如何使用 Dapper 从数据库中检索单个值

Question

I'm using Dapper and trying to retrieve a short from the database and would like to do this without grabbing it from a collection.

I've got the following code, which doesn't work because QueryAsync<short> returns IEnumerable<short> .

short status;
using (var sqlConnection = new SqlConnection(connectionString))
{
    var parameters = new DynamicParameters();
    parameters.Add("@ID", ID, DbType.Int32, ParameterDirection.Input);

    await sqlConnection.OpenAsync();
    status = await sqlConnection.QueryAsync<short>("SELECT [StatusID] FROM [MyTable] WHERE [ID] = @ID", parameters, commandTimeout: _sqlCommandTimeoutInSeconds);
}

Can I get Dapper to return a single value, instead of a collection?

Answer 1

Either you can use ExecuteScalarAsync or Single() with ExecuteScalarAsync you can retrieve a single value from database using Dapper.

short status;
using (var sqlConnection = new SqlConnection(connectionString))
{
  var parameters = new DynamicParameters();
  parameters.Add("@ID", ID, DbType.Int32, ParameterDirection.Input);

  await sqlConnection.OpenAsync();
  status = await sqlConnection.ExecuteScalarAsync<short>("SELECT [StatusID] FROM     [MyTable] WHERE [ID] = @ID", parameters, commandTimeout: _sqlCommandTimeoutInSeconds);
}

Single() can be used in this way

short status;
using (var sqlConnection = new SqlConnection(connectionString))
{
  var parameters = new DynamicParameters();
  parameters.Add("@ID", ID, DbType.Int32, ParameterDirection.Input);

  await sqlConnection.OpenAsync();
  status = await sqlConnection.QueryAsync<short>("SELECT [StatusID] FROM [MyTable] WHERE [ID] = @ID", parameters, commandTimeout: _sqlCommandTimeoutInSeconds).Single();
}

Answer 2

仅供参考，Dapper 现在添加了QuerySingle和QuerySingleAsync以及它们相应的OrDefault变体...... QuerySingleOrDefaultAsync用法是：

await connection.QuerySingleOrDefaultAsync<short>(sql);

Answer 3

您应该使用ExecuteScalar ：

status = await sqlConnection.ExecuteScalarAsync<short>("SELECT [StatusID] FROM [MyTable] WHERE [ID] = @ID", parameters, commandTimeout: _sqlCommandTimeoutInSeconds)