DbTransaction class and about interfaces in C#

Question

I came across an example of the implementation an an interface. Portion of code is

public partial interface IDataProvider
{
    DataTable GetEmployeeAbsenceDurationTypes();

    void AlterEmployeeAbsenceDurationTypes(DataTable lookUpTable);
}

public partial class DataProvider : IDataProvider
{    

    public DataTable GetEmployeeAbsenceDurationTypes()
    {
        return GetEmployeeAbsenceDurationTypes((DbTransaction)null);
    }
    public DataTable GetEmployeeAbsenceDurationTypes(DbTransaction tran)
    {
        //Db Operations
    }
}

My first question is about this "DbTransaction" class. Its not in my project, is it a build in class?

My second question is, why in the DataProvider (the implementing class), the function is calling another overload of itself?

Answer 1

DbTransaction is a common base-class for representing database transactions in ADO.NET; each actual ADO.NET provider subclasses this (typically) - for example SqlTransaction : DbTransaction (the sql-server client).

Calling an overload of self is a common way of implementing optional parameters, without code duplication prior to their addition in C# 4.0. In this case, that is essentially a pre-4.0 way of implementing:

public DataTable GetEmployeeAbsenceDurationTypes(DbTransaction tran = null) {...}

either implementation (overloads or optional parameter) allows usage of the form:

obj.GetEmployeeAbsenceDurationTypes(); // without transaction
obj.GetEmployeeAbsenceDurationTypes(tran); // with transaction