How do you include Xml Docs for a class library in a NuGet package?

I am creating a NuGet package for a C# class library, and I would like to include generated Xml Documentation with the library. This is my nuspec file:

<?xml version="1.0" encoding="utf-8"?>
<package>
  <metadata>
    <id>MyLibrary</id>
    <version>1.0.0.0</version>
    <authors>John Nelson</authors>
    <language>en-US</language>
    <description>A C# class library</description>
  </metadata>
  <files>
    <file src="..\..\build\MyLibrary.dll" target="lib\Net40" />
    <file src="..\..\build\MyLibrary.xml" target="lib\Net40" />
  </files>
</package>

When I build the package with this command:

nuget pack MyLibrary.nuspec

It generates an error. If I remove the line:

<file src="..\..\build\MyLibrary.xml" target="lib\Net40" />

NuGet.exe successfully creates the nupkg. I can even unzip the package, and verify that the contents are correct. What am I doing wrong? Should the xml file go into a different target directory?

The problem was that I didn't check "Generate Xml Documentation" for the build configuration I was using. That nuspec is correct.

In .NET Core/Standard you can do this by editing the project XML file, for example:

<PropertyGroup>
	<TargetFramework>netstandard2.0</TargetFramework>
	<GenerateDocumentationFile>true</GenerateDocumentationFile>
</PropertyGroup>

<PropertyGroup>
	<DocumentationFile>bin\$(Configuration)\$(TargetFramework)\$(AssemblyName).xml</DocumentationFile>
</PropertyGroup>

This will output the documentation as an XML file next to your output assembly.

EDIT: As a side note once you enable GenerateDocumentationFile you will probably get lots of warnings on your public methods for not having added full documentation tags. If you want to disable these warnings simply add in the PropertyGroup:

<NoWarn>$(NoWarn);1591</NoWarn>