Parameter Validation Best Practices

Question

Imagine you have an application which is some kind of front-end to all your business logic. This front-end has a lot of DLLs upon which it depends, and the methods in t

Answer 1

Unless the validation of the parameter is going to be expensive, I would go with #1. Fail-fast behavior lets you catch bugs in a fraction of the time, which will save you a lot more time than it takes to write a few guard statements at the beginning of each method.

One technology you may be interested to help with this is .NET's Code Contracts, which allow you to create quasi-compile-time checks to ensure that nobody calls a method without ensuring that the inputs match the expected patterns.

I personally tried using Code Contracts, and found that there was a little too much overhead for my needs. However, I appreciated the syntax, so I made a class to help with these guard statements, but which only works at run-time. It works like this:

public void ChangeUserName(int userId, string name)
{
    Require.ThatArgument(userId > 0);
    Require.ThatArgument(!string.IsNullOrWhitespace(name,
        () => "Usernames must be non-empty strings");
    var user = GetUser(userId);
    Require.That(user != null, 
        () => new UserDoesNotExistException("No user exists with ID " + userId));
    user.Name = name;
    ...
}

And one final technology that helps a lot for these checks is Resharper's Annotations. For example, consider the following method:

[CanBeNull]
public User GetUser(int userId)
{
    var user =  ... // Get the user from the db
    return user;
}

By telling Resharper that the method might return a null value, it will know to warn you if you haven't done a null check on user before trying to access user.Name. Another annotation is available to tell Resharper that Require.That(user != null) constitutes a null check. You could also re-write your method like this:

[NotNull]
public User GetUser(int userId)
{
    Require.ThatArgument(userId > 0);
    var user =  ... // Get the user from the db
    Require.That(user != null)
    return user;
}

By marking this method as NotNull, Resharper can automatically tell you that user != null will always resolve to true so you don't have to check for it. There are all kinds of fun stuff you can do to make validation easier.