c# - bool doesExist = command.ExecuteScalar() != null 由于某种原因不断评估为 true

Question

Possible Duplicate:
ExecuteScalar returns null or DBNull (development or production server)

我有一个存储过程，用于检查预先存在的文件 ID 是否与某个项目关联。如果 select 语句返回值，则该值应该为 true，并将“true”分配给 bool。但是，当 select 语句由于不存在而返回 null 时，我后面的代码仍然使 .Execute 返回“true”

这是我的存储过程:

ALTER PROCEDURE [dbo].[Events_TaskIDExists] 
@EventID int
AS
BEGIN
    select TaskID from Events where EventID = @EventID
END

这是我的代码:

public void hasTaskAssociatedToNote()
{
    String[] Notes = hidSelectedEventIDs.Value.Split(',');
    bool exists = false;
    foreach (var note in Notes)
    {
        var connection = new SqlConnection(ConfigurationManager.ConnectionStrings["OSCIDConnectionString"].ToString());
        var command = new SqlCommand("Events_TaskIDExists", connection);
        command.Parameters.Add(new SqlParameter("@EventID", SqlDbType.Int));
        command.Parameters["@EventID"].Value = Convert.ToInt32(note.Trim());
        command.CommandType = CommandType.StoredProcedure;
        try
        {
            connection.Open();
            exists = command.ExecuteScalar() != null;//causes true when it returns null......
            var temp = command.ExecuteScalar();//this was just to check something else
            if (exists)
            {
                exhibitWarning.Visible = true;
                Warning1.Text = "There is an existing Task associated 0.";
            }
        }
        catch (SqlException sql)
        {
            lblStatus.Text = "Couldn't connect to the Database - Error";
            lblStatus.ForeColor = System.Drawing.Color.Red;
        }
        catch (Exception ex)
        {
            lblStatus.Text = "An error occured";
            lblStatus.ForeColor = System.Drawing.Color.Red;
        }
        finally
        {
            if (connection.State == ConnectionState.Open)
                connection.Close();
        }
    }
}

Answer 1

您的 exists 变量应设置为:

object result = command.ExecuteScalar();
exists = result != DBNull.Value && result != null;

SqlCommand.ExecuteScalar() 的 null 结果返回 DBNull.Value，而不是 null。只有空结果集才会返回 null。

由于您是根据 EventID 选择 TaskID，所以我的猜测是您的数据库没有限制为每个事件都需要一个 TaskID，因此您的 TaskID 字段为空。换句话说，您的事件记录包含 @EventID，但没有关联的任务记录(基于 TaskID)。此条件将返回 DBNull.Value 而不是 null。

"Return Value Type: System.Object The first column of the first row in the result set, or a null reference (Nothing in Visual Basic) if the result set is empty. Returns a maximum of 2033 characters." - MSDN - SqlCommand.ExecuteScalar()