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); }