Question

Can anyone help me in understanding to know the difference between decimal and decimal? or int and int??.

When should I use them and what are the positives of using either of them.

Answer 1

data types with ? are nullable types. They can hold null value as well
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• Nullable types represent value-type variables that can be assigned the value of null. You cannot create a nullable type based on a
  reference type. (Reference types already support the null value.)
• The syntax T? (in C#) is shorthand for System.Nullable, where T is a value type. The two forms are interchangeable.

• Assign a value to a nullable type in the same way as for an ordinary value type, for example:
  C#: int? x = 10; or double? d = 4.108;

  VB.NET: Dim x As Nullable(Of Integer) = 10 or Dim d As Nullable(Of Double) = 4.108

• Use the System.Nullable.GetValueOrDefault method to return either the assigned value, or the default value for the underlying type if
  the value is null, for example

  C#: int j = x.GetValueOrDefault();

  VB.NET: Dim j as Integer = x.GetValueOrDefault()

• Use the HasValue and Value read-only properties to test for null and retrieve the value.

• The HasValue property returns true if the variable contains a value, or false if it is null.
• The Value property returns a value if one is assigned, otherwise a System.InvalidOperationException is thrown.
• The default value for a nullable type variable sets HasValue to false. The Value is undefined.
  In C# 4.0
• Use the ?? (C#) operator to assign a default value that will be applied when a nullable type whose current value is null is assigned
  to a non-nullable type, for example

int? x = null; 
int y = x ?? -1;