GridView with an empty image handler returns an error

Question

User17798125 posted

Hi forum,

I have a gridview that holds an image, but when I click my button to try to find all my info, an error returns with the next error:
System.InvalidCastException: 'No se puede convertir un objeto de tipo 'System.DBNull' al tipo 'System.Byte[]'.' 

My WebHandler is this, and the debugger points to the line in yellow.

<%@ WebHandler Language="VB" Class="Handler" %>

Imports System.Web
Imports System.Configuration
Imports System.Data.SqlClient

Public Class Handler
    Implements IHttpHandler
    Public Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
        Dim imageid As String = context.Request.QueryString("ImID")
        Dim connection As New SqlConnection(ConfigurationManager.ConnectionStrings("std").ConnectionString)
        connection.Open()
        Dim command As New SqlCommand("select img from stdplusreport1 where ImageID=" & imageid, connection)
        Dim dr As SqlDataReader = command.ExecuteReader()
        dr.Read()
        context.Response.BinaryWrite(DirectCast(dr(0), [Byte]()))
        connection.Close()
        context.Response.[End]()
    End Sub
    Public ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
        Get
            Return False
        End Get
    End Property
End Class

I have tried to convert pass to the null what is null, like the next but without success, I cannot find the the correct sentence.

If dr.Read() Then
            context.Response.BinaryWrite(DirectCast(dr(0), [Byte]()))
        Else
            Convert.IsDBNull(dr(0))
        End If

Any Ideas? 
Thanks! In advance!

Answer 1

User-1716253493 posted

Before casting, chek

        If Not IsDBNull(dr(0)) Then

        End If

another option

Dim command As New SqlCommand("select img from stdplusreport1 where not img is null 
and ImageID=" & imageid, connection)

Answer 2

User17798125 posted

Hi, 

Thanks, but it did not work… Any other suggestions?

Answer 3

User-1174608757 posted

Hi Jonsry10

According to your description,it seems that the data of sqldataReader exists null data.So, I suggest you to use reader.HasRows to check whether the row  exists

You could write as below:

Dim dr As SqlDataReader = Command.ExecuteReader()

            If dr.HasRows Then

                While dr.Read()
                    Context.Response.BinaryWrite(DirectCast(dr(0), [Byte]()))
                End While
            End If

Best Regards

Wei Zhang