HomeSearch

C# finally (Always Executed After a Try Block)

Use the finally keyword to ensure a block of code always runs. Finally is part of exception handling.
Finally. Code in finally blocks is always executed. In this way, the finally keyword helps ensure the correct execution of C# programs.
This keyword ensures a block of statements are always reached before the enclosing method is exited. It is part of the exception handling control flow.Keywords
An example. This program shows how the finally clause is part of the control flow in programs. In this program, a random number is generated.

And: This random value is used to determine whether to throw an exception, immediately return, or do nothing.

Return

Info: In each of the 3 cases, the finally block is reached immediately after processing completes.

Thus: The finally block can be used to ensure that some logic is always executed before the method is exited.

Important: In all cases, the string "Control flow reaches finally" is printed to the console window.

Console
C# program that uses finally and control flows using System; class Program { static void Main() { try { // Acquire random integer for use in control flow. // ... If the number is 0, an error occurs. // ... If 1, the method returns. // ... Otherwise, fall through to end. int random = new Random().Next(0, 3); // 0, 1, 2 if (random == 0) { throw new Exception("Random = 0"); } if (random == 1) { Console.WriteLine("Random = 1"); return; } Console.WriteLine("Random = 2"); } finally { // This statement is executed before Main exits. // ... It is reached when an exception is thrown. // ... It is reached after the return. // ... It is reached in other cases. Console.WriteLine("Control flow reaches finally"); } } } Possible output #1 Unhandled Exception: System.Exception: Random = 0 at Program.Main() ... Control flow reaches finally Possible output #2 Random = 1 Control flow reaches finally Possible output #3 Random = 2 Control flow reaches finally
Finally, syntax error. A try block must come before finally. We cannot put a finally in a block all by itself (a prelude part, the "try" is always first).
C# program that shows finally syntax error class Program { static void Main() { // Try must come before finally. finally { } } } Output Error CS1003 Syntax error, 'try' expected Error CS1514 { expected Error CS1513 } expected
Finally, syntax 2. We cannot place 2 finally blocks in an exception handling construct. Unlike "try" only 1 finally is allowed—a syntax error is reported if 2 finally blocks are used.
C# program that shows try expected error class Program { static void Main() { // Only one finally is allowed. try { string value = "HELLO"; } finally { } finally { } } } Output Error CS1003 Syntax error, 'try' expected Error CS1514 { expected Error CS1513 } expected
Leave instruction. A C# program is compiled to an intermediate language. An important instruction is the "leave" instruction, which is generated whenever the try block is exited.

Info: Each method contains an exception-handling section that indicates where the finally handler is defined.

Then: When the leave instruction is encountered, that section is used to direct the flow of control.

Also: The IL also defines the endfinally instruction. This is an implementation detail.

Specification. In the C# specification, we encounter many points regarding the finally block and how it conflicts with language constructs.

Goto: For example, the goto statement has many restrictions and you cannot go to a label outside of the finally block.

Goto

Error: If you use a goto in a finally block, you will get an error: "Control cannot leave the body of a finally clause."

Console program. One use for the finally clause in C# programs is to wrap the body of the Main method with a try-finally construct.

Then: In the finally clause, you can print a message that indicates that the program is finished processing.

Tip: Additional flags can be used to report status messages in finally. Or multiple blocks can be placed in Main().

C# program that uses try, finally in console using System; class Program { static void Main() { try { Console.WriteLine("Start"); // Perform important computations in try block. string temp = "x"; for (int i = 0; i < 1000; i++) { temp += "x"; } } finally { // Notify user when program is done. Console.WriteLine("Finished"); } } } Output Start Finished
Catch. How does the catch block relate to the finally block? The two constructs are separate. The catch block is for handling errors.

Meanwhile: The finally block can be used to perform important resource management tasks such as closing files.

Tip: The finally block does not involve an error. But it relies on the same alternate control flow mechanism as catch.

Catch
A summary. A finally block is a way to ensure a piece of logic is executed before the method is exited. The finally construct is separate conceptually from the catch block.
Final notes. Finally uses the exception-handling control flow mechanism. The finally block is used to free system resources in libraries, or for cleanup mechanisms or messages.Exception
© 2007-2019 Sam Allen. Every person is special and unique. Send bug reports to info@dotnetperls.com.
Home
Dot Net Perls