Unable to cast object of type 'System.DBNull' to type 'System.Byte[]'.
Hi,
I have developed a custom server control for .NET Framework 2.0. The server control has a property named BinaryData of type byte[]. I marked this property to be data bindable. Now, I have varbinary(Max) type of field in my SQL Database and I have used SQLDataSource and bound this varbinary(Max) field with the property BinaryData (byte[]) of my control. It is working fine as long as the data value is not NULL. Now, In my control, I have handled the NULL value so that no Exception is thrown. Still, when I bind this property using the SQLDataSource, I get Error "Unable to cast object of type 'System.DBNull' to type 'System.Byte[]'." I am not sure if I can do anything to stop this erro within my control. If it is not possible from the control, then what is the workaround that I can do in my ASPX page in order to stop this error ?
Thanks a lot in advance.
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emran834 
asp.net.sql-datasource
29906 articles. 
0 followers.
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You are reading in these values from a reader and casting them to their appropriate values. However, since you allow null values in your database table, if there is a null value in your resultant set, it will return DBNull.Value to your reader. This value cannot be casted into a primitive type. Thus you must check for it when you are reading it in and assign it accordingly. For instance,
this._strSomeValue = (sdrReader["SomeField"] == DBNull.Value)? "assign default value" : sdrReader["SomeField"].ToString();
basically check if it is DBNull.Value when you are reading it in, if it is, then manually set it to a default value, in your case you would use a empty byte array.
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triggered .
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emran834 .
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emran834 Dear Ryan,
Thanks a lot for your reply. Actually I am the control developer and I am supplying my control to my client. Now, within my code, i have the property of byte[] type. And this property is databoundable. Now, where can I place your snippet in the code of my control ? I mean, my code has following definition of the byte[] type property,
1 private byte[] binData;
2
3 [Bindable(true)]
4 [Category("Data Input fields")]
5 [Localizable(true)]
6 public byte[] BinData
7 {
8 get
9 {
10 try
11 {
12
13 }
14 catch
15 {
binData = null;
16 }
17
18 return binData;
19 }
20 set
21 {
22 try
23 {
24 binData = value;
25 }
26 catch
27 {
28 binData = null;
29 }
30
37 }
38 }
39Now, How can I access the datareader of the SQLDataSource or any datasource that has bound this property and override this error message ?
Regards.
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emran834 hmm. why do you try catch everything? Heavy on the processing. Let the error get thrown so the user knows why they were stupid. LOL. Anyways, the responsibility lies within the class that is binding to your datasource. Its not like .net protects us from trying to bind a string to an interger variable. So why should you do the same? You can give them a dope car, but cant teach them how to drive. Sorry, thats really all I can say on this. Perhaps, maybe if you have a class that manages the assignment of the data source I can see something in a different light. Take care.
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triggered Dear Ryan,
Thanks a lot for your kind response.
Actually, I was confused and I thought, may be I could do something within my sourcecode which could check the NULL value for the databound property. Because, when I have a label's Text property bound to a SQL varchar(x) type and if the SQL contains NULL for that varchar(x), we dont get this "Unable to cast....." Exception, rather, the Label shows Empty String. Same thing like CheckBox. If I bind "Checked" property of a checkbox to a SQL bit type field, and if SQL contains NULL value for that bit field, we dont get Exception, rather we see unchecked state of the CheckBox. So, I thought, the control developer made some smart way to handle databound NULL values for the Label and CheckBox. If so, as a developer, I also could do something for my Byte[] property.
I understood that it was my client's responsibility to find out the solution. But as a developer, I myself even could not find out the workaround for this problem in my page. For example, I want to use my own control for my varbinary(x) fields in SQL. And I used SQLDataSource to bind the Byte[] BinData property. It is very usual that I may have NULL values for some of the records in my SQL database and I want to use this control right a way to show the binary content. In this case, I would need to know either within Control's Source Code or Outside the control (Page's source code) I should use some snippet of code to handle this issue. And thats what I would like to know what is the soluition for this Error.
Regards.
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emran834 how about ....
public byte[] BinData
{
get { return this._bBinData; }
set
{
if(value.Equals(DBNull.Value) || value == null)
{
// set to initialized byte array or byte[] representation of a empty string like// this._bBinData = System.Text.Encoding.UTF8.GetBytes(String.Empty);
}
else
{
this._bBinData = value;
}
}
}
basically, when the client is setting the value, if it is
any value of null, DBNUll or null, set it to a new instance
of a byte array, which will be set to its default value by
nature.What do you think of this for a solution? It will fix your issue and still throw exceptions when needed.
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triggered Hi Ryan,
Thanks a lot for the solution. I have tried your solution and modified my code as follows :
private byte[] binData;
[Bindable(true)]
[Category("Data Input fields")]
[Localizable(true)]
public byte[] BinData
{
get
{
return binData;
}
set
{
if (value.Equals(DBNull.Value) || value == null)
{
binData = System.Text.Encoding.UTF8.GetBytes(String.Empty);
}
else
{
binData = value;
}
}
}Still I found error when I ran the web application. Here is the screenshot:
It looks like, the FrameWork tries to cast the read value (in this case DBNULL) to byte[] type before it assigns to the control's property so the control is not getting chance to override this process before the SQLDataSorce, am I right ? If so, is there any other idea available for this ?
Thanks again for your kind patience.
Regards
Nishat
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emran834 Correct. I mentioned before that this is going to occur when the value is being read in, and I thought you responded with, you dont have control of this. You are using an actual datasource control or your own class? I would recommend creating your own class that generates a list of objects you need from an internal constructor where you can handle this with the SqlReader. Let me know which way you want to go or if you need further details. If you absolutely want to use a data source control, then handle the event during OnDataBinding.
The picture just loaded for me... Can you also just possibly add a public function in there like BinData='<%# MyPublicFunction("imagedata") %>
where your public function will filter your data. This will work, but not recommended. Your class really should have all this handled behind the scenes and act as a interface to the user. So i would recommend the above method with internal constructor.
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triggered Hi Ryan,
Thanks again. Yes, I tried using public function for the binding method to filter the data and it works. But it works only for ITEM template of the GridView. I want to use this control in the Edit Template and Insert template too and in those cases, I can not use a public function in place of BIND statement, right ? So, this method is not useful in general.
I wont have any datasource control of my own class, rather, I always use SQLDataSource Control that comes with Visual Studio. And all my problem is with this data source control. As my users are most likely using SQLDataSource, and I will need to instruct them about the workaround for NULL values. So, in this case, would you please kindly give me the snippet for handling event of OnDataBinding, and which control's DataBinding Event should be handled ? Sqldatasource's ? or My Control's ?
Regards
Nishat
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emran834 Please take a look at this tutorial to its full extent. It explains nicely what I am talking about.
http://msdn2.microsoft.com/en-us/library/Aa479341.aspx
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triggered 
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