A good solution shouldn’t just work, it should also defend itself against unexpected inputs.

Most bugs come from invalid data, missing values, or unexpected states.

These small issues can lead to unpredictable behavior, hidden exceptions and painful debugging sessions.

One of the simplest ways to prevent such problems early is by adding guard clauses.

Guard Clauses

Guard clauses are small checks, usually placed at the beginning of a method, that validate arguments or state before the rest of the code runs.

They follow a simple principle, fail fast.

Without guard clauses, code often becomes cluttered with deeply nested checks that are harder to read and maintain.

By using guard clauses for common validations, you also follow the DRY principle (don’t repeat yourself), since you avoid copy-pasting the same checks everywhere.

.NET supports this approach very well, it includes many built-in guard clauses and several rich libraries provide even more.

On top of that, you can easily extend libraries with your own checks specific to your domain and business logic.

Since this concept is simple to implement, you can easily add your own guard clauses from scratch for full control.

Built-In Guard Clauses

Starting with .NET 6, C# introduced built-in methods that make it easier to throw common exceptions.

Instead of writing code like this:

public void ProcessOrder(Order order)
{
    if (order is null)
    {
        throw new ArgumentNullException();
    }
    if (order.Quantity <= 0)
    {
        throw new ArgumentOutOfRangeException();
    }
    if (order.ProductId is null)
    {
        throw new ArgumentException();
    }

    // Continue processing...
}

You can now use the built-in guard clause methods to make your code shorter and cleaner:

public void ProcessOrder(Order order)
{
    ArgumentNullException.ThrowIfNull(order);
    ArgumentOutOfRangeException.ThrowIfNegativeOrZero(order.Quantity);
    ArgumentException.ThrowIfNullOrWhiteSpace(order.ProductId);

    // Continue processing...
}

Starting with .NET 8, even more guard clause helpers were added, covering additional common cases:

ArgumentOutOfRangeException.ThrowIfEqual<int>(value, 42, nameof(value));
ArgumentOutOfRangeException.ThrowIfGreaterThan<int>(amount, max, nameof(amount));
ArgumentOutOfRangeException.ThrowIfLessThan<decimal>(price, 0, nameof(price));
ArgumentOutOfRangeException.ThrowIfNegative<int>(count, nameof(count));
ArgumentOutOfRangeException.ThrowIfZero<double>(factor, nameof(factor));

If you’re looking for an even more elegant solution, you should definitely check out third-party libraries that provide a wide range of ready to use guard clauses.

Ardalis Guard Clauses

The Ardalis.GuardClauses package is an excellent library that brings a rich set of ready to use guard clauses to your .NET projects.

Install-Package Ardalis.GuardClauses

It includes most of the common checks you’ll ever need:

Guard.Against.Null // (throws if input is null)
Guard.Against.NullOrEmpty // (throws if string, guid or array input is null or empty)
Guard.Against.NullOrWhiteSpace // (throws if string input is null, empty or whitespace)
Guard.Against.OutOfRange // (throws if integer/DateTime/enum input is outside a provided range)
Guard.Against.EnumOutOfRange // (throws if an enum value is outside a provided Enum range)
Guard.Against.OutOfSQLDateRange // (throws if DateTime input is outside the valid range of SQL Server DateTime values)
Guard.Against.Zero // (throws if number input is zero)
Guard.Against.Expression // (use any expression you define)
Guard.Against.InvalidFormat // (define allowed format with a regular expression or func)
Guard.Against.NotFound // (similar to Null but for use with an id/key lookup; throws a NotFoundException)

One of the best part is that you can easily add your own custom guards that fit your domain logic.

// Using the same namespace will make sure your code picks up your
// extensions no matter where they are in your codebase.
namespace Ardalis.GuardClauses
{
    public static class FooGuard
    {
        public static void Foo(this IGuardClause guardClause,
            string input, 
            [CallerArgumentExpression("input")] string? parameterName = null)
        {
            if (input?.ToLower() == "foo")
                throw new ArgumentException("Should not have been foo!", parameterName);
        }
    }
}

This makes your validation code clean, expressive and consistent across the entire codebase.

Guard Clauses Toolkit

Another great package I’ve used and can easily recommend is the Guard Clauses Toolkit:

Install-Package Microsoft.Toolkit.Diagnostics.Guard

It offers a modern, fluent syntax for guard clauses, making your code more expressive and easier to read:

Guard
    .Against.Null(order)
    .Against.OutOfRange(order.Quantity, 1, 100)
    .Against.NullOrWhiteSpace(order.ProductId);

This fluent style allows you to chain validations, reducing repetitive static calls and improving overall readability.

Some versions of the toolkit also support non-throwing validations, returning results instead of exceptions, a great fit if you use the Result pattern in your projects for which I've written a blog here: Manage Errors with Result Pattern in .NET

Conclusion

Guard clauses are one of those small techniques that make a big impact on code quality.

They help you write methods that are safer, clearer and easier to maintain.

Start by adding guard clauses to your existing methods and you’ll quickly notice how much easier it becomes to reason about your code.

If you want to check out examples I created, you can find the source code here:

If you find this topic interesting, I’ll create a follow-up blog showing how to build your own guard clauses step by step.

I hope you enjoyed it, subscribe and get a notification when a new blog is up!