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try-finally (C# Reference)

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By using a finally block, you can clean up any resources that are allocated in a try block, and you can run code even if an exception occurs in the try block. Typically, the statements of a finally block run when control leaves a try statement. The transfer of control can occur as a result of normal execution, of execution of a break, continue, goto, or return statement, or of propagation of an exception out of the try statement.

Within a handled exception, the associated finally block is guaranteed to be run. However, if the exception is unhandled, execution of the finally block is dependent on how the exception unwind operation is triggered. That, in turn, is dependent on how your computer is set up. The only cases where finally clauses don't run involve a program being immediately stopped. An example of this would be when InvalidProgramException gets thrown because of the IL statements being corrupt. On most operating systems, reasonable resource cleanup will take place as part of stopping and unloading the process.

Usually, when an unhandled exception ends an application, whether or not the finally block is run is not important. However, if you have statements in a finally block that must be run even in that situation, one solution is to add a catch block to the try-finally statement. Alternatively, you can catch the exception that might be thrown in the try block of a try-finally statement higher up the call stack. That is, you can catch the exception in the method that calls the method that contains the try-finally statement, or in the method that calls that method, or in any method in the call stack. If the exception is not caught, execution of the finally block depends on whether the operating system chooses to trigger an exception unwind operation.

Example

In the following example, an invalid conversion statement causes a System.InvalidCastException exception. The exception is unhandled.

public class ThrowTestA
{
    public static void Main()
    {
        int i = 123;
        string s = "Some string";
        object obj = s;

        try
        {
            // Invalid conversion; obj contains a string, not a numeric type.
            i = (int)obj;

            // The following statement is not run.
            Console.WriteLine("WriteLine at the end of the try block.");
        }
        finally
        {
            // To run the program in Visual Studio, type CTRL+F5. Then
            // click Cancel in the error dialog.
            Console.WriteLine("\nExecution of the finally block after an unhandled\n" +
                "error depends on how the exception unwind operation is triggered.");
            Console.WriteLine("i = {0}", i);
        }
    }
    // Output:
    // Unhandled Exception: System.InvalidCastException: Specified cast is not valid.
    //
    // Execution of the finally block after an unhandled
    // error depends on how the exception unwind operation is triggered.
    // i = 123
}

In the following example, an exception from the TryCast method is caught in a method farther up the call stack.

public class ThrowTestB
{
    public static void Main()
    {
        try
        {
            // TryCast produces an unhandled exception.
            TryCast();
        }
        catch (Exception ex)
        {
            // Catch the exception that is unhandled in TryCast.
            Console.WriteLine
                ("Catching the {0} exception triggers the finally block.",
                ex.GetType());

            // Restore the original unhandled exception. You might not
            // know what exception to expect, or how to handle it, so pass
            // it on.
            throw;
        }
    }

    static void TryCast()
    {
        int i = 123;
        string s = "Some string";
        object obj = s;

        try
        {
            // Invalid conversion; obj contains a string, not a numeric type.
            i = (int)obj;

            // The following statement is not run.
            Console.WriteLine("WriteLine at the end of the try block.");
        }
        finally
        {
            // Report that the finally block is run, and show that the value of
            // i has not been changed.
            Console.WriteLine("\nIn the finally block in TryCast, i = {0}.\n", i);
        }
    }
    // Output:
    // In the finally block in TryCast, i = 123.

    // Catching the System.InvalidCastException exception triggers the finally block.

    // Unhandled Exception: System.InvalidCastException: Specified cast is not valid.
}

For more information about finally, see try-catch-finally.

C# also contains the using statement, which provides similar functionality for IDisposable objects in a convenient syntax.

C# language specification

For more information, see The try statement section of the C# language specification.

See also