.NET Exceptions – System.ArgumentException

Continuing through our .NET Exception Handling series, today we’ll take a little gander at the System.ArgumentException. As the name implies, the System.ArgumentException is commonly thrown when there is an issue with a provided argument. As with a few other exceptions of its type, the System.ArgumentException is not thrown during normal execution by internal .NET classes or API calls but, instead, is commonly used by developers as an indication of an improper argument attempt.

Throughout this article we’ll further explore the System.ArgumentException, including where it sits in the .NET exception hierarchy, along with a bit of C# example code to see how we might go about using System.ArgumentExceptions in our own projects. Let’s get to it!

The Technical Rundown

When Should You Use It?

As described in the introduction, the System.ArgumentException is among a handful of .NET exceptions that were added into the .NET API, but are not typically triggered during the course of normal execution. That is to say, if you encounter a System.ArgumentException during the execution of an application, the act of throwing that particular error was put there intentionally by a developer, as a means of alerting you to a problem with your arguments, as opposed to some sort of system-level issue.

In most cases, the common language runtime (CLR), or the particular class library you’re using, will have chosen to throw a System.ArgumentException, based on the arguments you’re providing. For example, here we’ve created a simple GetFullName() method, which accepts two string parameters named first and last. As the name implies, we’re attempting to combine both of them and return the result:

private static string GetFullName(string first, string last)
{
    // Check if first value is null or has no characters.
    // Null must be checked prior to length to avoid checking an invalid object.
    if (first is null || first.Length == 0)
    {
        throw new System.ArgumentException($"GetFullName() parameter 'first' must be a valid string.");
    }
    // Check if last value is null or has no characters.
    // Null must be checked prior to length to avoid checking an invalid object.
    if (last is null || last.Length == 0)
    {
        throw new System.ArgumentException($"GetFullName() parameter 'last' must be a valid string.");
    }
    // Return full name.
    return $"{first} {last}";
}

Now, the first option that comes to mind when attempting to pass invalid arguments to a method may be to simply pass nothing where a required parameter belongs:

GetFullName("John")

However, most code editors, or at least the CLR itself, will catch this issue during build and not allow the application to be executed, since it recognizes a required parameter is missing. Here is the particular build error reported to me after a failed build attempt:

Error CS7036  There is no argument given that corresponds to the required formal parameter 'last' of 'Program.GetFullName(string, string)'

The other way we might provide invalid arguments to a method is to pass all the correct types where the method expects them, but using invalid values. This is where our if blocks within the GetFullName() method come in:

// Check if first value is null or has no characters.
// Null must be checked prior to length to avoid checking an invalid object.
if (first is null || first.Length == 0)
{
    throw new System.ArgumentException($"GetFullName() parameter 'first' must be a valid string.");
}
// Check if last value is null or has no characters.
// Null must be checked prior to length to avoid checking an invalid object.
if (last is null || last.Length == 0)
{
    throw new System.ArgumentException($"GetFullName() parameter 'last' must be a valid string.");
}

Here we’ve decided that neither parameter can be null, nor can their Length be zero. This is a basic form of validation, but it gets the job done for our purposes, and ensures that an empty String or null value are not acceptable. If either parameter matches those criteria, we throw a new System.ArgumentException to inform the user of the issue.

With GetFullName() ready to go, we have a couple methods to test it out with. First is ValidExample(), which executes our code within a try-catch block and attempts to combine the names John and Doe then output the result:

private static void ValidExample()
{
    try
    {
        // Get full name with two valid strings.
        var fullName = GetFullName("John", "Doe");
        // Output name result.
        Logging.Log($"Full name is: {fullName}.");
    }
    catch (System.ArgumentException e)
    {
        Logging.Log(e);
    }
}

As expected, this works just fine and the output shows our combined name:

Full name is: John Doe.

On the other hand, our InvalidExample() method passes a null value to the second parameter of GetFullName(), which should throw a System.ArgumentException like we’ve specified:

private static void InvalidExample()
{
    try
    {
        // Get full name with a valid first name, but an invalid last name.
        // Expectation is a thrown ArgumentException.
        var fullName = GetFullName("John", null);
        // Output name result.
        Logging.Log($"Full name is: {fullName}.");
    }
    catch (System.ArgumentException e)
    {
        Logging.Log(e);
    }
}

Sure enough, while there’s no compilation/build issue, since the CLR and code editor both recognize null as a valid type to be passed to a String parameter field, our backup code catches that the last value is null, and throws our System.ArgumentException at us:

[EXPECTED] System.ArgumentException: GetFullName() parameter 'last' must be a valid string.

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