什么是“未提供参数化查询...。”错误？

Question

I have experienced an error on

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

I am doing a HttpHandler.

I want to retrieve the image from the database. My codes are as below.

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); 
}

The error occurred at

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

Answer 1

Use AddWithValue instead of Add method.

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

You should provide your parameter with @ in your command.

Soner Gönül, image is null. How can I correct it?

So, you should return DBNull.Value if your image is null . You can't pass a null on a required parameter. You can use a method as an alternative like;

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;
}