System.InvalidOperationException：读取器关闭时调用 FieldCount 的尝试无效

Question

1. 我希望 queryOutput 能够实现？
2. 为什么 当 fieldcount 已经有 IEnumerable 时，是否尝试调用 fieldcount 无效？

---

/// <summary>
   /// Get all columns for a certain table
   /// </summary>
   public async Task<List<Tuple<string, string, int?>>> GetAllColumnsFromTableAsync(string tableName)
{
    List<Tuple<string, string, int?>> result;
    string query = "SELECT COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME= @tableName";
    using (IDbConnection db = DbConnection) 
    { 
        IEnumerable<Tuple<string, string, int?>> queryOutput = await db.QueryAsync<string, string, int?, Tuple<string, string, int?>>
            (query, Tuple.Create, new { tableName = tableName }, null, false, splitOn: "*");
        result = queryOutput.ToList(); // System.InvalidOperationException: Invalid attempt to call FieldCount when reader is closed.
    }

    if (result is not null && result.Count > 0)
    {
        return result;
    }
    else
    {
        return default;
    }
}

---

• 将此作为非异步方法有效。所以它必须是“tuple”+“dapper”+“async”

参考：https://github.com/DapperLib/Dapper/issues/745

更新：

1. 当用命名元组替换元组时，查询本身 有效。

   public async Task<List<(string COLUMN_NAME, string DATA_TYPE, int? CHARACTER_MAXIMUM_LENGTH)>> GetAllColumnsFromTableAsync2(string tableName)
   {
       const string query = "SELECT COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME= @tableName";
       using var connection = new SqlConnection(_connectionstring);
       var output = await connection.QueryAsync<(string COLUMN_NAME, string DATA_TYPE, int? CHARACTER_MAXIMUM_LENGTH)>
               (query, new { tableName = tableName });
       return output.ToList();
   }
   

我还有一个问题，但这是另一个问题。

Answer 1

Dapper 应该让你的生活更轻松，而代码看起来很复杂。

如果您使用带有命名字段的Tuple，您可以只使用 Dapper 的自动映射来实现。例如：

static class DbExtensions
{
    public static async Task<List<(string COLUMN_NAME, string DATA_TYPE, int? CHARACTER_MAXIMUM_LENGTH)>> GetAllColumnsFromTableAsync(this IDbConnection db, string tableName, string schema = "dbo")
    {
        string query = "SELECT COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME= @tableName and TABLE_SCHEMA=@schema";
        var queryOutput = await db.QueryAsync<(string COLUMN_NAME, string DATA_TYPE, int? CHARACTER_MAXIMUM_LENGTH)>(query, new { tableName = tableName, schema = schema });
        return queryOutput.ToList();
    }
}

另请注意，如果您有多个同名表，则查询中存在错误，因此您应该指定架构。