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POIs this a covariance problem? Not sure if brick wall
 

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  1. POIs this a covariance problem? Not sure if brick wall
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    <p>I wrote ASP.NET pages which will manage forms. They're based on the following base class.</p> <pre><code>public abstract class FormPageBase&lt;TInterface, TModel&gt; : Page, IKeywordProvider where TModel:ActiveRecordBase&lt;MasterForm&gt;, TInterface, new() where TInterface:IMasterForm { public TInterface FormData { get; set; } } </code></pre> <p>And a sample SubClass is here:</p> <pre><code>public partial class PersonalDataFormPage : FormPageBase&lt;IPersonalDataForm, PersonalDataForm&gt;, IHasFormData&lt;IPersonalDataForm&gt;, IHasContact { } </code></pre> <p>Below I have a usercontrol on the page which I want to "consume" the "FormData" from the page so that it can read/write to it.</p> <p>I then, have a more "common" user control that I want to have operate on the base Interface of all my form subclasses... IMasterForm </p> <p>But when the usercontrol tries casting Page.FormData (having tried to cast page to <code>IHasFormData&lt;IMasterForm&gt;</code> it tells me that the page is <code>IHasFormData&lt;IFormSubclass&gt;</code> even though I have a constraint on the IFormSubclass that says it is also IMasterForm</p> <p>Is there anyway that i can cast from the generic subclass to the generic superclass or is this "covariance" and a C# 4.0 thing?</p> <pre><code>public abstract class FormControlBase&lt;T&gt; : UserControl, IKeywordProvider where T:IMasterForm { protected T FormData { get; set; } protected override void OnLoad(EventArgs e) { base.OnLoad(e); //This cast is failing when my common control's T does not exactly match // the T of the Page.. even though the common controls TInterface is a base interface to the //pages TInterface FormData = ((IHasFormData&lt;T&gt;) Page).FormData; if (!IsPostBack) { PopulateBaseListData(); BindDataToControls(); } } protected abstract void PopulateBaseListData(); protected abstract void BindDataToControls(); public abstract void SaveControlsToData(); #region IKeywordProvider public List&lt;IKeyword&gt; GetKeywords(string categoryName) { if(!(Page is IKeywordProvider )) throw new InvalidOperationException("Page is not IKeywordProvider"); return ((IKeywordProvider) Page).GetKeywords(categoryName); } #endregion } </code></pre>
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    1. USEric Lippert
    1. USFamous Nerd
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    1. COSkeeter talks about this in c# in depth. It's worth a look.
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      1. USJason Down
    2. COI did think of that. Maybe for my own personal Apps I may do it for the lulz
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      1. USFamous Nerd
 

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