Entity Framework Fluent API - Unique key for multiple columns

PropertyConfigurationExtensions.cs

public static class PropertyConfigurationExtensions
{
    public static PrimitivePropertyConfiguration IsUnique(this PrimitivePropertyConfiguration configuration,
        string indexName, int indexOrder)
    {
        var indexAttribute = new IndexAttribute(indexName, indexOrder)
        {
            IsUnique = true
        };

        return configuration.HasColumnAnnotation(IndexAnnotation.AnnotationName, new IndexAnnotation(indexAttribute));
    }
}