Exception `System.ArgumentNullException: Value cannot be null. (Parameter 'bytes')` when UseCompatibilityAsyncBehaviour=false

[kaiohenrique]

Hi, @Wraith2
I just tested 7.0.0-preview3.25342.7 on my codebase
I am getting this error:

System.ArgumentNullException: Value cannot be null. (Parameter 'bytes')
   at Microsoft.Data.SqlClient.TdsParserStateObject.TryReadStringWithEncoding(Int32 length, Encoding encoding, Boolean isPlp, String& value)
   at Microsoft.Data.SqlClient.TdsParser.TryReadSqlStringValue(SqlBuffer value, Byte type, Int32 length, Encoding encoding, Boolean isPlp, TdsParserStateObject stateObj)
   at Microsoft.Data.SqlClient.TdsParser.TryReadSqlValue(SqlBuffer value, SqlMetaDataPriv md, Int32 length, TdsParserStateObject stateObj, SqlCommandColumnEncryptionSetting columnEncryptionOverride, String columnName, SqlCommand command)
   at Microsoft.Data.SqlClient.SqlDataReader.TryReadColumnInternal(Int32 i, Boolean readHeaderOnly, Boolean forStreaming)
   at Microsoft.Data.SqlClient.SqlDataReader.ReadAsyncExecute(Task task, Object state)
   at Microsoft.Data.SqlClient.SqlDataReader.ContinueAsyncCall[T](Task task, SqlDataReaderBaseAsyncCallContext`1 context)
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location ---
 

Set at the beggining of application

AppContext.SetSwitch("Switch.Microsoft.Data.SqlClient.UseCompatibilityAsyncBehaviour", false);
AppContext.SetSwitch("Switch.Microsoft.Data.SqlClient.UseCompatibilityProcessSni", false);

Originally posted by @kaiohenrique in #593

[kaiohenrique]

@Wraith2 FYI

Min-repro

using Microsoft.Data.SqlClient;
using System.Data;
using Testcontainers.MsSql;

namespace SqlClientRepro;

class Program
{
    static async Task Main(string[] args)
    {
        Console.WriteLine("=== SqlClient ArgumentNullException Bug - Minimal Reproduction ===");
        Console.WriteLine("Testing large result set with empty NVARCHAR(MAX) columns");
        Console.WriteLine();

        // Required for async snapshot continuation
        AppContext.SetSwitch("Switch.Microsoft.Data.SqlClient.UseCompatibilityAsyncBehaviour", false);
        AppContext.SetSwitch("Switch.Microsoft.Data.SqlClient.UseCompatibilityProcessSni", false);

        // Start SQL Server container
        var container = new MsSqlBuilder()
            .WithImage("mcr.microsoft.com/mssql/server:2019-CU28-ubuntu-20.04")
            .WithPassword("YourStrong@Passw0rd")
            .Build();

        Console.WriteLine("Starting SQL Server container...");
        await container.StartAsync();
        
        var connectionString = container.GetConnectionString();
        Console.WriteLine("✓ SQL Server started");
        Console.WriteLine();

        try
        {
            Console.WriteLine("=== Test: Large Result Set (Production Scenario) ===");
            await TestProductionScenario(connectionString);
            Console.WriteLine();

            Console.WriteLine("✅ Test Passed - No bug detected");
            Console.WriteLine("Note: Bug may require specific SQL Server versions or connection settings");
        }
        catch (ArgumentNullException ex)
        {
            Console.WriteLine();
            Console.WriteLine("❌ BUG REPRODUCED!");
            Console.WriteLine($"   {ex.GetType().Name}: {ex.Message}");
            Console.WriteLine($"   Parameter: {ex.ParamName}");
            Console.WriteLine();
            Console.WriteLine("Stack trace:");
            Console.WriteLine(ex.StackTrace);
            return;
        }
        finally
        {
            Console.WriteLine();
            Console.WriteLine("Cleaning up container...");
            await container.StopAsync();
            Console.WriteLine("✓ Cleanup complete");
        }
    }

    /// <summary>
    /// Reproduces production scenario: Large result set with empty NVARCHAR(MAX) columns
    /// Simulates 50k rows × 58 columns that trigger snapshot continuation and the bug
    /// </summary>
    static async Task TestProductionScenario(string connectionString)
    {
        // Small packet size forces multi-packet reads, triggering snapshot continuation
        connectionString += ";Packet Size=512";
        
        await using var conn = new SqlConnection(connectionString);
        await conn.OpenAsync();

        Console.WriteLine("  Creating test data: 1000 rows with large NVARCHAR(MAX) columns...");
        
        // Create table and insert test data
        await using var setupCmd = new SqlCommand(@"
            CREATE TABLE #TestData (
                ID INT,
                EmptyVarchar VARCHAR(MAX),
                LargeNVarchar NVARCHAR(MAX),
                Data2 NVARCHAR(MAX)
            );

            DECLARE @i INT = 0;
            DECLARE @largeData NVARCHAR(MAX) = REPLICATE(N'X', 8000);
            
