C#查询数据库显示进度条

在C#中查询数据库并显示进度条，可以使用`BackgroundWorker`控件来实现。以下是实现步骤：

1. 创建一个`BackgroundWorker`控件和进度条控件。

BackgroundWorker worker = new BackgroundWorker();
ProgressBar progressBar = new ProgressBar();

2. 在`DoWork`事件中编写查询数据库的代码，并在查询过程中更新进度条的值。

worker.DoWork += (sender, e) =>
{
    using (SqlConnection conn = new SqlConnection(connectionString))
    {
        conn.Open();
        SqlCommand cmd = new SqlCommand("SELECT COUNT(*) FROM MyTable", conn);
        int count = (int)cmd.ExecuteScalar();
        cmd.CommandText = "SELECT * FROM MyTable";
        using (SqlDataReader reader = cmd.ExecuteReader())
        {
            int i = 0;
            while (reader.Read())
            {
                // 处理查询结果
                i++;
                int progress = (int)((double)i / count * 100);
                worker.ReportProgress(progress);
            }
        }
    }
};

3. 在`ProgressChanged`事件中更新进度条的值。

worker.ProgressChanged += (sender, e) =>
{
    progressBar.Value = e.ProgressPercentage;
};

4. 启动`BackgroundWorker`控件并显示进度条。

worker.RunWorkerAsync();
progressBar.ShowDialog();

完整的代码如下：

BackgroundWorker worker = new BackgroundWorker();
ProgressBar progressBar = new ProgressBar();

worker.DoWork += (sender, e) =>
{
    using (SqlConnection conn = new SqlConnection(connectionString))
    {
        conn.Open();
        SqlCommand cmd = new SqlCommand("SELECT COUNT(*) FROM MyTable", conn);
        int count = (int)cmd.ExecuteScalar();
        cmd.CommandText = "SELECT * FROM MyTable";
        using (SqlDataReader reader = cmd.ExecuteReader())
        {
            int i = 0;
            while (reader.Read())
            {
                // 处理查询结果
                i++;
                int progress = (int)((double)i / count * 100);
                worker.ReportProgress(progress);
            }
        }
    }
};

worker.ProgressChanged += (sender, e) =>
{
    progressBar.Value = e.ProgressPercentage;
};

worker.RunWorkerAsync();
progressBar.ShowDialog();