c# argument suffix question

        public override DBProductCollection GetAllProducts(int CategoryID, int ManufacturerID,
            bool? FeaturedProducts, decimal? PriceMin, decimal? PriceMax, string Keywords, 
            bool SearchDescriptions, int PageSize, int PageIndex, out int TotalRecords, bool showHidden)

What does the "?" and the end of  bool and decimal do?  I've done a google search and without surprise the results were useless.

How is "bool FeaturedProducts" different from "bool? FeaturedProducts"?

The "?" means that the variable can hold the value type's normal range of values or null.

The variable "bool FeaturedProducts" can be either true or false.

The variable "bool? FeaturedProducts" can be true or false, or null.

More information about nullable types:
http://msdn.microsoft.com/en-us/library/1t3y8s4s%28VS.80%29.aspx
http://msdn.microsoft.com/en-us/library/2cf62fcy%28VS.80%29.aspx

.

as it says above, the ? is a nullable Type, Parameter, object or value

Thanks all.