            WHILE @i < 1000
            BEGIN
                -- Every 10th row has empty string (simulating production data pattern)
                IF @i % 10 = 0
                    INSERT INTO #TestData VALUES (@i, '', @largeData, @largeData);
                ELSE
                    INSERT INTO #TestData VALUES (@i, 'data', @largeData, @largeData);
                    
                SET @i = @i + 1;
            END;
        ", conn);
        
        setupCmd.CommandTimeout = 120;
        await setupCmd.ExecuteNonQueryAsync();
        
        Console.WriteLine("  Reading data (NO SequentialAccess)...");
        
        await using var selectCmd = new SqlCommand(
            "SELECT EmptyVarchar, LargeNVarchar, Data2 FROM #TestData ORDER BY ID", 
            conn);
        await using var reader = await selectCmd.ExecuteReaderAsync(CommandBehavior.Default);
        
        int rowCount = 0;
        int emptyCount = 0;
        
        while (await reader.ReadAsync())
        {
            rowCount++;
            
            // Read columns in order (standard production pattern)
            var col0 = await reader.GetFieldValueAsync<string>(0).ConfigureAwait(false);
            var col1 = await reader.GetFieldValueAsync<string>(1).ConfigureAwait(false);
            var col2 = await reader.GetFieldValueAsync<string>(2).ConfigureAwait(false);
            
            if (string.IsNullOrEmpty(col0)) emptyCount++;
            
            if (rowCount % 200 == 0)
            {
                Console.WriteLine($"    Processed {rowCount} rows, {emptyCount} with empty strings...");
            }
        }

        Console.WriteLine($"  ✓ Completed: {rowCount} rows processed, {emptyCount} had empty strings");
    }
}

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <EnableDefaultCompileItems>true</EnableDefaultCompileItems>
    <EnableDefaultItems>true</EnableDefaultItems>
  </PropertyGroup>
 
  <ItemGroup>
    <PackageReference Include="Microsoft.Data.SqlClient" Version="7.0.0-preview3.25342.7" />
    <PackageReference Include="Testcontainers.MsSql" Version="4.2.0" />
  </ItemGroup>

</Project>

Exception being thrown

❌ BUG REPRODUCED!
   ArgumentNullException: Value cannot be null. (Parameter 'bytes')
   Parameter: bytes

Stack trace:
   at System.Text.EncodingNLS.GetString(Byte[] bytes, Int32 index, Int32 count)
   at Microsoft.Data.SqlClient.TdsParserStateObject.TryReadStringWithEncoding(Int32 length, Encoding encoding, Boolean isPlp, String& value)
   at Microsoft.Data.SqlClient.TdsParser.TryReadSqlStringValue(SqlBuffer value, Byte type, Int32 length, Encoding encoding, Boolean isPlp, TdsParserStateObject stateObj)
   at Microsoft.Data.SqlClient.TdsParser.TryReadSqlValue(SqlBuffer value, SqlMetaDataPriv md, Int32 length, TdsParserStateObject stateObj, SqlCommandColumnEncryptionSetting columnEncryptionOverride, String columnName, SqlCommand com
   at Microsoft.Data.SqlClient.SqlDataReader.TryReadColumnInternal(Int32 i, Boolean readHeaderOnly, Boolean forStreaming)
   at Microsoft.Data.SqlClient.SqlDataReader.TryReadColumn(Int32 i, Boolean setTimeout, Boolean allowPartiallyReadColumn, Boolean forStreaming)
   at Microsoft.Data.SqlClient.SqlDataReader.ReadAsyncExecute(Task task, Object state)
   at Microsoft.Data.SqlClient.SqlDataReader.ContinueAsyncCall[T](Task task, SqlDataReaderBaseAsyncCallContext`1 context)
   at Microsoft.Data.SqlClient.SqlDataReader.SqlDataReaderBaseAsyncCallContext`1.ExecuteAsyncCallCallback(Task task, Object state)
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location ---
   at SqlClientRepro.Program.TestProductionScenario(String connectionString) in C:\AgileContent\SqlClientRepro\Program.cs:line 112
   at SqlClientRepro.Program.TestProductionScenario(String connectionString) in C:\AgileContent\SqlClientRepro\Program.cs:line 129
   at SqlClientRepro.Program.TestProductionScenario(String connectionString) in C:\AgileContent\SqlClientRepro\Program.cs:line 129
   at SqlClientRepro.Program.TestProductionScenario(String connectionString) in C:\AgileContent\SqlClientRepro\Program.cs:line 129
   at SqlClientRepro.Program.TestProductionScenario(String connectionString) in C:\AgileContent\SqlClientRepro\Program.cs:line 129
   at SqlClientRepro.Program.Main(String[] args) in C:\AgileContent\SqlClientRepro\Program.cs:line 35

[Wraith2]

Excellent, thanks. I'll take a look as soon as I can.

[Wraith2]

I managed to reproduce the problem. The fix is very small. Writing a test for it is more complex because of timing in the CI.