Validation Logic software Pattern

What you can do is create an interface:

public interface IProductValidator
{
    bool CanProcess(string productCode);
    void Validate(string keyCode);
}

Then implement this interface for every product you want to validate:

public class XYZProductValidator : IProductValidator
{
     public bool CanProcess(string productCode)
     {
         return productCode == "XYZ";
     }

     public void Validate(string keyCode)
     {
         //validation logic
     }
}

If you're using dependency injection, go and register this as a singleton. If you're not using DI, then you need some factory class that will be responsible for the creation of every class.

In your calling code, if you are using DI, you can inject an IEnumerable<IProductValidator> productValidators.

public class Calling
{
    private readonly IEnumerable<IProductValidator> _productValidators;

    public Calling(IEnumerable<IProductValidator> productValidators)
    {
        _productValidators = productValidators;
    }

    public void Validate(string productCode, string keyCode)
    {
        //find the right validator based on productCode
        var validator = _productValidators.Where(v => v.CanProcess(productCode));
        validator.Validate(keyCode);
    }

}

This way in the future when you add more products, all you have to do is implement the IProductValidator interface and all your calling code will just work. Now, if you're worried about looping through and IEnumerable to find the correct IProductValidator, you can have another class e.g. ProductValidatorProvider, inject the IEnumerable<IProductValidator> in there, convert that into a Dictionary<string, IProductValidator> and then use that to find the correct IProductValidator.