1. Constants

Aconstantis a symbol that has a never-changing value. When defining a constant symbol, its value must be determinable at compile time. The compiler then saves the constant’s value in the assembly’s metadata. This means that you can define a constant only for types that your compiler considers primitive types. In C#, the following types are primitives and can be used to define constants:Boolean, Char, Byte, SByte, Int16, UInt16, Int32, UInt32, Int64, UInt64, Single, Double, Decimal,andString. However, C# also allows you to define a constant variable of a non-primitive type if you set the value tonull: 

using System;

public sealed class SomeType {

// SomeType is not a primitive type but C# does allow

// a constant variable of this type to be set to 'null'.

public const SomeType Empty = null;

}

When code refers to a constant symbol, compilers look up the symbol in the metadata of the assembly that defines the constant, extract the constant’s value, and embed the value in the emitted Intermediate Language (IL) code. Because a constant’s value is embedded directly in code, constants don’t require any memory to be allocated for them at runtime. 

If the developer changes the constant and only rebuilds the DLL assembly, the application assembly which refers the constant is not affected. For the application to pick up the new value, it will have to be recompiled as well.

2. Fields

A field is a data member that holds an instance of a value type or a reference to a reference type.

Filed modifiers

CLR Term          C# Term

Static                static               The field is part of the type’s state, as opposed to being part of an object’s state.

Instance            (default)           The field is associated with an instance of the type, not the type itself.

InitOnly            readonly    The field can be written to only by code contained in a constructor method.

Volatile            volatile  Code that accessed the field is not subject to some thread-unsafe optimizations that may be performed
                                                    by the compiler, the CLR, or by hardware. Only the following types can be marked volatile: all reference
                                                    types, Single, Boolean, Byte, SByte, Int16, UInt16, Int32, UInt32, Char, and all enumerated types with
                                                    an underlying type of Byte, SByte, Int16, UInt16, Int32, or UInt32.

For type fields, the dynamic memory required to hold the field’s data is allocated inside the type object, which is created when the type is loaded into an AppDomain, which typically happens the first time any method that references the type is just-in-time (JIT)–compiled. 

For instance fields, the dynamic memory to hold the field is allocated when an instance of the type is constructed.

Because fields are stored in dynamic memory, their value can be obtained at runtime only. Fields also solve the versioning problem that exists with constants. In addition, a field can be of any data type, so you don’t have to restrict yourself   to your compiler’s built-in primitive types (as you do for constants).