ArgumentNullException

ArgumentNullException is a specific exception type in the .NET Framework and .NET Core that indicates a method received a null argument for a parameter that does not accept null. It is a subclass of ArgumentException, and by extension of SystemException. The exception signals a programming error due to a missing required value rather than a recoverable runtime condition.

Typically thrown when a public method requires a non-null value for a parameter, the check is performed at the start of the method and, if the argument is null, an ArgumentNullException is raised. The exception exposes the ParamName property, identifying the offending parameter; the message often references that parameter by name.

Constructors for ArgumentNullException include a parameterless form, one that accepts the parameter name, and one that accepts both the parameter name and a custom message. There is also support for serialization via a protected constructor used by the .NET serialization framework.

Usage notes include distinguishing this exception from other argument-related exceptions. ArgumentNullException should be used specifically for null arguments, whereas a NullReferenceException indicates an attempt to use a null value within code, such as dereferencing a null object, rather than an invalid parameter passed to a method.

Best practices emphasize guarding public APIs and using the nameof operator to supply the parameter name, which improves maintainability and reduces errors from hard-coded strings. This helps callers diagnose faults quickly by indicating exactly which parameter was null.