Two more reserved attributes for easier error handling

[thargol1]

Two more reserved attributes for easier error handling

A common pattern is to use Throw helpers for generating consistent error checks and error messages.

An example:

public static class Throw
{
	public static void WhenIsNullOrEmpty(string value, string paramName)
	{
		if (string.IsNullOrEmpty(value))
		{
			throw new ArgumentException($"'{paramName}' cannot be null or empty.", paramName);
		}
	}
}

And is called like this:

void MyDemo(string? name)
{
	Throw.WhenIsNullOrEmpty(name, nameof(name));
}

This example has two problems:

1. The null checks in the compiler don't see that this function does not return when value is null or empty.
2. It's a lot of typing for little functionality.

Suggestion 1: Add a new attribute to indicate that a function does not return if an argument is null or empty:

public static void WhenIsNullOrEmpty([DoesNotReturnWhenNullOrEmpty] string value, string paramName)

or

public static void WhenIsNull([DoesNotReturnWhenNull] object value, string paramName)

Suggestion 2: Add a new attribute that gets the source code from a parameter like CallerMemberNameAttribute returns the calling function name. The (0) indicates which parameter to get.

public static void WhenIsNullOrEmpty(string value, [ParameterSource(0)] string paramName = "")

So

Throw.WhenIsNullOrEmpty(name);

Lowers to:

Throw.WhenIsNullOrEmpty(name, "name");

Or

Throw.WhenIsNullOrEmpty(options.Name);

Lowers to:

Throw.WhenIsNullOrEmpty(options.Name, "options.Name");

[HaloFour]

See: #287

[jnm2]

The null checks in the compiler don't see that this function does not return when value is null or empty.

There's already an annotation that achieves the null side of this. Put [NotNull] on the parameter; it's a postcondition that applies if the function returns. It means exactly what you want [DoesNotReturnWhenNull] to mean.

[333fred]

The combination of this and HaloFour's answer is exactly what you're looking for @thargol1 (with C# 10 support for CallerExpressionAttribute):

public static void ThrowWhenNull([NotNull] object value, [CallerExpression("value")] string valueExpression = "")
{
    if (value is null) throw new ArgumentNullException(valueExpression);
}

[thargol1]

Thanks for the answers.

Perhaps https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.notnullattribute?view=net-5.0 should be expanded with an example or better clarification.

It's exactly what I asked for, but I didn't understand that from the documentation.

(This issue can be closed.)

[333fred]

@BillWagner making sure you see this feedback.

[BillWagner]

Thanks. This article explains the use of all these attributes. (It hasn't been updated for C# 10 though).

I like the sample @thargol1 showed above. I'll add that (unless @thargol1 wants to submit a PR as this was his idea).

I'll work with our team to ensure that the pages on each of these attributes links to the conceptual doc that explains how to use all the attributes.

[thargol1]

Go ahead Bill and add it. I don't need kudos.