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A First Look at Partial Methods

To see the implications of defining a partial method, create a new Console Application project

named PartialMethods. Now, define a new class named CarLocator within a C# file named

CarLocator.cs:

// CarLocator.cs

partial class CarLocator

{

// This member will always be part of the

// CarLocator class.

public bool CarAvailableInZipCode(string zipCode)

{

// This call *may* be part of this method

// implementation.

VerifyDuplicates(zipCode);

// Assume some interesting database logic

// here...

return true;

}

// This member *may* be part of the CarLocator class!

partial void VerifyDuplicates(string make);

}

Notice that the VerifyDuplicates() method has been defined with the partial modifier and

does not define a method body within this file. Also notice that the CarAvailableInZipCode()

method is making a call to VerifyDuplicates() within its implementation.

If you were to compile this application as it now stands and open the compiled assembly 

into a tool such as ildasm.exe or reflector.exe, you will find no trace of the VerifyDuplicates()

method in the CarLocator class, and no trace of the call to VerifyDuplicates() within

CarAvailableInZipCode()! Given the project as it now stands, you really authored the following

definition of the CarLocator class as far as the compiler is concerned:

internal class CarLocator

{

public bool CarAvailableInZipCode(string zipCode)

{

return true;

}

}

The reason for this strange stripping away of code has to do with the fact that our partial

VerifyDuplicates() method was never given a true implementation. If we were to now add a new

file to our project (named perhaps CarLocatorImpl.cs) that defined the remainder of our partial

method:

// CarLocatorImpl.cs

partial class CarLocator

{

partial void VerifyDuplicates(string make)

{

// Assume some expensive data validation

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