Here are some security best practices to consider when writing C# code:

1. Input validation: Always validate and sanitize user input before using it in any way. This will help prevent common security vulnerabilities such as SQL injection, cross-site scripting (XSS), and command injection.

2. Parameterized queries: Use parameterized queries or stored procedures instead of dynamic SQL queries to prevent SQL injection attacks.

3. Password storage: Never store passwords in plain text. Use salted hashes with a secure hashing algorithm, such as bcrypt or Argon2, to store passwords securely.

4. Authentication and authorization: Implement proper authentication and authorization mechanisms to ensure that only authorized users can access sensitive data or functionalities.

5. Secure transmission: Use secure communication protocols (e.g., HTTPS) and encrypt sensitive data when transmitting over network connections to prevent eavesdropping and man-in-the-middle attacks.

6. Secure configuration management: Avoid hardcoding sensitive information such as passwords and API keys in your code. Instead, use configuration files or environment variables to store and retrieve such values.

7. Principle of least privilege: Limit the permissions and privileges of your application and its components to the minimum necessary to perform their intended tasks. This helps reduce the impact of potential security vulnerabilities.

8. Error handling: Be cautious about the level of detail you expose in error messages. Avoid providing sensitive information that could be exploited by an attacker.

9. Secure session management: Implement secure session management techniques, such as using secure cookies with secure attributes and expiring sessions after a certain period of inactivity.

10. Security updates: Stay up to date with security patches and updates for your development tools, frameworks, libraries, and dependencies. Regularly review and update your code to incorporate any necessary security fixes.

11. Logging and monitoring: Implement logging and monitoring mechanisms to detect and respond to potential security breaches or suspicious activities in a timely manner.

12. Secure coding practices: Follow secure coding practices, such as avoiding buffer overflows, input/output validation, secure file handling, and safe object serialization.

13. Security testing: Conduct regular security testing, such as penetration testing and code reviews, to identify and address security vulnerabilities in your code.

By following these best practices, you can help protect your C# applications from common security threats and vulnerabilities.