I am developing an add-in for Microsoft Excel that offers a free trial. What are some common ways that people crack / get around the end of a free trial, and how can I mitigate them?
There are several ways that people may attempt to bypass the end of a free trial for a Microsoft Excel add-in, including:

1. Modifying the system clock: People may change the system clock on their computer in order to extend the trial period.

2. Cracking the software: Some individuals may try to use cracking tools or modify the code of the software to remove the trial limitation.

3. Using virtual machines: Some people may use virtual machines to reset the trial period each time the software is installed on a new virtual machine.

4. Sharing the software: Some individuals may share the software with others, who can then use the software beyond the trial period.

To mitigate these issues, you can consider the following methods:

1. Server-based validation: Implement a server-based validation system that checks the validity of the trial period. This makes it difficult for people to bypass the trial by modifying the system clock or using virtual machines.

2. Strong protection: Use strong protection measures such as obfuscation and code signing to make it difficult for people to crack your software.

3. Regular updates: Regularly update your software to address any vulnerabilities and fix any cracks that may have been discovered.

4. License enforcement: Implement a license enforcement system that checks the validity of the license before allowing users to use the software. This can prevent unauthorized sharing of the software.

It's important to keep in mind that it can be difficult to completely prevent people from bypassing the end of a free trial, but implementing the measures listed above can help to significantly reduce the risk.
Can you code me some C# that validates the trial period of my excel add-in?
Certainly! Here's some sample C# code that demonstrates how you can validate the trial period of your Excel add-in:

```
using System;
using Microsoft.Win32;

namespace MyExcelAddin
{
    public class TrialValidator
    {
        private const string REGISTRY_KEY = HKEY_CURRENT_USER\\Software\\MyExcelAddin;
        private const string INSTALL_DATE_VALUE_NAME = InstallDate;
        private const int TRIAL_PERIOD_DAYS = 30;
        
        public static bool IsTrialExpired()
        {
            DateTime installDate = GetInstallDate();
            if (installDate == DateTime.MinValue)
            {
                // No install date found, assume trial has expired.
                return true;
            }
            
            DateTime expirationDate = installDate.AddDays(TRIAL_PERIOD_DAYS);
            return (DateTime.Today >= expirationDate);
        }
        
        private static DateTime GetInstallDate()
        {
            object installDateObj = Registry.GetValue(REGISTRY_KEY, INSTALL_DATE_VALUE_NAME, null);
            if (installDateObj == null)
            {
                return DateTime.MinValue;
            }
            
            long installDateTicks;
            if (!long.TryParse(installDateObj.ToString(), out installDateTicks))
            {
                return DateTime.MinValue;
            }
            
            return new DateTime(installDateTicks);
        }
        
        public static void RecordInstallDate()
        {
            DateTime now = DateTime.Now;
            Registry.SetValue(REGISTRY_KEY, INSTALL_DATE_VALUE_NAME, now.Ticks, RegistryValueKind.QWord);
        }
    }
}
```

Here's a quick overview of what this code does:

It defines a TrialValidator class that contains two public methods: IsTrialExpired() and RecordInstallDate().
The IsTrialExpired() method checks whether the trial period has expired based on the installation date that is stored in the Windows registry. If there is no installation date found, it assumes that the trial has expired. If the installation date is found and the trial period has not yet expired, it returns false. If the trial period has expired, it returns true.
The RecordInstallDate() method records the current date and time in the Windows registry as the installation date for the add-in. This method should be called when the add-in is first installed.
The code assumes that the installation date is stored in the Windows registry under the key HKEY_CURRENT_USER\Software\MyExcelAddin and the value name InstallDate. You can customize these values to match your add-in's registry key and value name.
You can use these methods in your Excel add-in to validate the trial period. For example, you could call RecordInstallDate() when your add-in is first loaded and call IsTrialExpired() each time your add-in starts to determine whether the trial has expired. If the trial has expired, you can show a message to the user indicating that they need to purchase the add-in to continue using it.