c# - 空引用异常(从 mysql 下载 BLOB 文件)

Question

当单击链接按钮时，我尝试从 mysql 数据库下载 blob 文件。以下是我的代码:

Response.Buffer = true;
Response.Charset = "";
if (context.Request.QueryString["download"] == "1")
{
    context.Response.AppendHeader("Content-Disposition", "attachment; filename=" + filename);
}

try
{
    Response.BinaryWrite(bytes);
}
catch (NullReferenceException ex) {
    LinkButton2.Text = "No file was uploaded";
}

但是它给出了“bytes”上的空引用异常。字节在以下语句中初始化

if (!Convert.IsDBNull(r["proposalDoc"]))
{
    this.bytes = (byte[])r["proposalDoc"];
}

数据检索代码:

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {

            this.Time = String.Format("{0:HH:mm:ss}", DropDownList1.SelectedValue);
            String query = "Select * from event where time='" + this.Time + "'";
            MySqlConnection conn = new MySqlConnection(connection);

            MySqlCommand cmd = new MySqlCommand(query, conn);

            DateTime time = DateTime.Today;

            conn.Open();

            MySqlDataReader r = cmd.ExecuteReader();

            while (r.Read())
            {
                TextBox1.Text = r["name"].ToString();
                TextBox2.Text = r["Proposedby"].ToString();
                if (!Convert.IsDBNull(r["proposalDoc"]))
                {
                    this.bytes = (byte[])r["proposalDoc"];
                }

                TextBox5.Text = Calendar1.SelectedDate.ToString("d");
                TextBox6.Text = r["time"].ToString();
                TextBox7.Text = r["Society"].ToString();
                TextBox8.Text = r["venue"].ToString();

            }

此外，该记录存在于proposalDoc的数据库中

Answer 1

如果您的目的是获取proposalDoc的第一个非空值，请注意( per the MSDN ):

DBNull.Value is used to indicate a value that is missing. It is not equivalent to null or to String.Empty. Therefore, code such as Convert.IsDBNull(null) in C# or Convert.IsDBNull(Nothing) in Visual Basic returns false.

因此，如果 r["proposalDoc"] 为 null，您将把 null 分配给 this.bytes.

克服这个问题的最快方法是对分配进行空合并:

this.bytes = this.bytes ?? (byte[])r["proposalDoc"];