How to use guard clauses in C#

• You can use guard clauses in your methods to prevent runtime exceptions by handling null reference exceptions.
• You can use guard clauses to validate input data and enforce data integrity by only processing valid inputs.
• You can use guard clauses to define preconditions for your methods and properties, enhancing the readability and maintainability of your code.

We’ll examine each of these aspects of guard clauses using code examples in the sections that follow.

Use a guard clause to avoid null reference exceptions in C#

Null reference exceptions are often encountered in applications when you use an object reference that has not been initialized. In this section, we will examine how we can use a guard clause to prevent such exceptions from being thrown at runtime. The following code snippet shows how you can check if the parameter of a method is null and throw an ArgumentNullException, preventing a null reference exception from being thrown at runtime and allowing the method to complete gracefully.

public void CheckIfNull(object obj)
{
    if (obj is null)
    {
        throw new ArgumentNullException(nameof(obj), "Method parameter cannot be null.");
    }
    //Other code
}

Use a guard clause to enforce input validation rules in C#

Input validation enables you to maintain data integrity by enforcing validation rules and constraints in your application. You can implement guard clauses in your application’s source code to allow only valid data to be processed by your application.

The following code example shows how you can use a guard clause in C# to prevent invalid input. Note how an exception is thrown if the input string is null or empty.

public void CheckIfNullOrEmpty(string str)
{
    if(!string.IsNullOrEmpty(str))
    {
        throw new ArgumentException("Invalid data: The string passed in the method argument cannot be empty or null");
    }
    //Other code
}

Use a guard clause to enhance code readability and maintainability in C#

Guard clauses help you write maintainable and readable code by centralizing your application’s validation rules. They provide an elegant way to prevent unexpected behavior in your application’s code, making it consistent, organized, and easy to maintain.

Let us understand this with a code example. In the console application project we created earlier, create a new class named Author and enter the following code.

class Author
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Address { get; set; }
    public string Email { get; set; }
}

The following code snippet illustrates how you can take advantage of the constructor of the Author class to validate input.

 public Author(string firstName, string lastName, string address, string email)
 {
     if (string.IsNullOrEmpty(firstName))
     {
         throw new ArgumentException("First name cannot be null or empty", nameof(firstName));
     }
     if (string.IsNullOrEmpty(lastName))
     {
         throw new ArgumentException("Last name cannot be null or empty", nameof(lastName));
     }
     if (string.IsNullOrEmpty(address))
     {
         throw new ArgumentException("Address cannot be null or empty", nameof(address));
     }
     if (string.IsNullOrEmpty(email))
     {
         throw new ArgumentException("Email cannot be null or empty", nameof(email));
     }
 }

As you can see, the preceding code snippet is quite verbose. We can make it much more concise and readable by using guard clauses. Note that the following code can replace all of the if statements used in the constructor of the Author class above.