From time to time at my Windows 10 system background program in C# I receive NullReferenceException at this.connection.Open().

My class:

private SqlConnection connection;
private string connectionString;
private StringBuilder trace = new StringBuilder();
private int counter = 0;

public void InitConnectionIfNeeded()
{
    trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [0]");
    // Check if connection is already established but connection string changed, purge it.
    if (this.connection != null && this.connection.ConnectionString != this.connectionString) {
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [1]");
        if (this.connection.State == ConnectionState.Open) {
            trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [2]");
            this.connection.Close();
            trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [2+]");
        }
        this.connection = null;
    }
    // Create instance of SqlConnection object by connection string
    if (this.connection == null)
    {
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [3]");
        this.connection = new SqlConnection(this.connectionString);
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [3+]nConnectionString: {this.connectionString}");
    }
    // If ConnectionState != Open then open it.
    if (this.connection.State != ConnectionState.Open)
    {
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [4]");
        this.connection.Open();
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [4+]");
    }
    trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} InitConnectionIfNeeded [5]");
}

My code in function:

message = "";
errorStackTrace = "";
success = false;
resultsCount = 0;
results = new DataTable();

try
{
    bool connectionEstablished = false;
    Exception lastException = null;
    for (int i=0; i<3; i++) {
        try {
            trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} Try InitConnectionIfNeeded Attempt: {i+1}");
            InitConnectionIfNeeded();
            connectionEstablished = true;
            break;
        } catch(Exception ex)
        {
            System.Threading.Thread.Sleep(1000);
            lastException = ex;
        }
    }
    if(!connectionEstablished) {
        throw lastException;
    }
    using (SqlCommand cmd = new SqlCommand(query, connection))
    {
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} Get Collection [1]");
        cmd.CommandTimeout = (int)queryTimeout;
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} Get Collection [2]");
        
        SqlDataReader reader = cmd.ExecuteReader();
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} Get Collection [3]");
        results.Load(reader);
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} Get Collection [4]");
        resultsCount = results.Rows.Count;
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} Get Collection [5]");
        ChangeDate(results);
        trace.AppendLine($"{counter++}t{DateTime.Now.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss.fff")} Get Collection [6]");
        success = true;
    }
}
catch (SqlException ex) when (ex.Number == -2)
{
    message = "The command has timed out.";
}
catch (Exception ex)
{
    message = ex.Message;
    StringBuilder sb = new StringBuilder();
    sb.AppendLine($"-----------------------nError at: {DateTime.Now.ToLocalTime()}");
    sb.Append(this.trace + "n::n");
    sb.Append($"ex.Source: {ex.Source}n");
    sb.Append($"ex.HResult: {ex.HResult}n");
    if (ex.InnerException != null)
    {
        sb.Append($"ex.InnerExceptionMessage: {ex.InnerException.Message}n");
        sb.Append($"ex.InnerExceptionGetTypeFullName: {ex.InnerException.GetType().FullName}n");
        sb.Append($"ex.InnerExceptionStackTrace: {ex.InnerException.StackTrace}n");
    }
    sb.Append($"ex.GetType: {ex.GetType().FullName}n");
    if (ex.Data != null && ex.Data.Count > 0)
    {
        sb.Append("ex.Datan");
        foreach (DictionaryEntry dictionaryEntry in ex.Data)
        {
            sb.Append($"{dictionaryEntry.Key}t{dictionaryEntry.Value}n");
        }
    }
    sb.Append($"ex.StackTrace: {ex.StackTrace}n.n.n");
    sb.AppendLine("--------------------------------------");
    errorStackTrace = sb.ToString();
}

At 99% it works but sometimes I receive that result:

-----------------------
Error at: 2024-07-11 14:27:32
0   2024-07-11 14:27:29.555 Try InitConnectionIfNeeded Attempt: 1
1   2024-07-11 14:27:29.555 InitConnectionIfNeeded [0]
2   2024-07-11 14:27:29.555 InitConnectionIfNeeded [3]
3   2024-07-11 14:27:29.555 InitConnectionIfNeeded [3+]
ConnectionString: Data Source=MYSQLSERVER;Database=DB123;Trusted_Connection=Yes;Timeout=30
4   2024-07-11 14:27:29.555 InitConnectionIfNeeded [4]
5   2024-07-11 14:27:30.558 Try InitConnectionIfNeeded Attempt: 2
6   2024-07-11 14:27:30.558 InitConnectionIfNeeded [0]
7   2024-07-11 14:27:30.558 InitConnectionIfNeeded [4]
8   2024-07-11 14:27:31.560 Try InitConnectionIfNeeded Attempt: 3
9   2024-07-11 14:27:31.560 InitConnectionIfNeeded [0]
10  2024-07-11 14:27:31.560 InitConnectionIfNeeded [4]

::
ex.Source: do3wemmp
ex.HResult: -2147467261
ex.GetType: System.NullReferenceException
ex.StackTrace:    w SQLServerC_.Get_Collection(String query, Decimal queryTimeout, String& message, Boolean& success, Decimal& resultsCount, DataTable& results, String& errorStackTrace)
.
.
--------------------------------------

Full stackTrace:

   w System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions userOptions, DbConnectionInternal& connection)
   w System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection)
   w System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection)
   w System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
   w System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
   w System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource`1 retry)
   w System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry)
   w System.Data.SqlClient.SqlConnection.Open()
   w SQLServerC_.InitConnectionIfNeeded()
   w SQLServerC_.Get_Collection(String query, Decimal queryTimeout, String& message, Boolean& success, Decimal& resultsCount, DataTable& results, String& errorStackTrace)

As you see at string builder trace, my code fill this.connection class field and try to Open connection. Unfortunately Open operation failed three times with 1 second delay between.
This happen only at <1% cases. Have you got any idea how to deal with it?

I tried:

• add loop on InitConnectionIfNeeded method (better situation but still sometimes nullreferenceexception)
• add trace

I want to resolve these errors and Open connection with success result.