffer.Length + 2)-byte readBuffer, causing ArgumentException on real SPI devices. Fix: allocate writeBuffer with the same size as readBuffer, placing OpCode and register address in the first two bytes (remaining bytes are don't-care for the SPI read operation). Also fix the test mock's TransferFullDuplex to only pass command bytes to Write, preventing don't-care padding from corrupting register data. Agent-Logs-Url: https://github.com/dotnet/iot/sessions/7c410060-a096-49b1-8cfa-348f86e1f77b Co-authored-by: krwq <660048+krwq@users.noreply.github.com>

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Copilot AI assigned Copilot and krwq Apr 9, 2026

Copilot started work on behalf of krwq April 9, 2026 15:49 View session

Copilot AI linked an issue Apr 9, 2026 that may be closed by this pull request

System.ArgumentException: Parameters 'writeBuffer' and 'readBuffer' must have the same length #917

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dotnet-policy-service bot added the area-device-bindings Device Bindings for audio, sensor, motor, and display hardware that can used with System.Device.Gpio label Apr 9, 2026

Copilot AI changed the title ~~[WIP] Fix System.ArgumentException for buffer length mismatch~~ Fix SpiAdapter.Read buffer length mismatch in Mcp23xxx TransferFullDuplex Apr 9, 2026

Copilot finished work on behalf of krwq April 9, 2026 16:00

Copilot AI requested a review from krwq April 9, 2026 16:00

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Fix SpiAdapter.Read buffer length mismatch in Mcp23xxx TransferFullDuplex#2492

Fix SpiAdapter.Read buffer length mismatch in Mcp23xxx TransferFullDuplex#2492
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copilot/fix-argument-exception-buffers

Copilot AI commented Apr 9, 2026 •

edited

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Copilot AI commented Apr 9, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Copilot AI commented Apr 9, 2026 •

edited

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