如何在使用 SqlDatareader 读取记录时通知 UI

Question

我有以下方法

    public List<VatRate> GetAll( string cnString )
    {
        List<VatRate> result = new List<VatRate>();
        using (SqlConnection cn = new SqlConnection(cnString))
        {
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = cn;
            cmd.CommandText = SQL_SELECT;
            cmd.CommandType = System.Data.CommandType.Text;
            cn.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            if (reader.HasRows)
            {
                while (reader.Read())
                {
                    VatRate vr = new VatRate();
                    vr.IDVatRate = reader["IDVatRate"] == System.DBNull.Value ? Guid.Empty : (Guid)reader["IDVatRate"];
                    vr.Percent = reader["Percent"].XxNullDecimal();
                    vr.Type = reader["Type"].XxNullString();
                    vr.VatRateDescription = reader["VatRateDescription"].XxNullString();
                }
            }
            reader.Close();
            cn.Close();
        }
        return result;
    }

它将在 WPF 应用程序中使用，我希望能够通知 UI 阅读进度。我必须提出一个简单的事件吗？就像是 OnProgress(new MyProgressEventHandler(recordcounter));我肯定知道这样的方法会在执行时冻结 UI，有没有更好的方法可以做，例如使用异步方法仍然等待方法执行但能够通知用户它在做什么？

Answer 1

您可以传入IProgress<T> 并使用它的Report(T) 方法。如果支持接口的对象是Progress<T>，如果对象是在 UI 线程上创建的，它将自动在 UI 上编组回调。

//elsewhere
public class GetAllProgress
{
    public GetAllProgress(int count, int total)
    {
        Count = count;
        Total = total;
    }

    public Count {get;}
    public Total {get;}
}

public List<VatRate> GetAll( string cnString, IProgress<GetAllProgress> progress )
{
    List<VatRate> result = new List<VatRate>();
    using (SqlConnection cn = new SqlConnection(cnString))
    {
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = cn;
        cmd.CommandText = SQL_SELECT_COUNT;
        //You don't need to do set CommandType to text, it is the default value.
        cn.Open();

        var totalCount = (int)cmd.ExecuteScalar();
        progress.Report(new GetAllProgress(0, totalCount));

        cmd.CommandText = SQL_SELECT; 
        using(SqlDataReader reader = cmd.ExecuteReader())
        {
            //reader.HasRows is unnecessary, if there are no rows reader.Read() will be false the first call
            while (reader.Read())
            {
                VatRate vr = new VatRate();
                vr.IDVatRate = reader["IDVatRate"] == System.DBNull.Value ? Guid.Empty : (Guid)reader["IDVatRate"];
                vr.Percent = reader["Percent"].XxNullDecimal();
                vr.Type = reader["Type"].XxNullString();
                vr.VatRateDescription = reader["VatRateDescription"].XxNullString();
                result.Add(vr);
                progress.Report(new GetAllProgress(result.Count, TotalCount));
            }
            //I put reader in a using so you don't need to close it.
        }
        //You don't need to do cn.Close() inside a using
    }
    return result;
}

然后您可以在后台线程上调用 GetAll（请务必在 UI 线程上调用 new Progress<GetAllProgress>()）或将函数重写为异步。

public async Task<List<VatRate>> GetAllAsync( string cnString, IProgress<GetAllProgress> progress )
{
    List<VatRate> result = new List<VatRate>();
    using (SqlConnection cn = new SqlConnection(cnString))
    {
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = cn;
        cmd.CommandText = SQL_SELECT_COUNT;

        //The .ConfigureAwait(false) makes it so it does not need to wait for the UI thread to become available to continue with the code.
        await cn.OpenAsync().ConfigureAwait(false);

        var totalCount = (int)await cmd.ExecuteScalarAsync().ConfigureAwait(false);
        progress.Report(new GetAllProgress(0, totalCount));

        cmd.CommandText = SQL_SELECT; 
        using(SqlDataReader reader = await cmd.ExecuteReaderAsync().ConfigureAwait(false))
        {
            while (await reader.ReadAsync().ConfigureAwait(false))
            {
                VatRate vr = new VatRate();
                vr.IDVatRate = reader["IDVatRate"] == System.DBNull.Value ? Guid.Empty : (Guid)reader["IDVatRate"];
                vr.Percent = reader["Percent"].XxNullDecimal();
                vr.Type = reader["Type"].XxNullString();
                vr.VatRateDescription = reader["VatRateDescription"].XxNullString();
                result.Add(vr);
                progress.Report(new GetAllProgress(result.Count, TotalCount));
            }
        }
    }
    return result;
}

Answer 2

@Scott Chamberlain 的回答很棒，我建议使用 async/await 解决方案。

这里，我只是为 WPF 添加一些部分。

在WPF中，可以使用<ProgressBar>，只需设置值来表示进度。

<Grid>
    <ProgressBar x:Name="progressBar" HorizontalAlignment="Left" Height="22" Margin="59,240,0,0" VerticalAlignment="Top" Width="383"/>
</Grid>

当你调用GetAllAsync时，你实际上可以像下面这样简单地编写代码：

await GetAllAsync(new Progress<GetAllProgress>(progress=> { progressBar.Maximum = progress.Total; progressBar.Value = progress.Count; } ));

顺便说一句，最好使用 MVVM 模式并将 ADO 逻辑与 UI 解耦，你可以看看这个article。