C# - Partial Classes and Methods

In C#, you can split the implementation of a class, a struct, a method, or an interface in multiple .cs files using the partial keyword. The compiler will combine all the implementation from multiple .cs files when the program is compiled.

Consider the following EmployeeProps.cs and EmployeeMethods.cs files that contain the Employee class.

EmployeeProps.cs
public partial class Employee
{
    public int EmpId { get; set; }
    public string Name { get; set; }
}
EmployeeMethods.cs
public partial class Employee
{
    //constructor
    public Employee(int id, string name){
        this.EmpId = id;
        this.Name = name;
    }

    public void DisplayEmpInfo() {
        Console.WriteLine(this.EmpId + " " this.Name);
    }
}

Above, EmployeeProps.cs contains properties of the Employee class, and EmployeeMethods.cs contains all the methods of the Employee class. These will be compiled as one Employee class.

Example: Combined Class
public class Employee
{
    public int EmpId { get; set; }
    public string Name { get; set; }

    public Employee(int id, string name){
        this.EmpId = id;
        this.Name = name;
    }

    public void DisplayEmpInfo(){
        Console.WriteLine(this.EmpId + " " this.Name );
    }
}

Rules for Partial Classes

  • All the partial class definitions must be in the same assembly and namespace.
  • All the parts must have the same accessibility like public or private, etc.
  • If any part is declared abstract, sealed or base type then the whole class is declared of the same type.
  • Different parts can have different base types and so the final class will inherit all the base types.
  • The Partial modifier can only appear immediately before the keywords class, struct, or interface.
  • Nested partial types are allowed.

Partial Methods

Partial classes or structs can contain a method that split into two separate .cs files of the partial class or struct. One of the two .cs files must contain a signature of the method, and other file can contain an optional implementation of the partial method. Both declaration and implementation of a method must have the partial keyword.

EmployeeProps.cs
public partial class Employee
{
    public Employee() { 
        GenerateEmpId();
    }
    public int EmpId { get; set; }
    public string Name { get; set; }

    partial void GenerateEmployeeId();

}
EmployeeMethods.cs
public partial class Employee
{
    partial void GenerateEmployeeId()
    {
        this.EmpId = random();
    }
}

Above, EmployeeProps.cs contains the declaration of the partial method GenerateEmployeeId() which is being used in the constructor. EmployeeMethods.cs contains the implementation of the GenerateEmployeeId() method. The following demonstrates creates an object the Employee class which used the partial method.

EmployeeMethods.cs
class Program
{
    static void Main(string[] args)
    {
        var emp = new Employee();
        Console.WriteLine(emp.EmpId); // prints genereted id

        Console.ReadLine();
    }
}

Rules for Partial Methods

  • Partial methods must use the partial keyword and must return void.
  • Partial methods can have in or ref but not out parameters.
  • Partial methods are implicitly private methods, so cannot be virtual.
  • Partial methods can be static methods.
  • Partial methods can be generic.

Learn more about partial classes and methos here.

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