Sqlite ExecuteScalar抛出NullReferenceExcpetion

Question

我有一个自定义编写的DB提供程序.当我运行我的测试时,他们正在打破ExecuteScalar命令NullReferenceException.我可能会在这里失踪什么？我读过一些人有一个MultiThreading问题,但我并不"认为"这就是我遇到的问题.

这是我的GetOpenConnection方法

public SqliteConnection GetOpenConnection()
{
    var connection = new SqliteConnection(_connectionString);
    if (connection == null) throw new Exception("Could not create a database connection.");

    connection.Open();

    return connection;
}

和ExecuteScalar方法

public TKey ExecuteScalar<TKey> ( string commandText, IDictionary<string, object> parameters )
{
    using ( var connection = _connectionProvider.GetOpenConnection() )
    {
        using ( var command = connection.CreateCommand() )
        {
            command.CommandType = CommandType.Text;
            command.CommandText = commandText;
            foreach ( var parameter in parameters )
            {
                command.Parameters.Add( new SqliteParameter( parameter.Key, parameter.Value ?? DBNull.Value ) );
            }

            // BREAKING HERE
            return ( TKey )command.ExecuteScalar();
        }
    }
}

而这就是调用它的方法 ExecuteScalar

private const string CheckTableExists = "SELECT name FROM sqlite_master WHERE type='table' AND name='{0}'";

public bool CheckIfTableExists ( string tableName )
{
    var exists = ExecuteScalar<int>( string.Format( CheckTableExists, tableName ) ) == 1;
    return exists;
}

我在它上面设置了一个断点,并尝试进入它... ..代码只是中断并抛出异常......我无法跟踪它

Answer 1

ExecuteScalar如果查询未返回任何记录,则返回null.这似乎解决了NullReferenceException.

public TKey ExecuteScalar<TKey> ( string commandText, IDictionary<string, object> parameters )
{
    using ( var connection = _connectionProvider.GetOpenConnection() )
    {
        using ( var command = connection.CreateCommand() )
        {
            command.CommandType = CommandType.Text;
            command.CommandText = commandText;
            foreach ( var parameter in parameters )
            {
                command.Parameters.Add( new SqliteParameter( parameter.Key, parameter.Value ?? DBNull.Value ) );
            }

            if (typeof (TKey) != typeof (int))
            {
                return (TKey) command.ExecuteScalar();
            }

                var executeScalar = command.ExecuteScalar();
                var item = executeScalar == null ? 0 : 1;
                return (TKey)(object)item;

        }
    }
}