•  Abstraction 

1. Abstraction is "To represent the essential feature without representing the background details."
2. Abstraction lets you focus on what the object does instead of how it does it.
   
3. Abstraction provides you a generalized view of your classes or objects by providing relevant information
   
4. Abstraction is the process of hiding the working style of an object, and showing the information of an object in an understandable manner.

• Encapsulation 
1. Hiding the internal state and functionality of an objec...

You can solve the issue by adding below lines in web.config file.

 <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral"/>
        <bindingRedirect oldVersion="0.0.0.0-8.0.0.0" newVersion="8.0.0.0"/>
      </dependentAssembly>
    </assemblyBinding>
  </runtime>

A string is a sequential collection of characters that is used to represent text.

A String object is a sequential collection of System.Char objects that represent a string; a System.Char object corresponds to a UTF-16 code unit.

The value of the String object is the content of the sequential collection of System.Char objects, and that value is immutable (that is, it is read-only).

For more information about the immutability of strings, see the Immutability and the StringBuilder class section in ...