PrimaryKey struct

The structure PrimaryKey is used by Entity class (and MListElement) as the type of the Id property.

public abstract class Entity : ModifiableEntity, IEntity
{
    [Ignore]
    internal PrimaryKey? id = null; //primary key
}

The mission of PrimaryKey is to provide the flexibility to any class that inherits from Entity to choose the type of the primary key in the database (int, long, Guid... or any other IComparable) using PrimaryKeyAttribute, and at the same time let any code that manipulates the entity pass PrimaryKey structures around independently of this decision.

PrimaryKey only has one field, IComparable Object that contains the underlying value.

[Serializable]
public struct PrimaryKey : IEquatable<PrimaryKey>, IComparable, IComparable<PrimaryKey>, ISerializable 
{
    public readonly IComparable Object;

    public PrimaryKey(IComparable obj)
    {
        if (obj == null)
            throw new ArgumentNullException("obj");

        this.Object = obj;
    }
}

This Object can not be null, in order to represent an optional Id a nullable PrimaryKey? is required.

PrimaryKey is designed to get out of the way, letting you have the illusion that is the base class for all the possible value types (int, long, Guid...). In that sense:

• It implements IEquatable<T> and IComparable. You can compare two PrimaryKeys to know if they are equal or witch one is bigger (even GUIDs can be ordered). Note: Comparing two PrimaryKey of different types throws an exception.
• It provides implicit casting operators to convert from the three most common types (int, long and Guid), for any other type, the constructor can be used. There are also operators from the nullable versions of this three types, or the PrimaryKey.Wrap methods for the remaining.
• Is provides explicit casting operators to convert to the three most common types (int, long and Guid), for any other type, just access the Value property. There are also operators to the nullable versions of the types and the PrimaryKey.Unwrap for the remaining.
• It overloads the comparison operators (==, !=, <, <=, > and >=) with other PrimaryKey.

PrimaryKey struct contains a static Polymorphic data-structure to know what is the type of the primary key of each entity class.

You can call PrimaryKey.Parse passing the type of the entity and it will parse the underlying value accordly.

public static bool TryParse(string value, Type entityType, out PrimaryKey id)
public static PrimaryKey Parse(string value, Type type)

In order to convert the primary key to string, just use ToString or TryToString.