Read Onely and Consat Varribles are same but Read onely 
Variables you can assign the Values at Runtime

Ex. Date

variable used to store the data
Read only variable:-
Read only variable means read only but not allow to write
constant:
constant variable specify the constant value.

Read only variable value is changed at runtime.

but Constant variable value doesn't change.

Readonly Variable can be accessed while instantiating the
Class and CONSt - you have to manually code the constant in
each and every project. 

Like:
  ReadOnly Property age() As Integer
        Get
            Return 98
        End Get
         
    End Property

constants are dealt with at compile-time, while static 
readonly fields are set at the time they are evaluated at 
run-time. 
the constant value will need to be recompiled if the 
constant value changes. Libraries referencing a static 
readonly field will reference the field and not the value, 
thus they will pick up any change in the field without the 
need for recompilation
constants will only support value types plus the 
special .NET ones string and null while Static readonly 
fields are able to hold reference types too.
So, constants should be used when it is very unlikely that 
the value will ever change, or if no external apps/libs 
will be using the constant. Static readonly fields should 
be used when run-time calculation is required, or if 
external consumers are a factor