To generate an XML documentation file from documentation comments in the code, use the /doc option with the csc.exe C# compiler.

In Visual Studio 2013/2015, In Project -> Properties -> Build -> Output, check the XML documentation file checkbox:

https://i.stack.imgur.com/tXXQy.png

When you build the project, an XML file will be produced by the compiler with a name corresponding to the project name (e.g. XMLDocumentation.dll -> XMLDocumentation.xml).

When you use the assembly in another project, make sure that the XML file is in the same directory as the DLL being referenced.

This example:

/// <summary>
/// Data class description
/// </summary>
public class DataClass
{
    /// <summary>
    /// Name property description
    /// </summary>
    public string Name { get; set; }
}

/// <summary>
/// Foo function
/// </summary>
public class Foo
{
    /// <summary>
    /// This method returning some data
    /// </summary>
    /// <param name="id">Id parameter</param>
    /// <param name="time">Time parameter</param>
    /// <returns>Data will be returned</returns>
    public DataClass GetData(int id, DateTime time)
    {
        return new DataClass();
    }
}

Produces this xml on build:

<?xml version="1.0"?>
<doc>
    <assembly>
        <name>XMLDocumentation</name>
    </assembly>
    <members>
        <member name="T:XMLDocumentation.DataClass">
            <summary>
            Data class description
            </summary>
        </member>
        <member name="P:XMLDocumentation.DataClass.Name">
            <summary>
            Name property description
            </summary>
        </member>
        <member name="T:XMLDocumentation.Foo">
            <summary>
            Foo function
            </summary>
        </member>
        <member name="M:XMLDocumentation.Foo.GetData(System.Int32,System.DateTime)">
            <summary>
            This method returning some data
            </summary>
            <param name="id">Id parameter</param>
            <param name="time">Time parameter</param>
            <returns>Data will be returned</returns>
        </member>
    </members>
</doc>