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Property Explanation
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Current Position List Bindings Count Table 20-2
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Contains the value of the current item in the data source Returns the position of the current object in the data source Returns the data source Returns the collection of bindings that the CurrencyManager manages Returns the number of rows that are being managed
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Properties of the CurrencyManager
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20: Data in Windows Forms
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all the CurrencyManager objects, and there is one CurrencyManager object for every data source Consider the following code:
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MessageBoxShow ("We are at position " + thisBindingContext[dsStudents1, "StudentFirstName"]PositionToString()); CurrencyManager cm; cm = (CurrencyManager)thisBindingContext[dsStudents1, "StudentFirstName"]; MessageBoxShow ("There are " + cmCountToString() + " rows");
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The output with the first MessageBox is shown in Figure 20-24 EXAM TIP Notice that the Position property is zero-based
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The reason this works is that the code thistxtXXXDataBindingsAdd( ) creates a new instance of the Binding object and adds it to the Bindings collection
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PART III
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Figure 20-24
A MessageBox displaying the BindingContext object s Position property
MCAD/MCSD Visual C# NET Certification All-in-One Exam Guide
Exception Handling
Any application that accesses data needs the ability to prevent run-time errors from happening Consider the following scenario You design a Windows Form like the college application, and the database server is in a remote location from the user of the Windows Forms (which is a very typical arrangement) For some reason, the connection to the database server fails What happens to your application If you have followed good programming practices, nothing terminal should happen Obviously, the connection failure is a problem for the user, since the live connection to the data is now disconnected However, your application should not terminate with some cryptic message generated by the internal exception-handling mechanism Instead, you want to trap any exceptions and handle them properly (Refer to 4 for the basics of exception handling) What you need to do is enclose your code in try catch blocks to make sure that any exceptions that are beyond your control are handled As a reminder, here is what happens with good exception-handling code: 1 The code instructions that could potentially cause an exception to be thrown (for example, connecting to the database) are enclosed in a try catch finally block 2 The try section says try this code if it works without an exception being thrown, move to the finally block 3 If an exception occurs, execution moves to the catch blocks and checks whether the exception is listed If so, the code within that block is executed Once the exception code has been executed, execution moves to the finally block and the code within that block is run EXAM TIP The finally block always executes (unless the power plug has been pulled!) The type of exceptions that can occur when working with data are shown in Table 20-3 All exceptions found in ADONET have SystemDataDataException as their base class
Exception Explanation
ConstraintException
DataException DBConcurrencyException
This exception indicates that a constraint has been violated For example, a primary key record was deleted when foreign key records still exist This is the base class This exception means database write conflicts have occurred For example, this could happen if one user is updating the database while another is trying to
Table 20-3
Classes Derived from SystemDataDataException
20: Data in Windows Forms
Exception Explanation
DuplicatedNameException NoNullAllowedException ODBCException OleDbException SqlException ReadOnlyException SyntaxErrorException Table 20-4
This exception happens when an object in the database is given a name that already exists This exception occurs when an INSERT into the database contains a NULL value that is not allowed These are general exceptions for the three different data providers This exception means an attempt to update read-only data has occurred This exception happens when a call to SQL instructions contains syntax errors
Classes Derived from SystemDataDataException (continued)
PART III
Although we have not tried to access data programmatically in our application so far, the code section found in Figure 20-25 demonstrates good practice when connecting to a database The try block attempts to create the connection and tests it using a try block If the connection cannot be made, the catch block catches the exception Without this coding, we would receive the message shown in Figure 20-26, which forces the user to ignore the message or quit the application If the connection fails, the code within the catch block executes (assuming an SqlException has been thrown)
Figure 20-25
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