How to create Custom Field Types in SharePoint

There are situation in which it is required that we need to create controls which can be combination of one or more controls,usually controls are .net controls and SharePoint controls,in SharePoint also provides to create SharePoint custom field types by inheriting from one of the  builrt-in field type classes, such as SPFieldText, SPFieldNumber, SPFieldDateTime, and SPFieldMultiColumn. 

For Example

public class EmployeeJoinDate : SPFieldDateTime {

//Code comes here custom field type implementation

}

public class EmployeePANNumber : SPFieldText {

//Code comes here custom field type implementation

}

public class EmployeeSkills : SPFieldMultiColumn {

// custom field type implementation

}

public class EmployeementStatus : SPFieldText {

// custom field type implementation

}