C# Language Tutorial => partial

Example

The keyword partial can be used during type definition of class, struct, or interface to allow the type definition to be split into several files. This is useful to incorporate new features in auto generated code.

File1.cs

namespace A
{
    public partial class Test
    {
        public string Var1 {get;set;}
    }
}

File2.cs

namespace A
{
    public partial class Test
    {
        public string Var2 {get;set;}
    }
}

Note: A class can be split into any number of files. However, all declaration must be under same namespace and the same assembly.

Methods can also be declared partial using the partial keyword. In this case one file will contain only the method definition and another file will contain the implementation.

A partial method has its signature defined in one part of a partial type, and its implementation defined in another part of the type. Partial methods enable class designers to provide method hooks, similar to event handlers, that developers may decide to implement or not. If the developer does not supply an implementation, the compiler removes the signature at compile time. The following conditions apply to partial methods:

Signatures in both parts of the partial type must match.

The method must return void.

No access modifiers are allowed. Partial methods are implicitly private.

-- MSDN

File1.cs

namespace A
{
    public partial class Test
    {
        public string Var1 {get;set;}
        public partial Method1(string str);
    }
}

File2.cs

namespace A
{
    public partial class Test
    {
        public string Var2 {get;set;}
        public partial Method1(string str)
        {
            Console.WriteLine(str);
        }
    }
}

Note: The type containing the partial method must also be declared partial.

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