Enable Compiler Extensions in .NET 2.0

If you get an error like shown below; this is probably due to the downgrade of a project from .NET 3.5 or higher to .NET 2.0

Cannot define a new extension method because the compiler required type 'System.Runtime.CompilerServices.ExtensionAttribute' cannot be found. Are you missing a reference to System.Core.dll?

Extension methods are a new feature in .NET 3.5 (C#3/VB9) that let you appear to "spot weld" new methods on to existing classes.
If you think that the "string" object needs a new method, you can just add it and call it on instance variables.

But how do we accomplish this in .NET 2.0?

VB.NET

' you need this once (only), and it must be in this namespace

Namespace System.Runtime.CompilerServices

    <AttributeUsage(AttributeTargets.Assembly Or AttributeTargets.[Class] Or AttributeTargets.Method)> _

    Public NotInheritable Class ExtensionAttribute

        Inherits Attribute

    End Class

End Namespace

' you can have as many of these as you like, in any namespaces

Public NotInheritable Class MyExtensionMethods

    Private Sub New()

    End Sub

    <System.Runtime.CompilerServices.Extension> _

    Public Shared Function MeasureDisplayStringWidth(graphics As Graphics, text As String) As Integer

        ' ... 

    End Function

End Class

C#

// you need this once (only), and it must be in this namespace

namespace System.Runtime.CompilerServices

{

    [AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class

         | AttributeTargets.Method)]

    public sealed class ExtensionAttribute : Attribute {}

}

// you can have as many of these as you like, in any namespaces

public static class MyExtensionMethods {

    public static int MeasureDisplayStringWidth (

            this Graphics graphics, string text )

    {

           /* ... */

    }

}