System.InvalidOperationException: Invalid attempt to call FieldCount when reader is closed

Question

1. I would have expected the queryOutput to be materialized?
2. Why is there the invalid attempt to call fieldcount when it alreadyhas the IEnumerable?

---

/// <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;
    }
}

---

• making this a non async method works. so it MUST be "tuple"+"dapper"+"async"

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

update:

1. when replacing the tuple with named tuples the query in itself works.

   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();
   }
   

I have a further problem but that is another question.

Answer 1

Dapper's supposed to make your life easier, and that code looks complicated.

If you use a Tuple with named fields, you can just use Dapper's auto mapping to materialize. EG:

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();
    }
}

Also note there's a bug in your query if you have multiple tables with the same name, so you should specify the schema.