C# Partial Types and Methods

Partial Types

Partial types allow a type definition to be split—typically across multiple files. A common scenario is for a partial class to be autogenerated from some other source (such as a Visual Studio template or designer), and for that class to be augmented with additional hand-authored methods:

// PaymentFormGen.cs - auto-generated
partial class PaymentForm { ... }

// PaymentForm.cs - hand-authored
partial class PaymentForm { ... }

Each participant must have the partial declaration;Participants cannot have conflicting members. A constructor with the same parameters, for instance, cannot be repeated. Partial types are resolved entirely by the compiler, which means that each participant must be available at compile time and must reside in the same assembly.

You can specify a base class on one or more partial class declarations, as long as the base class, if specified, is the same.

Partial Methods

A partial type can contain partial methods. These let an autogenerated partial type provide customizable hooks for manual authoring. For example:

partial class PaymentForm    // In auto-generated file
{
  ...
  partial void ValidatePayment (decimal amount);
}

partial class PaymentForm    // In hand-authored file
{
  ...
  partial void ValidatePayment (decimal amount)
  {
    if (amount > 100)
      ...
  }
}

A partial method consists of two parts: a definition and an implementation. The definition is typically written by a code generator, and the implementation is typically manually authored.

Partial methods must be void and are implicitly private. They cannot include out parameters.