More on Partial Methods

Thank you everyone for the feedback.  If you have any more to say then please do express your opinion (and yes it is valued ;).  I think there has been a bit of misunderstanding about what partial methods really are.  So let's set the matter straight.  Here is the current spec (quoting from here on out):

Partial methods are a new feature complementing partial classes with the option of declaring “hook” – or compile time events – in the code.

Here is an example

partial class Customer


  public string Name {

    get { … }

    set {


      _name = value;




  partial void OnNameChanging(string value);

  partial void onNameChanged();


Partial methods are only allowed in partial types; either classes or structs. There are no other kinds of partial members.

There are two parts of a partial method; the defining and the implementing partial method declaration. There can be at most one defining declaration, and at most one implementing declaration, and the implementing declaration is only allowed if there is a defining declaration. The defining declaration is distinguished by having a “;” instead of a method body. The two can both appear in the same partial type declaration.

Partial methods must always return void. The contextual keyword partial has to appear right before void – that is how we recognize it. They can have ref but not out parameters; both params and this modifiers are allowed in the usual positions.

Access modifiers on partial methods are not allowed, but partial instead implies private. The only modifiers allowed on a partial method declaration (apart from partial) are unsafe and static. Thus a partial method declaration cannot declare or override a virtual method. Also, partial methods cannot be extern, because the presence of the body determines whether they are defining or implementing.

If both a defining and an implementing partial method declaration are given, all modifiers on parameters and the method declarations themselves must be given in both declarations, although not necessarily in the same order.

Partial methods can be generic. Constraints are placed on the defining partial method declaration, and must be repeated on the implementing one. Parameter and type parameter names do not have to be the same in the implementing declaration and the defining one.

Whether or not there is an implementing declaration, a partial method participates in overload resolution and type checking in the normal way. If there is no implementing declaration, however, the method will be removed. Furthermore calls to the method are removed, including evaluation of arguments to the call, in a manner similar to conditional methods.

You can only make a delegate to a partial method if there is an implementing declaration; otherwise a compile time error occurs. Similarly, calls to a partial method occurring within the body of a lambda which is converted to an expression tree are only allowed if there is an implementing declaration of the partial method.

Because partial methods are always private, they cannot implement interface methods, neither implicitly or explicitly.

If both a defining and an implementing partial method declaration are given, attributes on the resulting method and its return type, parameters and type parameters are determined by assembling, in unspecified order, the attributes from both the defining and implementing declarations. Duplicates are not removed. Thus, for attributes that can only occur once on a given declaration, having that attribute on both declarations will result in a compile time error.

Definite assignment is only checked after the possible removal of partial method calls and evaluation of the argument expressions. Thus, if assignments occur within these argument expressions, and the method is removed, they will not affect definite assignment. This can potentially lead to compile time errors.

The Main method of a program can be a partial method, but will only be considered as the entry point for the program if there is an implementing declaration.

XML doc comments are allowed on both defining and implementing partial method declarations, but only the ones that appear on the implementing declaration will be used by the compiler. Other tools may make use of the comments on defining declarations.