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` is only valid for *reference* types: types that are a `class` rather than a `structure`.
* .Net also has value types: int, double, DateTime, etc. Value types cannot be null, you so you normally compare those to their default value, which is very often `type.MinValue`, but might be something (consider Boolean, for example).
* `Nullable<T>` is for when you have a value type that might genuinely be null. The default value (maybe MinValue) is also valid and you need to distinguish it from when the variable has not been assigned yet. In C#, you can use a ? with a value type as a short hand notation (for example: `int?`), but you're still creating the same `Nullable<T>`.
* `DBNull` specifically refers to NULL values from a database. It's not the same thing as using null elsewhere in the language: it's only for talking to a database so you can know when a query returned a null value.
* When using generics, you'll also see the `default(T)` construct used, where T is a type parameter. This allows you to set a type's default value without knowing whether that type is a reference type or a value type, much less what a specific value type's default value might be.
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.