[Feature]: Custom assertions in .NET

🚀 Feature Request

A mechanism for defining custom assertions in Playwright .NET. The custom assertions would take advantage of the same retry logic (and any other good stuff) that the built-in assertions have.

This is possible in JS/TS, though I wouldn't expect .NET custom assertions to be registered in the same way.

Example

Expect(locator).ToContainNumberAsync(42)

Motivation

Currently, if you want to assert something that isn't covered by the built-in assertions, then you must resort to a method like ILocator.TextContentAsync() and then make a (non-Playwright) assertion on the result. In doing so, you no longer have this benefit:

Playwright includes async matchers that wait until the expected condition is met. Using these matchers makes tests non-flaky and resilient

Indeed, custom assertions/matchers are already supported in Node.js.

Start by reading the linked Playwright JS/TS custom matcher documentation and reviewing the built-in .NET assertion behavior described in the issue. Compare that with the ILocator.TextContentAsync workaround and the ToContainNumberAsync example. Done means a defined .NET mechanism for custom assertions that preserves Playwright's retry behavior.