Is there any way we could make use of CallerArgumentExpressionAttribute?

[Dreamescaper]

C#10 has a nice addition of CallerArgumentExpressionAttribute, which we could use to print actual argument expression in error messages.

I can see two problems though:

• We need to add optional string parameter to methods. But we already have overloads with message string parameter.
• We have methods with params parameter. And we can't combine params and optional parameters in the same method.

Is there any way we could make use of this attribute?

[manfred-brands]

We cannot do this with breaking binary compatibility, but the attribute is only effective at compile time anyhow.
For source code we have currently two methods for each test, e.g.:

Assert.True(bool condition)
Assert.True(bool condition, string? message, param object?[]? args)

We can replace the first with:

Assert.True(bool condition, [CallerArgumentExpression("condition")] string? message = null)

It will not be confused with the 2nd. Similar like Console.WriteLine has different overloads.

[Dreamescaper]

Problem is that existing code like that:

Assert.True(myCondition, "MyCondition should certainly be true!");

will pick up the first method now.

Assert.True(bool condition, [CallerArgumentExpression("condition")] string? conditionExpression = null)

[manfred-brands]

Which is correct. The CallerArgumentExpression only evaluates to the condition string when not specified, otherwise it will be the value passed as an argument:

Assert.True(args.Length == 0);
Assert.True(args.Length == 0, "args");
Assert.True(args.Length == 0, "{0} != 0", args.Length);

Gives:

Expected 'true' for 'args.Length == 0'
Expected 'true' for 'args'
Expected 'true' for '1 != 0'

Based on the following dummy class:

public static class Assert
{
    public static void True(bool condition, [CallerArgumentExpression("condition")] string? message = null)
    {
        if (!condition)
        {
            Console.Error.WriteLine($"Expected 'true' for '{message}'");
        }
    }

    public static void True(bool condition, string message, params object?[] args)
    {
        if (!condition)
        {
            Console.Error.WriteLine($"Expected 'true' for '{string.Format(message, args)}'");
        }
    }
}

[Dreamescaper]

Problem is that existing method usages rarely provide actual expression, but usually some failure description.
So for my example

Assert.True(myCondition, "MyCondition should be true when the sky is blue!");

it will print

Expected 'true' for 'MyCondition should be true when the sky is blue!'

which is probably not what's expected.

[manfred-brands]

Yes, this only helps where there was no message given previously.

[manfred-brands]

This one helps with your situation:

public static void True(bool condition, string? userMessage = null, [CallerArgumentExpression("condition")] string? conditionMessage = null)

This however will conflict in one case with the params overload if a user passes a string argument as params, e.g.:
Assert.True(variable, "Expected: '{0}' to be true.", nameof(variable));

Potentially this can be detected by scanning for '{0' in the string.

[jamers99]

This would be a really great feature to have. Is there any option to do this ourselves?