当数组参数的值为空时，无法确定其类型。

Question

使用PostgreSQL数据库和Dapper。

这是一个失败的代码的最小例子。

await using var connection = new NpgsqlConnection(_configuration.ConnectionString);

return await connection.QueryAsync<Diamond>(
  @"SELECT
      id,
      name
    FROM
      product
    WHERE
      @Names IS NULL OR name = ANY(@Names);",
  new
  {
      Names = Names,
  });

如果 Names = new { "Tom", "Dick", "Harry" }.

然而，它失败了，如果 Names = (string[]) null.

我得到的错误是

Npgsql.PostgresException (0x80004005): 42P18: could not determine data type of parameter $1
   at Npgsql.NpgsqlConnector.<>c__DisplayClass160_0.<<DoReadMessage>g__ReadMessageLong|0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at Npgsql.NpgsqlConnector.<>c__DisplayClass160_0.<<DoReadMessage>g__ReadMessageLong|0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at Npgsql.NpgsqlDataReader.NextResult(Boolean async, Boolean isConsuming)
   at Npgsql.NpgsqlCommand.ExecuteReaderAsync(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken)
   at Npgsql.NpgsqlCommand.ExecuteDbDataReaderAsync(CommandBehavior behavior, CancellationToken cancellationToken)
   at Dapper.SqlMapper.QueryAsync[T](IDbConnection cnn, Type effectiveType, CommandDefinition command) in /_/Dapper/SqlMapper.Async.cs:line 419

能否在Dapper中发送可为空的数组参数到PostgreSQL中 或者我需要找到一个变通方法?

Answer 1

@PeterCsala是正确的。问题是PostgreSQL在服务器端准备查询，所以在C#世界中铸造参数并没有帮助。铸造必须在SQL世界中完成。

工作代码就变成了。

await using var connection = new NpgsqlConnection(_configuration.ConnectionString);

return await connection.QueryAsync<Diamond>(
  @"SELECT
      id,
      name
    FROM
      product
    WHERE
      @Names::text[] IS NULL OR name = ANY(@Names);",
  new
  {
      Names = Names,
  });