CA2208 - Undocumented check: ArgumentNullException paramName argument should match an actual method parameter

[jacob-finn-hs1]

Type of issue

Missing information

Description

The following compiler warning is not reflected in the documentation.

warning CA2208: Method Foo passes 'bar' as the paramName argument to a ArgumentNullException constructor. Replace this argument with one of the method's parameter names. Note that the provided parameter name should have the exact casing as declared on the method. (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca2208)

Example:

public void DoFoo(Foo foo) {
    Bar? bar = foo.Bar;
    if (bar is null) throw new ArgumentNullException(nameof(bar), $"Foo named {foo.Name} had no Bar!"); // CA2208
}

The documentation says a lot about not mixing up the parameter name with the message, but doesn't say anything about the parameter name needing to match a method parameter.

Attempting to use NullReferenceException results in CA2201. I'm left wondering what to do if I want to raise an exception when some non-parameter variable is null.

Page URL

https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca2208

Content source URL

https://github.com/dotnet/docs/blob/main/docs/fundamentals/code-analysis/quality-rules/ca2208.md

Document Version Independent Id

cc2b5015-89a9-02ff-a585-fca3449dfa9a

Platform Id

09de10c1-92b4-8a2d-2ff4-db0e71226d62

Article author

@gewarren

Metadata

• ID: ff82be90-2d17-5880-4edd-a89b18a3ae2b
• PlatformId: 09de10c1-92b4-8a2d-2ff4-db0e71226d62
• Service: dotnet-fundamentals

Related Issues

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Associated WorkItem - 502087

[gewarren]

In your example, foo.Bar is not an "argument", so throwing ArgumentNullException doesn't seem like the right exception to throw. If you must throw an exception, perhaps something like ApplicationException is more appropriate.

[jacob-finn-hs1]

That seems like reasonable advice. I think it would be appropriate for the CA2208 documentation to provide advice like that in the "How to fix violations" section, and to mention this paramName issue more explicitly in the Causes section. (I guess it's technically covered by "An incorrect string argument is passed" but that phrase is too vague to be very helpful).

It may be worth noting here that there's no such warning for ArgumentNullException.ThrowIfNull, which happily accepts both non-argument variables as the first argument and non-argument paramNames as the second argument. I don't know if that's worth calling out in the doc or not.