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Error Handling and Debugging

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In this section, we will cover the essential concepts of error handling and debugging in Delphi/Object Pascal. Proper error handling and debugging are crucial for developing robust and reliable applications. This module will help you understand how to manage errors gracefully and debug your code effectively.

Key Concepts

  1. Error Handling:

    • Understanding exceptions
    • Using try...except blocks
    • Using try...finally blocks
    • Raising exceptions

  2. Debugging:

    • Setting breakpoints
    • Inspecting variables
    • Using the call stack
    • Step-by-step execution

Error Handling

Understanding Exceptions

Exceptions are runtime errors that disrupt the normal flow of a program. Delphi provides a robust mechanism to handle these exceptions using try...except and try...finally blocks.

Using try...except Blocks

The try...except block is used to catch and handle exceptions. Here’s a basic example:

procedure DivideNumbers(a, b: Integer);
var
  result: Integer;
begin
  try
    result := a div b;
    Writeln('Result: ', result);
  except
    on E: EDivByZero do
      Writeln('Error: Division by zero!');
    on E: Exception do
      Writeln('An error occurred: ', E.Message);
  end;
end;

begin
  DivideNumbers(10, 0);
end.

Explanation:

  • The try block contains the code that might raise an exception.
  • The except block handles specific exceptions (EDivByZero in this case) and a general exception handler for any other exceptions.

Using try...finally Blocks

The try...finally block ensures that certain code is executed regardless of whether an exception occurs. This is useful for resource cleanup.

procedure OpenFile;
var
  FileHandle: TextFile;
begin
  AssignFile(FileHandle, 'example.txt');
  try
    Reset(FileHandle);
    // Perform file operations
  finally
    CloseFile(FileHandle);
  end;
end;

begin
  OpenFile;
end.

Explanation:

  • The finally block ensures that CloseFile is called whether an exception occurs or not.

Raising Exceptions

You can raise exceptions using the raise keyword. This is useful for custom error handling.

procedure CheckAge(Age: Integer);
begin
  if Age < 18 then
    raise Exception.Create('Age must be 18 or older.');
  Writeln('Age is valid.');
end;

begin
  try
    CheckAge(16);
  except
    on E: Exception do
      Writeln('Error: ', E.Message);
  end;
end.

Explanation:

  • The raise keyword is used to create and raise a new exception.

Debugging

Setting Breakpoints

Breakpoints allow you to pause the execution of your program at specific points. This helps you inspect the state of your application.

  1. Open your Delphi IDE.
  2. Click on the left margin next to the line number where you want to set a breakpoint.
  3. A red dot will appear, indicating a breakpoint.

Inspecting Variables

While debugging, you can inspect the values of variables to understand the state of your application.

  1. Run your application in debug mode.
  2. When execution pauses at a breakpoint, hover over a variable to see its value.
  3. You can also use the "Watch List" to monitor specific variables.

Using the Call Stack

The call stack shows the sequence of function calls that led to the current point of execution. This is useful for understanding the flow of your program.

  1. When execution pauses at a breakpoint, open the "Call Stack" window.
  2. The call stack will display the list of function calls.

Step-by-Step Execution

Step-by-step execution allows you to execute your code one line at a time.

  • Step Over (F8): Executes the next line of code but does not step into functions.
  • Step Into (F7): Steps into the function call.
  • Step Out (Shift+F8): Steps out of the current function.

Practical Exercise

Exercise: Handling Division by Zero

Write a procedure that takes two integers and divides them. Handle the division by zero exception and print an appropriate message.

Solution:

procedure SafeDivide(a, b: Integer);
var
  result: Integer;
begin
  try
    result := a div b;
    Writeln('Result: ', result);
  except
    on E: EDivByZero do
      Writeln('Error: Division by zero!');
  end;
end;

begin
  SafeDivide(10, 0);
end.

Exercise: File Handling with Exception

Write a procedure to open a file, read its content, and ensure the file is closed properly using try...finally.

Solution:

procedure ReadFileContent;
var
  FileHandle: TextFile;
  Line: string;
begin
  AssignFile(FileHandle, 'example.txt');
  try
    Reset(FileHandle);
    while not Eof(FileHandle) do
    begin
      Readln(FileHandle, Line);
      Writeln(Line);
    end;
  finally
    CloseFile(FileHandle);
  end;
end;

begin
  ReadFileContent;
end.

Common Mistakes and Tips

  • Forgetting to Close Resources: Always use try...finally to ensure resources like files and database connections are closed properly.
  • Catching General Exceptions: Avoid catching general exceptions unless necessary. Handle specific exceptions to provide more meaningful error messages.
  • Overusing Exceptions: Use exceptions for exceptional conditions, not for regular control flow.

Conclusion

In this section, we covered the basics of error handling and debugging in Delphi/Object Pascal. You learned how to use try...except and try...finally blocks to handle exceptions and ensure resource cleanup. Additionally, you explored debugging techniques such as setting breakpoints, inspecting variables, and using the call stack. These skills are essential for developing robust and reliable applications. In the next module, we will delve into working with data, including arrays, strings, and records.

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Delphi/Object Pascal Programming Course

Module 1: Introduction to Delphi/Object Pascal

Module 2: Control Structures and Procedures

Module 3: Working with Data

Module 4: Object-Oriented Programming

Module 5: Advanced Delphi Features

Module 6: GUI Development with VCL and FMX

Module 7: Web and Mobile Development

Module 8: Best Practices and Design Patterns

Module 9: Final Project

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