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Debugging LINQ to DataSet Queries

Visual Studio 2012 supports the debugging of LINQ to DataSet code. However, there are some differences between debugging LINQ to DataSet code and non-LINQ to DataSet managed code. Most debugging features work with LINQ to DataSet statements, including stepping, setting breakpoints, and viewing results that are shown in debugger windows. However, deferred query execution in has some side effects that you should consider while debugging LINQ to DataSet code and there are some limitations to using Edit and Continue. This topic discusses aspects of debugging that are unique to LINQ to DataSet compared to non-LINQ to DataSet managed code.

You can view the result of a LINQ to DataSet statement by using DataTips, the Watch window, and the QuickWatch dialog box. By using a source window, you can pause the pointer on a query in the source window and a DataTip will appear. You can copy a LINQ to DataSet variable and paste it into the Watch window or the QuickWatch dialog box. In LINQ to DataSet, a query is not evaluated when it is created or declared, but only when the query is executed. This is called deferred execution. Therefore, the query variable does not have a value until it is evaluated. For more information, see Queries in LINQ to DataSet.

The debugger must evaluate a query to display the query results. This implicit evaluation occurs when you view a LINQ to DataSet query result in the debugger, and it has some effects you should consider. Each evaluation of the query takes time. Expanding the results node takes time. For some queries, repeated evaluation might cause a noticeable performance penalty. Evaluating a query can also cause side effects, which are changes to the value of data or the state of your program. Not all queries have side effects. To determine whether a query can be safely evaluated without side effects, you must understand the code that implements the query. For more information, see Side Effects and Expressions.

Edit and Continue does not support changes to LINQ to DataSet queries. If you add, remove, or change a LINQ to DataSet statement during a debugging session, a dialog box appears that tells you the change is not supported by Edit and Continue. At that point, you can either undo the changes or stop the debugging session and restart a new session with the edited code.

In addition, Edit and Continue does not support changing the type or the value of a variable that is used in a LINQ to DataSet statement. Again, you can either undo the changes or stop and restart the debugging session.

In Visual C# in Visual Studio, you cannot use Edit and Continue on any code in a method that contains a LINQ to DataSet query.

In Visual Basic in Visual Studio, you can use Edit and Continue on non-LINQ to DataSet code, even in a method that contains a LINQ to DataSet query. You can add or remove code before the LINQ to DataSet statement, even if the changes affect line number of the LINQ to DataSet query. Your Visual Basic debugging experience for non-LINQ to DataSet code remains the same as it was before LINQ to DataSet was introduced. You cannot change, add, or remove a LINQ to DataSet query, however, unless you stop debugging to apply the changes.

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