[Feature]: Support custom results in GenerateAssertion

[ascott18]

Problem Statement

It would be nice if GenerateAssertion could generate assertions similar to HasSingleItemAssertion, where the assertion is "terminal" and returns the matched item. Currently this requires building the entire assertion class by hand.

Proposed Solution

Something like this:

public class UnitTest1
{
    [Test]
    public async Task TestWithMemberData()
    {
        string[] items = new[] { "food", "bard" };
        var matched = await Assert.That(items).ContainsMessage("foo", exact: false);
        // Do other stuff with `matched`, whose value will be "food".
    }
}

public static partial class Assertions
{
    [EditorBrowsable(EditorBrowsableState.Never)]
    [GenerateAssertion(ExpectationMessage = "to contain message '{needle}'")]
    // New type: AssertionResult<T>
    public static AssertionResult<string> ContainsMessage(this IEnumerable<string> strings, string needle, bool exact = true)
    {
        var result = strings.FirstOrDefault(x => exact ? x == needle : x.Contains(needle));
        if (result is not null)
            return AssertionResult.Passed(result); // New method: Produces an AssertionResult<T>
        else
            // AssertionResult would have an implicit conversion to AssertionResult<T>,
            // allowing this to work? The conversion from AssertionResult => AssertionResult<T>
            // should throw if AssertionResult.Passed because there's no suitable `Result` to use?
            return AssertionResult.Failed(exact
                ? $"{needle} not found (exact matching)"
                : $"{needle} not found  (loose matching)");
    }
}

Alternatives Considered

Build the Assertion by hand

Feature Category

Assertions

How important is this feature to you?

Nice to have - would improve my experience

Additional Context

No response

Contribution

• I'm willing to submit a pull request for this feature