Write fluent guard in your C# methods

Intoduction

What are guard clauses ? Guard clauses are true/false expressions (predicates) found at the top of a method or function that determine whether the function should continue to run. Guard clauses test for preconditions and either immediately return from the method or throw an exception, preventing the remaining body of code from executing.

Example

A common example is to check if an argument is null and throw an ArgumentNullException. However, one may be performing other forms of validation on arguments such as if dates fall within a certain date range, integer values are positive, strings are a certain length, etc. Any assumptions about the arguments or state of the object that are being made by the remaining block of code are typically made explicit by the guard clauses.

void Add(User user)
{
    // Guard clause  
    if (user == null)
        throw new ArgumentNullException(nameof(user));

    // rest of the code...
}

What is Fluent Guard?

Fluent Guard is a simple library that helps you to use Microsoft.Toolkit.Diagnostics.Guard extensions with more fluency.

Logo

logo of fluent guard library

The idea is not new, I tried to copy some code and personalize some code from the project FluentAssertions.

Installation

You can download and install the package manually from Nuget org:

Nuget

Or run this command on the package manager of your Visual studio:

Install-Package Mahdhi.GuardFluently.Core

Usage

Let’s say we have a service SomeService that defince the following method:

 public void DoSomeCall(string name, object length)
{ 
  // do something
}

Here you will sure need to make some checks for your parameters name and length.

Let’s say :

  • the name shouldn’t be null and should have at least 10 chars.
  • the length should be assignable to the type int

In this case your check can look like this,

public void DoSomeCall(string name, object length)
{
  //guard
  name.Should()
      .NotBeNullOrWhiteSpace()
      .And
      .HaveLengthGreaterThan(10);

  length.Should().BeAssignableTo(typeof(int));

 // do something
}

In this example there are three exceptions that can be thrown:

  • System.ArgumentNullException: Thrown if name is null.
  • System.ArgumentException: Thrown if name is empty or whitespace.
  • System.ArgumentException: Thrown if length is not assignable to the type int.

If you call this method as following:

SomeService someService = new();
try
{
    someService.DoSomeCall("A", 7);
    Console.WriteLine("No exception...");
}
catch (Exception ex)
{
    Console.WriteLine(ex.Message);
}

Here the name ‘A’ is shorter than 10 letters, in this case an exception will be thrown:

But with this example, where name and length are valid:

try
{
    someService.DoSomeCall("My size is over that 10", 7);
    Console.WriteLine("No exception...");
}
catch (Exception ex)
{
    Console.WriteLine(ex.Message);
}

In this case no exception will be thrown.

I hope this blog could give you some new infos 🙂

References

David Hayden – Guard Clauses in Computer Science

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