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POWhy does a generic type constraint result in a no implicit reference conversion error?
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  1. POWhy does a generic type constraint result in a no implicit reference conversion error?
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    copied!<p>I have created a couple of interfaces and generic classes for working with agenda appointments:</p> <pre><code>interface IAppointment&lt;T&gt; where T : IAppointmentProperties { T Properties { get; set; } } interface IAppointmentEntry&lt;T&gt; where T : IAppointment&lt;IAppointmentProperties&gt; { DateTime Date { get; set; } T Appointment { get; set; } } interface IAppointmentProperties { string Description { get; set; } } class Appointment&lt;T&gt; : IAppointment&lt;T&gt; where T : IAppointmentProperties { public T Properties { get; set; } } class AppointmentEntry&lt;T&gt; : IAppointmentEntry&lt;T&gt; where T : IAppointment&lt;IAppointmentProperties&gt; { public DateTime Date { get; set; } public T Appointment { get; set; } } class AppointmentProperties : IAppointmentProperties { public string Description { get; set; } } </code></pre> <p>I'm trying to use some constraints on the type parameters to ensure that only valid types can be specified. However, when specifying a constraint defining that <code>T</code> must implement <code>IAppointment&lt;IAppointmentProperties&gt;</code>, the compiler gives an error when using a class that is <code>Appointment&lt;AppointmentProperties&gt;</code>:</p> <pre><code>class MyAppointment : Appointment&lt;MyAppointmentProperties&gt; { } // This goes wrong: class MyAppointmentEntry : AppointmentEntry&lt;MyAppointment&gt; { } class MyAppointmentProperties : AppointmentProperties { public string ExtraInformation { get; set; } } </code></pre> <p>The error is:</p> <p><code>The type 'Example.MyAppointment' cannot be used as type parameter 'T' in the generic type or method 'Example.AppointmentEntry&lt;T&gt;'. There is no implicit reference conversion from 'Example.MyAppointment' to 'Example.IAppointment&lt;Example.IAppointmentProperties&gt;'.</code></p> <p>Could anybody explain why this does not work?</p>
 

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