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613 ADONET Error Handling Before diving into SQL Server CE programming in general, we take a moment to examine error handling In the NET Compact Framework, ADONET error handling uses the standard try catch finally technique used in any NET application If an error occurs, a SqlCeException is thrown There is only one SqlCeClient exception class, SqlCeException, not one derived class for each possible error or category of error Because SQL Server CE is written in unmanaged (native) code, it is written for the world of error numbers and HResults, not exceptions Thus, each error represented by a SqlCeException contains its own error information The SqlCeException class has an Errors collection property, consisting of one or more SqlCeErrors (each containing the Source, Message, NativeError, and HResult properties, which are also properties of the SqlCeException class) plus three string parameters and three numeric parameters In the SqlCeException class properties, Source, NativeError, and HResult are equal to the properties of the same name of SqlCeExceptionErrors[0], while Message is equal to StringEmpty The SqlCeException class also has an InnerException property, which is often empty So, your best plan when handling a SQL Server CE exception is to examine its Errors collection For instance, if a SELECT statement contains a misspelled column name, you receive the following information via the SqlCeExceptionErrors[0] object:
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Source: Microsoft SQL Server Windows CE Edition Message: The column name is not valid NativeError: 25503 HResult: -214721900 ErrorParameters[0]: Empty ErrorParameters[1]: [The name of the misspelled column] ErrorParameters[2]: Empty NumericErrorParameters[0]: 0 NumericErrorParameters[1]: 0 NumericErrorParameters[2]: 0
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You should always use error handling If nothing else, error handling allows your application to present the most meaningful message possible to the user and to exit gracefully, as shown in the following code:
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61 Examining ADONET
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private string strConn = "Data Source=" + @"My Documents\ourProduceCosdf";
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: : SqlCeEngine dbEngine = new SqlCeEngine(); dbEngineLocalConnectionString = strConn; : : try { dbEngineCreateDatabase(); } catch( SqlCeException exSQL ) { MessageBoxShow("Unable to create database at " + dbEngineLocalConnectionString + " Reason: " + exSQLErrors[0]Message ); }
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We mention error handling because it is a necessary part of production code Without it, your code is brittle to the touch of all but the most casual user With it, your code becomes robust enough to withstand the most brutal attack That being said, you may notice that our sample code in this book has only a minimal amount of error handling (and sometimes none at all) We do this to keep the focus on the subject at hand and to help make the sample programs easier to understand
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In any programming environment it is advantageous to have a set of utility routines for performing generic tasks This is certainly true in the ADONET environment, where data is repetitively moved from one tier to another or converted from one format to another For instance, several ADONET methods return an array of data rows, arrays that you might like to convert into a data table so that you can bind it to a control This convert from row array to data table task is a good candidate for being handled by a generic routine We have written some generic routines for use in this chapter and placed them in the UtilData class located in the UtilData project at the book s Web site, wwwpaulyaocom/cfbook/code
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62 Working with Data Sets
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The ADONET DataSet class lies at the center of the three-tiered approach and at the heart of this chapter Understanding the DataSet class and its contained objects is a key to successful data management in managed code Let s recap what we have already said about data sets (including Table 63)
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A DataSet object is a memory-resident database A DataSet object can contain multiple DataTable objects DataTable objects can have constraints defined for them (eg, primary key, foreign key, and unique) Parent/child relationships can be specified between the DataTable objects of a DataSet object A DataView object sorts and filters the rows of its associated DataTable object Each DataView is a view of just one table A DataTable can have multiple DataViews defined on it
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Remember that the DataSet class and associated classes are located in the SystemData namespace and therefore are not provider-specific That is, a single DataSet object can be used to hold the data regardless of the provider from which the data was obtained and regardless of the environment in which it is being used The DataSet class is used today in Web services, Web applications, and Windows applications, as well as mobile applications DataSet objects provide four major benefits for the mobile application developer 1 They can contain data from a variety of remote data sources 2 They function while disconnected from those data sources 3 They can persist their data locally as XML files through their ReadXml/WriteXml methods 4 They display their data through data binding Although DataSet objects do not have all the functionality of SQL Server CE (see the DataSet Class Limitations sidebar), their capability to retrieve, disconnect, data-bind, persist locally, reconnect, and update remotely is ideal for lightweight remote applications that do not need the full functionality of SQL Server CE Additionally, if the application does
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