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Guard Clauses

A simple extensible package with guard clause extensions.

A guard clause is a software pattern that simplifies complex functions by "failing fast", checking for invalid inputs up front and immediately failing if any are found.

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public void ProcessOrder(Order order)

    // process order here

// OR

public class Order
    private string _name;
    private int _quantity;
    private long _max;
    private decimal _unitPrice;
    private DateTime _dateCreated;

    public Order(string name, int quantity, long max, decimal unitPrice, DateTime dateCreated)
        _name = Guard.Against.NullOrWhiteSpace(name);
        _quantity = Guard.Against.NegativeOrZero(quantity);
        _max = Guard.Against.Zero(max);
        _unitPrice = Guard.Against.Negative(unitPrice);
        _dateCreated = Guard.Against.OutOfSQLDateRange(dateCreated, dateCreated);

Supported Guard Clauses

  • 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)

Extending with your own Guard Clauses

To extend your own guards, you can do the following:

// 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);

// Usage
public void SomeMethod(string something)
    Guard.Against.Foo(something, nameof(something)); // optional - provide parameter name

YouTube Overview

Ardalis.GuardClauses on YouTube

Breaking Changes in v4

  • OutOfRange for Enums now uses EnumOutOfRange
  • Custom error messages now work more consistently, which may break some unit tests

Nice Visualization of Refactoring to use Guard Clauses


via Nicolas Carlo


Commercial Support

If you require commercial support to include this library in your applications, contact NimblePros

Build Notes (for maintainers)

  • Remember to update the PackageVersion in the csproj file and then a build on main should automatically publish the new package to
  • Add a release with form 1.3.2 to GitHub Releases in order for the package to actually be published to Nuget. Otherwise it will claim to have been successful but is lying to you.