public async Task ExistsAsync_WhenRepositoryIsNull_ThrowsArgumentNullException <TKey>(TKey defaultValue)
{
// Arrange & Act & Assert
var exception = await Assert.ThrowsAsync <ArgumentNullException>(
() => RepositoryExtensions.ExistsAsync <IEntity <TKey>, TKey>(null, x => x.Id.Equals(defaultValue))
);
Assert.Equal("repository", exception.ParamName);
}