Question: I am always confused with the different ways of expressing nulls. There is the null reference type (aka "null"). Then I've seen that throughout my application, developers have used MinValue to represent nulls. Ex: Double.MinValue or DateTime.MinValue except for a String for which they use "null"

Then there is System.DBNull (and System.DBNull.Value - not sure what to use when). To add to the confusion, there are also `System.Nullable` and `System.Nullable<T>` namespaces.

Can someone help me clear this null confusion?

Thanks

Answer: 
* null for reference types in the
actual null value
* Nullable, added in .NET 2.0 is a
way of expressing nulls for value
types, which by definition can not be
null.
* Same with DateTime.MinValue -
DateTime is a value type, and can not
be null, so you can have a convention
that a well known value, like
DateTime.MinValue is treated as if it
was null. It also has other usages.
The important difference here is that between value types and reference types. 
Value types represent directly a certain value, whereas reference types point to a memory location that should represent a value. When a a reference type actually does not point to a memory location with valid contents, the reference is Null.

Since value types are direct representations of value, they cannot be null. However, it might be the case that a actual value of a value type is unknown. For this case, .Net provides the Nullable types construct. When such a construct is not available, however, people tend to use special or default values, such as MinValue.

When communicating with databases, a lot of things we expect to be value types can actually be Null, since that it the way the database handles unknown values. This can also be solved by the Nullable types, but these were not always available. That's why DBNull exist, to deal with a possible null in a database.