什么是“未提供的参数化查询……”。错误？

Question

我遇到了一个错误

"The parametrized query '(@blue nvarchar(4000))SELECT blueBallImage FROM CorrespondingBal' expects the parameter '@blue',
which was not supplied."

我正在做一个 HttpHandler。

我想从数据库中检索图像。我的代码如下。

public void ProcessRequest (HttpContext context) {
    string image = context.Request.QueryString["image"];

    SqlConnection con = new SqlConnection(@"Data Source=localhost;Initial Catalog=MyCloudGames;Integrated Security=True"); 
    SqlCommand cmd = new SqlCommand("SELECT blueBallImage FROM CorrespondingBall WHERE objective = blue", con); 
    cmd.CommandType = CommandType.Text; 
    cmd.Parameters.Add("blue", image); 

    con.Open(); 
    byte[] ballImage = (byte[])cmd.ExecuteScalar(); 
    con.Close(); 

    context.Response.ContentType = "image/png"; 
    context.Response.OutputStream.Write(ballImage, 78, ballImage.Length - 78); 
}

错误发生在

byte[] ballImage = (byte[])cmd.ExecuteScalar();

Answer 1

使用AddWithValue 代替Add 方法。

SqlCommand cmd = new SqlCommand("SELECT blueBallImage FROM CorrespondingBall WHERE objective = @blue", con); 
cmd.CommandType = CommandType.Text; 
cmd.Parameters.AddWithValue("@blue", image);

您应该在命令中为您的参数提供@。

Soner Gönül，图像为空。我该如何纠正？

因此，如果您的 image 是 null，您应该返回 DBNull.Value。您不能在必需参数上传递 null。您可以使用方法作为替代方法；

SqlCommand cmd = new SqlCommand("SELECT blueBallImage FROM CorrespondingBall WHERE objective = @blue", con); 
cmd.CommandType = CommandType.Text; 
cmd.Parameters.AddWithValue("@blue", CheckForDbNull(image));

...

public static object CheckForDbNull(object value)
{
   if(value == null)
   {
       return DBNull.Value;
   }

   return value;
}