Troubleshooting Data Access in Visual Studio .NET
This topic lists the following common issues that arise when working with data access in Visual Studio .NET.
This is most likely the result of the user name and password passed at run time being denied access to the database. Resolution depends on the authentication and authorization methods being used in the database, as well as the connection string.
Some common configurations that cause this problem are:
- Selecting the Use a specific username and password option and not selecting the Allow saving password option when completing the Data Link Properties Dialog Box. In this scenario you are prompted for a username and password when attempting to connect to the data source at design time. For run-time connectivity, you need to incorporate functionality into your application to pass the proper username and password to the data source.
- Not setting up permission for the ASPNET user in your data source. For information on how ASP.NET requests permissions to data, see Access Permissions for Web Applications.
Things to check:
- Verify the username and password that your application is using have proper permissions to access the data source.
- Verify the user has the correct permission settings in the database.
- Verify there is network connectivity between the machine running the application and the data source.
Reconfiguring a data adapter does not automatically regenerate the dataset. The dataset needs to be synchronized with the new data-adapter configuration. Regenerating the dataset should fix this problem.
To regenerate a Dataset from a configured data adapter
- Select the form that contains the data adapter and open it in Design view.
- On the Data menu, click Generate Dataset.
The Generate Dataset Dialog Box opens.
- In the Choose which table(s) to add to the dataset area, select the table associated with the reconfigured data adapter.
- Click OK.
The following table provides links to related information.
|For more information on...||See...|
|DataSet Class||Introduction to Datasets|
|DataAdapter Class||Introduction to Data Adapters|
This is most likely the result of command objects that no longer contain a reference to a valid data connection object. This is typically caused by deleting and recreating an existing connection object.
Applications with Data Adapters
Reconfigure the data adapter to set the Connection property for the adapter's Command objects. For more information about data adapters, connections, and commands, see Using .NET Data Providers to Access Data.
To reconfigure a data adapter
- Select the form that contains the data adapter(s) and open it in Design view.
- Select the data adapter in the component tray.
- On the Data menu, click Configure Data Adapter .
The Data Adapter Configuration Wizard opens.
- Complete the wizard. For more information, see Data Adapter Configuration Wizard.
- Repeat this process from step 2 for each affected data adapter.
Applications with Individual Command Objects
Set the Connection property to a valid connection object.
To set the Connection property of a Command object
- Select the form that contains the command(s) and open it in Design view.
- Select the command object in the component tray.
- Set its Connection property to a valid connection object.
- Repeat this process from step 2 for each affected command.
This most likely results from incorrect setting of the parameter's Direction property. The parameter's Direction property is set to a value defined in the ParameterDirection Enumeration.
- Verify the parameter's Direction property is set to a value that that can receive data (Output for output parameters).
- Verify the parameter's data type is the same as the data type of the expected return value.
Note Output parameters are returned at the end of the data stream, so if you are using the DataReader object, you must close it or read to the end of the data before the output parameters are visible.
For more information on output parameters not being returned, as well as a code sample, search for Q308051 on the Microsoft Product Support Services Web site (http://support.microsoft.com). Select Article ID as the Search option.
Getting the 'The .NET Data SQL Provider (System.Data.SqlClient) requires Microsoft Data Access Components (MDAC) version 2.6 or later' Error
The Microsoft .NET Framework SDK and the .NET Framework redistributable package do not include the MDAC installation. All .NET applications that use data-access functionality require MDAC 2.6 or later (MDAC 2.7 is recommended). The latest version of MDAC is available as a download from the Microsoft Web site (http://www.microsoft.com).
Because Visual Studio .NET installs MDAC 2.7 by default, this error is most likely to occur when deploying to a machine that does not have Visual Studio .NET installed.
When you deploy your application, you can have the setup check the version of MDAC on the computer being deployed to. For more information, see Adding a Launch Condition for Microsoft Data Access Components.
When an error at the data source causes a transaction to be immediately rolled back, one of the following errors may be raised in your application:
- "The COMMIT TRANSACTION request has no corresponding BEGIN TRANSACTION"
- "The ROLLBACK TRANSACTION request has no corresponding BEGIN TRANSACTION"
Although this behavior is by design, it is always best to call methods that access external data sources (such as the Commit and Rollback methods) from within a Try...Catch statement. For more information, see Using the Try/Catch Block to Catch Exceptions.
When a dataset is bound to a XmlDataDocument calling the Clear method throws the following exception:
System.InvalidOperationException: Cannot add or remove tables from the DataSet once the DataSet is mapped to a loaded XML document.
Clearing the data from a dataset bound to a XmlDataDocument object requires navigating the table and removing the individual DataRow objects. For more information on removing data rows from a data table, see Deleting Records in a Dataset and DataRowCollection.RemoveAt Method.