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Compiler Error CS1110

Visual Studio 2008

Cannot use 'this' modifier on first parameter of method declaration without a reference to System.Core.dll. Add a reference to System.Core.dll or remove 'this' modifier from the method declaration.

Extension methods are supported on version 3.5 and later of the .NET Framework. Extension methods generate metadata which marks the method with an attribute. The attribute class is in system.core.dll.

To correct this error

  • As the message states, add a reference to System.Core.dll or remove the this modifier from the method declaration.

The following example generates CS1110 if the file is not compiled with a reference to System.Core.dll:

// cs1110.cs
// CS1110
// Compile with: /target:library
public static class Extensions
{
    public static bool Test(this bool b) { return b; }
}

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