no suitable method found to override error with partial class

Question

i need to define a class partially in seperate assemblies. actually i need to redefine a class partially, that is already defined in an assembly written in C++ Cli, but this is maybe a different question.
for the case, all codes written in c#, i have a baseclass definition in basenamespace assembly

using System;

namespace BaseNameSpace
{
  public class BaseClass
  {
    public int Num;
    public double dNum;
    public BaseClass(int s, double d)
    {
      Num = s;
      dNum = d;
    }

    public virtual void Wrt()
    {
      Console.WriteLine("{0},{1}", Num, dNum);
    }
  }
}

i add another assembly project called derivedclassSpace and declare a derivedclass derived from baseclass. i also add a partial class definition to this project.

using System;
using BaseNameSpace;

namespace BaseNameSpace
{
  public partial class BaseClass
  {
    public void Mult()
    {
      Num *= 2;
    }
  }
}

namespace DerivedNameSpace
{
  public class DerivedClass : BaseClass
  {
    public DerivedClass(int s)
      : base(s, 0)
    {

    }

    public override void Wrt()
    {

      Mult(); // mult line
      base.Wrt();
    }
  }
}

when i build the solution i get

'DerivedNameSpace.DerivedClass.Wrt()': no suitable method found to override

if i remove the partial class lines and mult line, everything is normal.
am i using the concept of partial keyword? how can i get rid of the error?

Solution

You cannot define a partial class across assemblies. Partial classes (and partial methods) are just compiler features that allow you to split up the definition across files. Once the assembly is compiled, there is a single class. Your only real option is to inherit from the base class, add your function, then have further derived classes inherit from this new class.

If all you're trying to do is add a method to an existing class, however, you might consider an extension method. While this doesn't actually add it to the class per se, it does allow you to write your code as if it had.

namespace BaseNameSpace
{
  public static class BaseClassExtensions
  {
    public static void Mult(this BaseClass instance)
    {
      instance.Num *= 2;
    }
  }
}

namespace DerivedNameSpace
{
  public class DerivedClass : BaseClass
  {
    public DerivedClass(int s)
      : base(s, 0)
    {

    }

    public override void Wrt()
    {

      Mult(); // mult line
      base.Wrt();
    }
  }
}

Comments

[Steve]

Indeed. Partial classes are a feature of C# language, not of the .Net platform, so once something is compiled to IL you no longer have the ability to use partial classes.

[abatishchev]

@Steve: Event better to say - of C# compiler

[ali_bahoo]

it says The name 'Mult' does not exist in the current context. i assume it will be "BaseClassExtensions.Mult(this);"

[Adam Robinson]

@bahadir: You can call them that way (they are just normal static methods), but the purpose of extension methods is to allow the appearance of calling an instance method. However, you must be using the namespace in which the class that defines the extension methods is declared.