public override Func<ExecutableTestCreationContext, TestMetadata, AbstractExecutableTest> CreateExecutableTestFactory
{
    get
    {
        if (InstanceFactory != null && InvokeTypedTest != null)
        {
            return (context, metadata) =>  // NEW LAMBDA EVERY ACCESS!
            {
                var typedMetadata = (TestMetadata<T>)metadata;
                Func<TestContext, Task<object>> createInstance = async testContext => { ... };
                Func<object, object?[], TestContext, CancellationToken, Task> invokeTest = async (...) => { ... };
                return new ExecutableTest(createInstance, invokeTest) { ... };
            };
        }
        throw new InvalidOperationException(...);
    }
}

private Func<ExecutableTestCreationContext, TestMetadata, AbstractExecutableTest>? _cachedExecutableTestFactory;

public override Func<ExecutableTestCreationContext, TestMetadata, AbstractExecutableTest> CreateExecutableTestFactory
{
    get
    {
        if (_cachedExecutableTestFactory != null)
            return _cachedExecutableTestFactory;

        if (InstanceFactory != null && InvokeTypedTest != null)
        {
            _cachedExecutableTestFactory = (context, metadata) => { ... };
            return _cachedExecutableTestFactory;
        }
        throw new InvalidOperationException(...);
    }
}